Synthetic resins are widely used by conservators. These resins are polymers constructed of a chain or network of repeating single units, called monomers, that combine with themselves, or with other similar molecules or compounds, to form polymers. Resins can be divided into two types of polymers: thermoplastic resins and thermosetting resins which are discussed below.

Thermoplastic resins are polymers in which the monomeric units are linked together to form two dimensional linear chains that are soluble in a range of solvents. They remain permanently fusible and soluble; however, some thermoplastic resins may form insoluble, infusible resins after long exposure to light or heat. Such exposure can cause chemical bonds or links, referred to as cross-linking, which become established between linear chains to form three dimensional networks characteristic of thermosetting resins.

Thermosetting resins are characterized by monomeric units that are linked together by chemical bonds to form three dimensional networks that are infusible and insoluble in all solvents. The three dimensional network will not allow solvents to flow between the chains so thermosetting resins remain permanently insoluble. However, some solvents may cause the resins to swell forming a gel. Originally thermosetting resins were hardened by the application of heat, thus the name thermosetting. At present, there are many cold setting resins, e.g., epoxy, polyurethane, and styrene, that congeal at room temperature when a catalyst is added.

There are innumerable adhesive/consolidants used in conservation and new ones are developed regularly. The ones most commonly used in conservation (Dowman 1970, UNESCO 1968) are:

1. Polyvinyl acetate (PVA) which is an organic solvent, e.g.,

Vinylite AYAA (V 12.5-14.5)

AYAC (V 14-16)

AYAF (V 17-21)

AYAT (V 24-30)

Gelva V7, V15 and V25;

2. Polyvinyl acetate (PVA) emulsions, e.g., CMBond M-2 (Elmer’s Glue All is not a true PVA emulsion.)

3. Acryloid B-72;

4. Cellulose Nitrate, also called nitrocelluloid, e.g., Ducco & HMG;

5. Polyvinyl butyral;

6. Various polymethacrylates in an organic solvent, e.g., Elvacite 20/3;

7. Polymethacrylate emulsions, e.g., Bedacryl;

8. Polyvinyl alcohol.

9. Elmer’s Glue All

In general polyvinyl acetate (PVA) is the most commonly used thermoplastic polymer resin for organic material recovered from archaeological excavations (Ashley-Smith 1983b, UNESCO 1968). This is true in the field as well as the conservation laboratory.

PVA is used both as a consolidant and as a glue. It comes in a range of viscosities (V) ranging from V1.5 to V60. The lower the number the less viscous the solution. The lower the viscosity, the lower the molecular weight and the lower the molecular weight the greater the penetration capability of the consolidant. However, the lower viscosity PVAs have less bonding strength than the more viscous ones. Also, the lower viscosity PVAs (below V7) form soft films that attract dust and are subject to cold flow. The finish of PVAs above V25 are very glossy and are often brittle if used alone. V7, V15 and V25 are the only ones commonly use in conservation. PVA V7, with its smaller molecules, is uses on denser material such as well preserved bone and ivory , PVA, V15 is the general purpose resin, and PVA, V25 is used as a glue.

PVA has good stability to light and does not yellow. It remains soluble and does not cross-link and become irreversible. PVA in strong concentrations, especially V25, can be used as a surface consolidant or as a glue. Many conservators prefer to use V25 PVA as a glue, especially for pottery reconstructions, with good success. In my own experience, I have found that ceramic vessels glued with V25 PVA have occasionally fallen apart because of excessive "cold flow of the resin" in hot, humid, adverse storage conditions.

PVA can be used on any non-metal object, e.g., bone, ivory, shell, antler, teeth, wood, botanical specimens, textiles, murals, stone, etc. In thin solutions the lower viscosity PVAs, V7 & V15, are used to penetrate and consolidate fragile objects by painting or spraying. In many cases the object is best consolidated by immersing several times in a dilute solution of PVA. Often, there is a tendency for the dried PVA film to have a gloss. This can be eliminated by allowing the object to dry while it is suspended over an open bowl or jar of the solvent used to dissolve the PVA. In addition, the gloss can usually be eliminated by wiping the surface with a lint free cloth saturated with one of its solvents, especially acetone.

On drying there is some shrinkage of PVA that exerts contractual forces on the treated object. This can distort fragile thin, pieces, textiles, thin painted surfaces, and other similar objects. PVA is heat sealable; for example, two pieces of cloth treated with PVA can be bound by ironing the two together.

PVA is soluble in a number of organic solvents. Solubility of PVA is directly related to the volatility of the solvent; the more volatile the solvent, the more soluble the PVA. The more soluble the PVA, the better the penetration of PVA into the object being treated. Some of the most common solvents, ranked in order from the most volatile to the least, are listed below:

The non-toxic, water miscible solvents are the most useful with acetone and ethanol being the most commonly used.

PVA FORMULATIONS

Slow Drying PVA Formula Fast Drying Formula

1. ethanol 1. acetone

2. 5-15% PVA 2. 5-15% PVA

PVA can also be purchased as an emulsion, such as CMBOND M-2. Emulsions are stabilized dispersions of finely divided particles of the resin in water. The resin is merely suspended in the water; it is not dissolved in the water. As long as the emulsions are liquid they can be thinned with water; for example, most water cleanable interior latex paints are actually PVA emulsions. PVA emulsions can be used directly on wet material without drying or driving off the water with a water miscible alcohol. Emulsions are miscible with water, but after drying the resin requires the same solvents as the nonemulsified resins. In the repair of pottery it has been found that PVA emulsions form better optical bridges across cracks than solvent glues. Most commercial PVA emulsions come in a viscosity suitable for use as a glue, so they must be diluted to use for impregnating material. For dilution use, CMBOND M-2 has approximately .6 grams of resin per 1 gram of stock mixture.

PVA V25 and even V15 is often used as a glue. When used as a glue, it only necessary that it be thick enough. One acceptable procedure for making glue is provided by Stephen Koob (1996) The procedure described here uses Acryloid B-72, but the process works equally well with any PVA formulation. This glue formulation, using Acryloid B-72 is the standard glue used at CRL.

It has generally been thought that Elmer's Glue All is a PVA emulsion. In its original formulation it was a casein glue, but about twenty years ago the Borden Co. changed the formula to a PVA emulsion. Recently, there has been an additional change. Now, unlike other PVA emulsions that are soluble only in PVA solvents after they are dried, the only solvent recommended by Borden, Inc. for Elmer's Glue All is water. They claim that any other solvent would only set the glue more. Because of the uncertainties about Elmer's Glue All, it is not recommended for conservation. It is, however, an excellent glue for wood that is not going to be exposed to outside environments and for consolidating quantities of faunal bone. The stock solution of Elmer’s Glue All works fine as glue, but it must be diluted with water in order to use it to impregnate and strengthen material. For dilution purposes, stock Elmer’s Glue All has approximately .9 grams of resin per one gram of stock mixture.

In conservation make sure at all times that you are working with a true PVA emulsions, such as Bulldog Grip White Glue. Innumerable problems and additional work have resulted from the use of "white glues" of unknown formulation.

ACRYLOID B-72

Acryloid B-72, in Europe referred to as Paraloid B-72, is a thermoplastic acrylic resin manufactured by Rohm & Haas which has replaced PVA in many applications and is preferred by many conservators over PVA. It is a methyl acrylate/ethyl methacrylate copolymer and is an excellent general purpose resin. Acryloid B-72 generally dries with less gloss than PVA. It dries to a water-clear transparency and is resistant to discoloration even at high temperatures. Acryloid B-72 is very durable and has excellent resistance to water, alcohol, alkalis and acid. Acryloid B-72 has exceptional resistance to mineral oil, vegetable oils and grease and it retains excellent flexibility. It can be applied in either clear or pigmented coatings by a variety of application methods and can be air-dried or baked. It has a very low reactivity with sensitive pigments. Acryloid B-72 is durable and non-yellowing. It is compatible with other film forming materials such as PVA and cellulose nitrate. It can be used in combination with them to produce coatings with a wide variety of characteristic with emphasis on stability and transparency. In stronger concentrations, it can be used as a glue. See Koob (1996) for details on preparing it for use as a glue.

Acryloid B-72 is unique in possessing a high tolerance for ethanol, e.g., after being dissolved in acetone or toluene, up to 40% ethanol can be added to the solution to control the working time. This property allows its use in applications where strong solvents cannot be tolerated. The alcohol dispersion may be cloudy or milky; however, clear, coherent films are formed on drying. It has also been found that the friable surfaces of porous, salt contaminated objects can be stabilized with Acryloid B-72 while the salts are being diffused out in water baths without the adverse effects resulting from the use of soluble nylon discussed below.

Krylon Clear Acrylic 1301 is a formulation of 20% Acryloid B-66 in toluene (NWM) that is easily obtained and is excellent for consolidating or sealing off the surfaces of a wide range of material. It is a ethyl methacrylate resin that is harder than Acryloid B-72. It can be used in most instances in place of Acryloid B-72.

Cellulose nitrate, formerly called nitrocelluloid, has a long history of use in conservation. Recently it has, to a large degree, been replaced by other synthetic resins. Cellulose nitrate is still used; especially as an adhesive. It has many of the same characteristics of PVA, but it is not internally plasticized as most PVAs are. Therefore, cellulose nitrate has a much greater tendency to become brittle, crack and peel off than does PVA.

Cellulose nitrate is soluble in acetone, methyl ethyl ketone and esters such as amyl acetate and n-butyl acetate. Since it is not soluble in the alcohols, e.g., ethanol and methanol, it is useful on compound objects requiring different consolidating resins with different solvents. A plasticizer is required to prevent the resin from becoming too brittle.

There are a number of proprietary adhesives on the market that utilize cellulose nitrate. Duco cement is one example marketed in the USA. Duco cement is cellulose nitrate dissolved in acetone and butyl acetate with oil of mustard added as a plasticizer. Because of its availability Duco Cement has been used extensively, with varying success, in pottery reconstructions and general artifact mending. In the past, I have used Duco and have found that while it is easy to use and is effective in the short run, over the years the glue has yellowed and has become brittle resulting in the breakup of the glued items. It is not recommended for use in archaeological conservation (Feller and Witt 1990; Moyer 1988b).

Cellulose nitrate is discussed here because of it availability and it general misuse in many conservation projects. In a few given cases where it might be necessary to use several resins with mutually exclusive solvents to consolidate some complex object. It use is this case is only on a temporary basis, and should be removed and substituted with a longer lasting, reversible resin. Cellulose nitrate should never be used as a glue. While it still has its drawbacks, diluted Duco Cement can be used to stabilize material such as bone by impregnation. For dilutions purposes, Duco Cement has approximately .8 grams of resin per one gram of stock mix in the tube.

POLYMETHYLMETHACRYLATE

There are a large number of Polymethylmethacrylate (PMM) resins that are easily obtained world wide under different trade names such as Perspex and Lucite (formally called Plexiglass). There are many different formulation. In the past I have had good results with Elvacite 20/3. The glue is commonly made from sheets of Lucite. Even safety mask shields and motorcycle wind shields can be dissolved in solvents. The toxicity of the solvents restrict PMM resins from being widely used.

A typical formulation for a "plexiglass glue is as follows:

Grind, cut, drill a sheet of Lucite to get a cup of shavings -- approx same volume of solvent. Place in a jar and add 50% chloroform /50% toluene. Caution heat is generated. Add acetone to thin to correct viscosity.

The PMM resins have similar properties to PVA. PMM resins are stronger but have fewer solvents. Two solvents for PMM resins are chloroform and ethyl acetate. Many PMM resins require mixed solvents such as 8 parts toluene and 2 parts methanol or a combination of chloroform and ethylene dichloride. In dilute solutions PMMs penetrate dense material very well. In most instance the PMM adhesives are prepared from sheets of resin. The PMM consolidants are particularly useful when more than one consolidant is required on the same object or cluster of objects. Like PVA, PMM can be purchased as a resin or as an emulsion. Bedacryl is one type of PMM emulsion.

Polyvinyl alcohol (PVAl) is a very useful resin in certain circumstances because water is the only suitable solvent (UNESCO 1968). PVAl resins are used as consolidants and adhesives. They come as a white powder in low, medium and high acetate grades with viscosities ranging from 1.3 to 60. Low and medium acetate grades with viscosities of 2 to 6 are most commonly used in conservation. Concentrations of 10-25% are used depending on the viscosity and penetration desired. In general, (depending on brand) PVAl dries clearer than PVA. It is more flexible and shrinks less; therefore, it exerts less contractile force than PVA when drying. For this reason it is often used in textile conservation. It can be used on damp or dry objects. PVAl has been particularly useful for treating wet bone, fragile textiles and for gluing fragile textiles to plastic supports. It has been used for conserving paper and textiles with water fast dyes that are alcohol soluble. PVAl is not recommended for wood.

Since PVAl is soluble only in water the solution requires a fungicide such as Mystox LPL (pentachlorophenol), Dowicide 1 (ortho-phenylphenol), or Dowicide A (sodium-o-phenylphenate tetrahydrate) to prevent mold growth.

There are indications of a slight tendency for some PVAl's to cross link in three to five years if exposed to strong light, dryness, and heat; especially temperatures over 100^o C. If cross-linking occurs the resin becomes less soluble, but probably never becomes completely insoluble. Some conservators recommend that objects treated with PVAl be retreated every 3-5 years to counteract any possible cross-linking.

The high acetate grades of PVAl are soluble in cold water, but the low and medium grades must be dissolved in water heated to 40-50^o C (ca. 125^o F). It is particularly useful when more than one consolidant is required on the same object. PVAl is very resistant to oils, greases, and organic solvents, but it has poor adhesion properties for smooth surfaces. Like PVA it is heat sealable at 120^o-150^o F.

There are innumerable thermosetting epoxy resin on the market with many varied properties and special characteristics. Each conservator, through experience, has his own favorites. Epoxy resins make excellent adhesives, consolidants, and gap-fillers. There are cold setting thermosetting resins that set up with the addition of a catalyst. The most desirable characteristic, aside from their strength, is that there is no shrinkage as they set. This is in contrast to all the thermoplastic resins that set through the evaporation of a solvent thereby undergoing some degree of shrinkage. The main disadvantages of epoxies are that they are essentially irreversible and often discolor with age. In some applications, optically clear resins should be selected. As a general rule epoxy resins should be avoided, but this is not to say they do not have a use in conservation. In many instances, epoxies are required because nothing else has the necessary strength. They are excellent when a very strong, permanent bond is required. They are often used in reconstructions of wooden and glass artifacts and are used extensively in all aspects of casting.

Various Araldite epoxy compounds are used extensively in glass conservation and in preparing fossil and other materials that require a permanently clear epoxy. In casting and replicating metal artifacts from marine sites, various Hysol casting epoxies have been used. In all cases, be sure and follow the directions of the manufacturer on the recommended hardeners, mixtures and thicknesses. If mishandled, a considerable amount of exothermic heat can be generated.

These are just a few of the most commonly used adhesives/consolidants used in conservation that have a long, successful track record and are, therefore widely used for a wide variety of conservation purposes. Specific applications are discussed in the following sections.

Approximately 70% of bone and ivory is made up of an inorganic lattice composed of calcium phosphate and various carbonates and fluorides. The organic tissue of both is ossein and it constitutes at least 30% of the total weight. It is often difficult to distinguish between bone and ivory unless examined microscopically. Bone is coarse grained with characteristic lacunae or voids while ivory is a hard, dense tissue with lenticular areas. Both bone and ivory are easily warped by heat and moisture and are decomposed by prolonged exposure to water.

In archeological sites, ossein is decomposed by hydrolysis and the inorganic framework is disintegrated by acids. In waterlogged sites bone and ivory can be reduced to a sponge-like material. In arid sites they become dry, brittle and fragmented. In some circumstances bone and ivory can become fossilized as the ossein is replaced by silica and mineral salts. Archeological bone and ivory can only be cleaned, strengthened and stabilized; satisfactory restoration is often impossible.

Removal of surface dirt from structurally sound bone/ivory

Structurally weak bone/ivory must be cleaned carefully and the cleaning method used is dictated by the specimen's condition.

Removal of soluble salts

Bone/ivory from a salty environment will invariably absorbs soluble salts that will crystalize out as the object drys. The action of salt crystallization will cause surface flaking and can, in some cases, destroy the specimen. The soluble salts have to be removed to make the object stable. For faunal bone, it is usually not necessary to remove all the soluble salt. It is safer to rinse then in tap water until the chloride level in the material being treated equalizes with the tap water. For more important artifacts made of any of these materials it advisable to remove all the soluble salts by rinsing for an appropriate length of time in tap water and then in deionized water.

In order to determine the level of salts in the rinse water, it is necessary to use a conductivity meter. In most cases, one can alternatively use the silver nitrate which tests for the presence of sodium chloride. When sodium chlorides are no longer present, one can be pretty sure -- but not absolutely sure -- that the bulk of the soluble salts have been removed. The conductivity meter measure the presence of all soluble salts and is thus a much more reliable indicator of the presence and absence of soluble salts in an aqueous solution.

In all cases, the question should be asked, is it necessary to remove the insoluble salts. If the answer is, Yes, then some means of mechanical removal using picks, or other tools is always recommend over any chemical treatment, for inevitably some damage is done to bone and related material when stains and insoluble salts are removed by chemical means. When chemical means are utilized, always make sure that the material is thoroughly wetted with water before any chemical are applied. If this is done, the chemical is not absorbed and the treating chemical stays on the surface.

Calcium carbonate

Iron stains

Sulfide stains

Unsound bone should be treated with localized applications of the acid with a brush or swab. If unsound bone is submerged, the evolution of carbon dioxide from the decomposition of the CaCO₃ will break up the specimen. Very fragile bone may require that the acid be applied locally to stubborn spots, scraped and blotted and then all the steps repeated until the area is cleaned.

It is then necessary to rinse in water to remove all residue of the treating chemical, dry in alcohol baths, and then consolidate with a resin as describe below.

Consolidation

1. The resin solutions must be diluted to decrease the viscosity and increase its ability to penetrate the material being treated. A 5% to 10% solution of a suitable transparent synthetic resin may be used. When large amounts of faunal bone needs to be consolidated, I have found that the water soluble, Elmer's Glue All works quite satisfactorily. When bone, ivory and teeth are treated, usually, slow dehydration in organic solvents, as described in I above, then consolidation with PVA (V7) or Acryloid B-72 is recommended. PVA with a viscosity of V7 is recommended, because the molecules are smaller and are able to penetrate denser material, yet the resin has enough strength to mechanically strengthen any object treated with it. For less dense material and a lot of faunal bone, PVA with a viscosity of V15 is recommended since it is a stronger, general purpose resin.

2. For surface consolidation apply resin by brush. Best results can be obtained by applying a light coat of resin, let dry and then apply a second coat. Such a procedure should be repeated several times in order to get sufficient resin absorbed by the material to accomplish consolidation. Repeated applications are necessary.

Complete immersion in the consolidating resin gives the best results. Complete immersion in a vacuum is even better.

3. Reconstruction or Gluing

The glue used to glue bone, ivory, or teeth together depends upon It depends, to some degree, on how it was treated. what the material being glued is It the bone or related material has been consolidated with a resin, then a thick viscous mixture of the same resin should be use. PVA with a viscosity of V25, Acryloid B-72, PVA emulsions such as Bull Dog, and in some cases Elmer's Glue All make serviceable glues.

The miscellaneous seeds and plant material are, for all intent and purposed treated in much the same way, if not the exact same way, as described above for bone and related material. Once recovered, it necessary to rinse them to remove any soluble salts that may be present, mechanically clean the material requiring it, chemically treating the material, rinsing out the treating chemical, then drying the material in a series of water/water miscible alcohol baths, and then consolidating the material.

The conservation of waterlogged bone and ivory, as well as most plant material, is a straight forward process. Seldom are there any complex problems except in cases where the bone is so badly deteriorated that it is can not be treated. In general, intensive rinsing in water to remove the soluble salts followed by complete dehydration through a graded series of water miscible solvents, and consolidation with an appropriate resin is all that is required. Stains can be removed, but the process can damage the bone if care is not taken. The only equipment required are the right size containers, a selection of resins, and a variety of solvents.

Generally pottery recovered from marine sites requires only minimal treatment since earthenware survives well in marine environments (Pearson 1987b). It is necessary, however, that the conservator be able to recognize earthenware, stoneware, and porcelain and to be familiar with the alternative treatments for conserving them (Olive and Pearson 1975; Pearson 1987d). Stoneware and porcelain are fired at such high temperature that they are impervious to liquids, and thus do not absorb soluble salts from their archaeological environment and it is not necessary to take then through long rinses to remove soluble salts. However, in some instances with certain kinds of stoneware and porcelain, glazes are applied in subsequent firings and sometimes salts get between the glaze and the body. If these salts are not removed the glaze can be flake off. So, even caution has to be exercised with stoneware and porcelain. Well fired pottery need only be washed in a mild detergent, scrubbing the edges and surfaces with a soft brush. Care should be taken not to remove traces of food, paint, pigments, and soot that is left on the interior or exterior surfaces. The conservator must be careful not to mark the pottery surface when using a brush or any other object during cleaning. Fragile, badly fired pottery requires more care, but the procedure is the same. Fragile pieces, pottery with friable surfaces, flaking surfaces or fugitive paints may require consolidation with a resin.

Earthenware excavated from marine sites becomes saturated with soluble salts and/or the surfaces often become covered with insoluble salts such as calcium carbonate and calcium sulfate. In many instances pottery adjacent to metal objects will be entrapped by the encrustation forming around the metal, especially iron. The soluble salts (chlorides, phosphates and nitrates) are potentially the most dangerous and they must be removed in order for the pottery to be stable. The soluble salts are hygroscopic and as the relative humidity rises and falls, the salts repeatedly dissolve and crystallize. These salts eventually reach the surface of the pot where extensive crystallization takes place causing exfoliation of the surface of the pot. Eventually, the pot will break as a result of internal stresses. At times, masses of needle-like crystals can cover the surface, hiding all details. Soluble salts can be removed by repeated rinsing in water (a running bath is the quickest and most effective but is very wasteful). There are any number of ways of setting up a series of vats so that water runs into one vat, and over flows into a second vat, and then another, etc. in order to waste the minimum amount of water, especially if you are using deionized water. One should always keep in mind that very simple procedures, such as putting soluble salt laden sherds in a mesh bag and simply place them in the reservoir of a toilet. Innumerable volunteers assist you each day in changing the water and the salt content in the sherd quickly equalizes with that contained in the supply water. Then, if necessary, the rinsing can then be continued in several baths of D.I. water to lower the salt content even further. This is a simple trick that is very effective.

Monitor the rinsing progress with a conductivity meter. If sherds or pottery are too fragile to withstand the rinsing process, surfaces can be consolidated first with Acryloid B-72 (formerly soluble nylon was used), then rinsed. Since Acryloid B-72 is somewhat water permeable, it will allow the salts to diffuse out. The diffusion process will be considerably slower if treated with any consolidant such as Acryloid B-72 or PVA.

In most cases the safest and most satisfactory method of removing insoluble salts from the surface of pottery is by hand. Most calcareous concretions can be removed easily when wet by scraping with a scalpel, dental tool or similar tool. Dental burrs and pneumatic air chisels are also quite useful - especially the latter.

The insoluble salts can also be removed chemically, but it is important to pre-wet the sherd. Nitric acid, hydrochloric acid and oxalic acid are most commonly used. Before using any acid on pottery make sure that the paste is thoroughly wetted so the acid will not be absorbed into the paste. Although 10-20% nitric acid can be used to remove calcareous concretion, it is potentially the most damaging acid of the three. More care should be exercised when it is used; dilute nitric acid dissolves lead (glazes). In most cases 10-20% hydrochloric acid is safer than nitric acid to clean glazed pottery. Glazed pottery from marine sites is often cleaned of calcareous encrustation with 10-20% hydrochloric acid. The sherds are left immersed until all gas evolution ceases--usually less than an hour--and repeated if necessary. Care must be exercised, for hydrochloric acid can discolor glazes, especially lead glazes which will turn milky. The samples are then washed thoroughly in tap water; then immersed in 10% oxalic acid (very toxic) for 10-20 hours to remove iron stains. A thorough rinsing follows and the sherds are then dried. It is imperative that pottery with a carbonate temper (shell, calcium carbonate) not be in immersed in hydrochloric or nitric acid for the tempering material will be removed from the paste resulting in the weakening of the pottery.

While nitric, oxalic and hydrochloric acid treatments will remove calcareous deposits (especially hydrochloric), they tend to dissolve the iron oxides from pottery containing iron oxides in the paste or in the glazes (many stoneware glazes contain iron oxides). The use of these acids on glazes containing iron oxides increases their tendency to exfoliate, especially if the glazes are friable. To avoid over cleaning, the sherd should always be pre-wetted by soaking in water and then apply the acid locally on the surface with a cotton swab or by drops. The excess acid is immediately removed when the effervescing action stops by either wiping the area or rinsing under running water to remove the acid. Earthenware and terra cotta often contain iron oxides, are more porous and more prone to deteriorate when treated with these acids; therefore acid treatments should be used with some discretion..

Useful chemicals for removing calcareous deposits from ceramics are ethylene-diaminetetraacetic acid (EDTA). A 5% solution of the tetra-sodium salts of EDTA (pH 11.5) works best for removing calcareous material without seriously affecting the iron content of the pottery. Iron is more soluble at pH 4 while calcareous deposits are more soluble at pH 13. In this treatment the sherds are immersed in the solution and left until the deposits are removed. Periodically, the solution may have to be replenished. In the process, the iron stains that are usually bound in with the calcium salts are removed along with the calcium. Generally speaking, 5% EDTA-tetra sodium is recommended. It is a slow but effective treatment.

Soaking calcareous encrusted sherds in a 5% aqueous solution of sodium hexametaphosphate has been used to remove calcareous deposits. Care must be taken however since a solution of sodium hexametaphosphate has a tendency to soften the paste of the sherd more readily than the calcareous encrustation.

Calcium sulfate is very difficult to remove from pottery. To test for the presence of calcium sulfate, drop dilute nitric acid on the deposits - then add 3 drops of 1% barium chloride solution. If a white precipitate forms, it indicates the presence of sulfates (Plenderleith and Werner 1971). These can be dissolved slowly by immersing in 20% nitric acid. As the sulfates dissolve, sulfuric acid is produced which cancels out the reaction of the nitric acid. The nitric acid has to be changed often. This technique is not generally recommended. Mechanical cleaning is preferred.

Silicates on the surface of pottery can be removed with hydrofluoric acid, but this acid is very dangerous and is not recommended to be used by amateurs. Again, mechanical cleaning is recommended.

Iron oxide stains can be removed with 10% oxalic acid applied locally with cotton swabs on the surface of pre-wetted pottery. This is generally a successful method of removing iron stains from stoneware and earthenware ceramics although a small amount of the iron in the paste might be removed. A 5% EDTA solution is often used to remove stains from pottery containing iron oxide in the glaze or paste in order to minimize the removal of the iron oxide (Olive & Pearson 1975; Pearson 1987d). The di-sodium salts or EDTA are the most efficient for removing iron oxide stains because of their lower pH.. Either oxalic acid or EDTA will remove iron stains. In all treatments, caution must be exercised to avoid over cleaning. Intensive rinsing after cleaning is required.

Black metallic sulfide stains are very common on pottery from marine shipwrecks. They can be removed by immersion in 10-25% by volume hydrogen peroxide solution until the stains disappear.. The time required to remove the stains ranges from just a few second to several hours. No rinsing is required after treating with hydrogen peroxide. Hydrogen peroxide can be applied directly to sherds that have been treated with nylon. The hydrogen peroxide permeates the nylon film. Hydrogen peroxide is also useful for removing organic stains. Carefully monitor the progress, especially on tin enamel wares (delft, majolica, faience) when the glaze is crazed. Bubbles generated during treatment may lift off the poorly attached glaze.

Glues such as PVA (V25 or equivalent) and Acryloid B-72 can be used to repair broken pottery. In the past celluloid glues such as Duco have been used, but they have too short of a serviceable life to be used in conservation. A thick PVA (V25) solution in acetone, acetone/toluene or acetone and amyl acetate can be used as a glue. Others prefer a PVA emulsion glue in an aqueous base for gluing together porous pottery. It forms a better optical bridge across cracks than a solvent glue, but it has a tendency to give way in damp climates or uncontrolled storage. Alpha cyanoacrylate glues (Super Glue) are very handy. These can be dissolved slowly in acetone and toluene after setting. In most instances, it is necessary to consolidate earthenware sherds with a dilute solution of PVA or Acryloid B-72 in order to thoroughly strengthen their surfaces before they can be glued or repaired. This can be accomplished simply by immersing the sherds in a dilute solution of PVA.

Small objects made of stone can be treated in essentially the same manner as described for pottery (once pottery has been fired it is actually a form of stone). Many of the sedimentary rocks can absorb soluble salts and be stained. The same treatments and chemicals described under pottery can be used, but the acids should be no stronger than 5%. Do not use any acids on any of the sedimentary rocks (e.g. limestone, marble, sandstone, etc.)- these can be quickly destroyed by acid treatments. The acids can be used effectively on metamorphic and igneous rock.

The conservation of ceramics recovered from a marine site is not complicated. When they are found encrusted, the most difficult part of the conservation is the removal of the adhering material without damaging the paste or glaze. For this reason, mechanical cleaning techniques are preferred, but HCl acid is used with some regularity to remove calcareous encrustation. The soluble salts that are invariably present in any porous material recovered from marine sites are removed by rinsing in water. In most instances, tap water is all that is needed, but to the use of D.I. water in the final baths will remove more soluble salts. Sulfide staining is easily removed with hydrogen peroxide, but other stains, such as iron stains are more difficult to remove with out adversely affecting the ceramic. If the decision is made to remove the more difficult stains, the material should be thoroughly wetted with water before immersing or applying the chemicals to remove the stains. Monitor the process carefully and rinse thorough in water after using any chemicals. After treating allow the potter to air dry. Solvent drying is not required, but it can be used in desired. After drying, consolidate by completely immersing in a dilute solution of PVA or Acryloid B-72. Any sherds or reconstructible vessel can them be reconstructed.

Equipment required to conserve ceramics include appropriate size vats, tap water, D.I. water, acetone, ethanol, PVA, Acryloid B-72, hydrogen peroxide, HCl, EDTA, dental picks, and a pneumatic chisel is very useful.

Glass is usually the most stable of archaeological materials, but it can undergo some complex disintegration - especially 17th-century glass. Ideally, glass should consist of 70-74% silica, 16-22% alkali or soda ash (sodium carbonate) or potash (potassium carbonate, usually derived from wood ash) and 5-10% flux [lime (calcium oxide). Soda-lime glass is the most common glass throughout the history of glass making and the modern equivalent is 74% SiO², 16% Na₂CO_3, and 55 lime added as stabilizer._. Soda glass is characteristic of southern Europe where it is made from crushed white pebbles and soda ash derived from burnt marine vegetation. Soda glass, which is often used for the manufacture of cheap glass, is twice as soluble in water as potash glass.

Potash glass is more characteristic of inland Europe where it is made from local sands, and potash derived from wood ash and burnt inland vegetation. A little salt and minute amounts of manganese is added to make the glass clear, but potash glass is less clear than soda glass. Most early glass is green because of iron impurities in the materials. The alkali lowers the melting point of the sand and the flux facilitates the mixture of the components. As long as the original glass mixture was kept in balance, the resulting glass will be stable. Problems arise when an excess of alkali and a deficiency in lime (calcium oxide is used as a stabilizer) is used in the mixture, for the glass will be especially susceptible to attack by moisture. RH higher than 40% can be dangerous. If old glass has 20%-30% sodium or potassium it may begin to weep and break down -- this is called glass disease.

In all glass, the sodium and potassium oxides in glass are hygroscopic, therefore, the surface of the glass absorbs moisture from the air. The absorbed moisture and exposure to carbon dioxide causes the Na0₂ or NaOH and KO₂ or KOH to convert to sodium or potassium carbonate. Both NaCO₂ and KCO₂ are extremely hygroscopic. At a relative humidity (RH) of 40% and above (and in some cases as low as 20% RH), drops of moisture appear on the glass surface. In water, especially salt water, the Na and K carbonates in unstable glass can leach out leaving only a fragile, porous hydrated silica (SiO₂) network. This causes the glass to craze, crack, flake and pit giving the glass a frosty appearing surface. In some cases there is an actual separation of layers of glass from the body. Fortunately, these problems are not commonly encountered in 19th-century and later glass. When problems are encountered it is on types not expected to be encountered on sites dating from this period. Pearson (1987b. 1987d) discusses glass deteriorations and reviews the various glass conservation procedures.

At our present state of knowledge, the decomposition of glass is imperfectly understood, but most glass technologists agree that glass decomposition is due to preferential leaching and diffusion of alkali ions (Na & K) across a hydrated porous silica network. Sodium ions are removed and replaced by hydrogen ions which diffuse into the glass to preserve the electrical balance. The silicates are converted into a hydrated silica network through which sodium ions diffuse out.

Decomposed glass often appears laminated, with iridescent layers on the surface. Glass retrieved from an acid environment often have an iridescent film which is formed by the leached silica layers. The alkali which leaches out is neutralized by the acid and fewer hydroxyl ions are available to react with the silica. This causes the silica layer to thicken and become gelatinized as the alkali leaches out. Glass excavated from an alkaline environment is less likely to have laminated layers because there is an abundance of hydroxyl ions to react with the silica network. Normally a protective layer does not form on glass exposed to alkaline solutions. The dissolution of the glass proceeds at a constant rate. The alkali ions are always extracted in excess of the silica, leaving an alkali deficient layer which continually thickens as the deterioration moves deeper into the glass.

There are considerable differences of opinion as to what to do with unstable glass. Some advise that the only treatment should be to keep the glass in low relative humidities so the glass does not have any moisture to react with. While a RH range of 40% to 55% is usually recommended, it varies in relationship to the stability of the glass. The weeping or sweaty condition is sometimes made worse by the application of a surface lacquer or sealant. No resin sealants are impervious to water vapor and the disintegration continues under the sealant until the glass falls apart. Other glass conservators try to remove the alkalinity from the glass to halt the deterioration.

When it comes to 18th-20th-century sites, we are on much more secure ground, for most, if not all, of the glass that might be found during this period was produced from a stable glass formulation, and there is not likely to be any problem glass present, other than normal devitrification. Thus, since the glass is impervious to salt contamination, no conservation treatment other than simple rinsing, removal of incidental stains, especially lead sulfide staining on any lead crystal, and removal of calcareous deposits is envisioned. The main problems will be related to gluing pieces together and possibly some restorations. All the problems likely to be encountered are discussed thoroughly in Conservation of Glass (Newton & Davidson 1989).

While, glass that is susceptible to weeping because of unstable glass formulations, is not expected, when it is encountered is treated in different ways, but the technique described by Plenderleith & Werner (1971:345) is representative of the treatments often recommended.

The above treatment does not attempt to remove any of the glass corrosion products which often results in layers of opaque glass, which can be removed with various acid treatments. The decision to remove surface corrosion products, which often mask the color of the glass, has to be made on a case by case basis. Removal of corrosion products, which can reveal the true color of the glass as it significantly reduces the thickness of the walls. At the same time, it can sometimes weaken the glass object significantly. Indiscriminate removal of surface corrosion products can weaken, blur or alter surface details. The corrosion layers of a glass object can be deemed a part of the history of the object, and thus a diagnostic attribute and should not be removed without good reason.

Devitrification is a natural process that occurs on siliceous material. It occurs naturally on flint and obsidian and is the basis for obsidian hydration dating. The surface of any glass from any time period usually becomes hydrated through time, especially soda glass. It can be expected to be present on any glass after a length of time. Devitrification occurs when the surface of the glass becomes partly crystalline as it adsorbs moisture from the atmosphere -- and from being submerged in water. As it becomes crystalline the surface becomes crazed and flakes from the body of the glass. Devitrified glass has a frosty or cloudy, iridescent appearance. Pane glass is especially susceptible. To prevent further devitrification and to consolidate the crazed surface, a coating of PVT or Acryloid B-72 is applied. Any of these surface adhesives will smooth out the irregularities in the pitted, crazed surface of the glass, making it appear more transparent, by filling in the small cracks and forming optical bridges. Merely wetting glass will cause it to be appear clearer for the same reason.

Leaded glass, which includes a wide variety of stem wares and forms of lead crystal can become badly stained by lead sulfide. Glass that is normally clear will come from marine sites and anaerobic sites with a very dense black film on its surfaces. A 10-15% solution of hydrogen peroxide is used, as with ceramics, to remove these sulfide stains. Other than stain removal, strengthening of glass artifacts with a consolidating resin is often required. Fragments can be glued together with a good glue, or if deemed necessary, an epoxy such as Araldite.

Glass can be repaired and reconstructed with the same glues as described for pottery, but optically-clear epoxy resins are generally used as they adhere to the smooth, non-porous glass more readily, they dry clearer and shrink less than the solvent resins and are, therefore, less noticeable and the bonds are stronger. The epoxy resins are, however, usually irreversible. Hysol Epoxy 2038 with Hardener 3416 and Araldite, are the two brands most commonly used in glass repair. The new "super glues" made of cyanoacrylate are used quite often to piece the glass together quickly. After using the cyanoacrylate, epoxy is flowed into the cracks with an artists brush to permanently glue the pieces. It is exceptionally difficult and time consuming to gap-fill glass. It requires considerable work and experience. The problem of matching transparent glass colors is equally difficult. All these problems are adequately discussed in greater detail in Conservation of Glass (Newton and Davison 1989).

As is the case in all conservation it necessary for the conservator to be able to recognize what the problems are and to know what can be used to counter them. In glass conservation when lead oxides are found on glass it can be removed with 10% nitric acid. 1-5% sulfuric acid can be used to remove iron oxide and to neutralize the alkalinity of glass that is breaking down and occasionally for removing calcareous deposits. Calcareous deposits are commonly removed with 10% hydrochloric acid and on some occasions by immersing in 5% EDTA, tetra sodium. Iron stains are commonly removed with 5% oxalic acid or 5% EDTA, di-sodium.

Realistically, few problems, other than reconstruction and restoration are likely to be encountered on any of the glass objects found in archaeological site dating from the mid-18th century to the present. Essentially the same chemicals and equipment required for treating ceramics are used for conserving glass.

Schematic diagram of hardwood and softwood illustrating the relative appearance of vessels and tracheids.

Being of organic origin, wood normally decays under combined biological and chemical attack when buried in the ground. It can, however, survive prolonged exposure to extremes of dryness or wetness. In shipwreck sites, the wooden components of the hull and small artifacts of wood often survive in good condition, although thoroughly waterlogged. The mechanisms of organic deterioration are succinctly presented in Florian (1987). Successful conservation depends upon a knowledge of wood structure and types. Trees are divided into two broad categories, hardwoods and softwoods. Hardwoods are in a taxonomic grouping called Angiosperms and which refers to broadleaf trees which are usually deciduous. They are referred to as porous woods because they have vessel pores. Oak and birch are typical examples of a hardwood. Softwoods, or Gynmosperms are needle bearing trees or conifers. They are called nonporous because they lack vessel pores. Pines are a typical example of a softwood. It is very important to know the category of wood and in many instances, it is critical that one knows the species in order to successfully conserve the waterlogged wood.

Generally speaking, freshly cut, sound wood will, through water loss, shrink ca. 3-6% radially, 5-10% tangentially, and - 0.5% longitudinally. Oak shrinks 4% radially and 8% tangentially when air dried after cutting, while waterlogged oak can shrink 12% radially and ca. 24% tangentially. Proper conservation will control the amount of shrinkage. In practice, a particular conservation technique is often selected because it is known that the treated wood will shrink a desired amount (Patton 1988:43).

The manner in which a board was originally sawn from the log will have an effect on how it is going to shrink after undergoing any conservation treatment. In fact, the method of sawing can make it very difficult to conserve the wood without cracking and warping occurring after treatment.

In order for lumber to exhibit true tangential and radial planes it must be rift or flitch-sawn. Because rift sawing is complex and inefficient, a modified pattern called quarter sawing was developed. Quarter-sawing results in lumber with predominantly radial surfaces on the faces, it dries with less tendency to warp. Because, flat or plain sawn lumber has similar proportions of radial and tangential surfaces with arched grain patterns, it tends to warp easily.

In anaerobic waterlogged environments there are profound chemical changes and alterations in the composition and microstructure of woods, resulting in great loss of strength while retaining overall shape and form. In other environments wood decays from; 1) physical action (changes in temperature, fluctuations in relative humidity, etc.), 2) insect attack, and 3) fungal decay. By far fungal decay, along with anaerobic bacteria, plays the largest role in the breakdown of wood. Fungal decay can be eliminated as long as the wood is kept in a relative humidity of less than 65%.

The tannin in wood protects it from degradation, and allows some wood, especially those with high tannin contents, to survive in good condition. In all wood, after long periods in wet soil, peat bogs and marine sites, bacterial action causes a degradation of the cellulosic components of cell walls. In general, water soluble substances such as starch and sugar disappear from wood first, along with mineral salts, coloring agents, tanning matters and other bonding materials. In time, through hydrolysis, cellulose in the cell walls disintegrates, leaving a lignin network to support the wood. Even the lignin will break down over a long period of time. As a result of the disintegration of cellulose and lignin, spaces between the cells and molecules increases and the wood becomes more porous and permeable to water. All the deteriorated parts, all cell cavities and intermolecular spaces are filled with water. The remaining lignin structure of wood cells and the absorbed water preserves the shape of the wood. The loss of the finer cellulose tissue does not cause much alteration in the gross volume of wood, but the porosity is increased and the wood absorbs water like a sponge. As long as the waterlogged wood objects are kept wet they will retain their shape. If the wood is exposed to the air, the excess water evaporates and the resulting the surface tension forces of the evaporating water causes the weakened cell walls to collapse, causing considerable shrinkage and distortion. As mentioned above, freshly cut, sound wood will, through water loss, shrink ca. 3-6% radially, 5-10% tangentially, and - 0.5% longitudinally. Oak shrinks 4% radially and 8% tangentially when air dried after cutting, while waterlogged oak can shrink 12% radially and ca. 24% tangentially. The amount of shrinkage is dependent upon the degree of disintegration and the amount of water present. The amount of water in waterlogged wood is determined by the formula:

%H₂0 = weight of wet wood - weight of oven dried wood X 100 weight of oven dried wood

Anything greater than 200% is considered to be degraded, and it is not uncommon to find wood that has more than 500% water. 1000% water is not uncommon. Waterlogged wood is often classed according to the amount of water is contains.

Class I, over 400% H₂0

Class II 185-400% H₂0

Class III less than 185% H₂0

The Class III hardwoods are the most difficult to conserve.

The conservation of waterlogged wood is a two-fold problem that involves (1) incorporation of a material into the wood which will consolidate and confer mechanical strength to the wood as the water is removed and (2) the removal of the excess water by a method which will prevent any shrinkage or distortion of the wood. All the bulking treatments that use PEG or sugar are examples of the first, while solvent drying and freeze-drying are examples of the last. The most common techniques for treating waterlogged wood are discussed below. In all the treatments discussed below, if the wood was recovered from a salt water environment, it is necessary that the bulk of the soluble salts be removed, otherwise, they will cause a white bloom on the conserved wood and will can adversely affect any remaining iron components in the wood and other material in the same room or environment where the treated wood is stored.

The polyethylene glycol is a synthetic materials that has the generalized formula H₂OCH (CH₂OH₂)_nCH₂OH. The low molecular weights (300 - 600) are liquids, the intermediate members are semi-liquids or have the consistency of Vaseline (1000-1500) and the higher molecular weights (3250-6000) are wax-like materials. The various PEGs are now designated by their average molecular weight, but this only came about in recent years and the names have been changed. What was once called PEG 1500 is now called 540 blend (it is equal parts PEG 300 and PEG 1500), PEG 1540 is now called PEG 1500 and PEG 4000 is now called PEG 3250. Although the PEGs have some of the physical properties of waxes, they are distinguished from true waxes by the fact that they are freely soluble alcohol (ethanol, methanol, isopropanol), as well as water.. PEG 4000, which has a melting point of 53-55^o C (ca. 125^o F), was once the most commonly used because it was the least hygroscopic, but its large molecules prevent it from penetrating dense wood. Now PEG 1500 and 540 blends are more commonly used.

The development of PEG as a conservation process was the first reliable method for treating waterlogged wood that was simple to carry out. The excess water in wood is removed and the wood is bulked in one operation. The waterlogged object is cleaned to remove all dirt on the surface and is placed in a ventilated vat where the temperature is gradually increased until, after a period of days to weeks, it has reached 60^o C (140^o F). During this time the PEG percentage of the solution increases as the solvent (water or alcohol) evaporates of by adding increments of PEG. In the process, the wax slowly diffuses into the wood, displacing the water. At the end of the operation, the wooden object is covered with molten 70- 100% PEG, depending upon the nature of the wood. The object is then removed, the excess wax wiped off and allowed to cool. When cooled, any excess wax on the surface is removed with a hot air gun or with hot water. When carrying out this method using a container in which the PEG concentration is increased by evaporation of the solvent, it is important that the dimensions of the container be such that the starting quantity of wax present will be more than enough to cover the object at the end of the process.

As mentioned above, PEG is soluble in both water and various alcohol. For large objects, water is always used, because it is the cheaper procedure. When using PEG in water it is necessary to use a fungicide such as Dowicide 1 (ortho phenylphenol) , 0.1% of weight of PEG used. On the Wasa, a fungicide consisting of 7 parts boric acid and 3 parts sodium borate was used -- 1% of weight of PEG (Barkman 1975:82). For smaller objects, it is often more convenient to use alcohol, such as ethanol. The treatment is considerably reduced, and it results in a wood that is lighter in weight as well as lighter in color. When alcohol is used, it is best to dehydrate the wood in at least three baths of ethanol before placing it in the first PEG mixture. It is not critical that all the water be removed however, for since PEG is soluble in both water and alcohol, the presence of water in the wood does not present a problem. Alcohol treatments save time, but obviously increase the cost and there is always the inherent danger of heating alcohol mixtures. Since all the alcohols are fungicide, no biocide is required when using them in PEG mixtures.

An important consideration that comes into play when PEG is used to conserve wood is the fact that PEG is corrosive to all metals, but especially with iron. For that reason, PEG treatments should not be used on wood that is going to be in contact with any metal, especially iron. Therefore, you would not want to use it on a gun stock that was going to be put back on the iron barrel after treatments, because the PEG would or could then cause the iron to start corroding.

It usually a simple matter to rig up any number of ways to treat the smaller waterlogged wood artifacts in the laboratory. Small vats (metal or glass) are readily available and they can be placed in a thermostatically controlled oven to maintain the correct temperature and only a small amount of PEG is required. In contrast, when large pieces of wood are treated, there is a substantial investment in PEG (sometimes measured in the tons) and a substantial vat has to be constructed that can be heated and the contents circulated with pumps. It is a major investment in both equipment and chemicals when a laboratory elects to get in the business of conserving large wood pieces. Quite often this capacity is the most costly investment of the laboratory. In addition, between treatments, the PEG has to be kept stored.

As am example of cost, if a single piece of wood 6 feet long and 1 foot wide was treated. While a stainless steel vat would be preferred, because PEG is corrosive to all metals, but most laboratories get by with using mild steel vats painted with a good epoxy paint. The cost of a mild steel vat 8 feet long, 2 feet wide, and 2 feet deep would cost approximately $400.00. A vat this size will hold 32 cubic feet, or 1984 pounds of water, and approximately the same amount of PEG. a ton of PEG costs approximately $2000. Either strip heaters or preferably a in-line heated pump is required to keep the solution heated (otherwise it solidifies when a 20-30% concentration is reached). A Chromalox circulation heater will cost approximately $900.00. The other pumps to circulate the solution will cost around $200.00. This comes to an investment of $3500.00 for equipment and PEG. Of the methods discussed in this section, any of the various PEG treatments with water is the most utilized because of its reliability and low cost.

The sucrose (sugar) method of conserving waterlogged wood was developed as an alternative to more expensive methods (Parrent 1983, 1985). In practice the procedure is for all intent and purpose, identical to that described for PEG, except that sucrose is used. Wood to be conserved should be carefully cleaned by rinsing in baths of fresh water to remove all ingrained dirt and to remove the bulk of any soluble salts that are present. Once the wood is clean the following procedure is recommended:

1. Prepare a solution with a sufficiently low sucrose (1-5%) concentration to prevent the dehydration of well preserved wood or regions of sound wood within an otherwise deteriorated piece. This necessitates the thorough examination of the wood to be treated in order to determine its state of preservation before treatment begins. With highly degraded wood it is possible to start with a higher concentration of sucrose; however, if in doubt, start with a 1% weight/volume solution. Commence a program of weighing a representative sample of wood in treatment to determine when the wood has reached equilibrium with its solution. Once saturation with a given x% solution is achieved increase the sugar concentration by 1% to 10%.
2. Select an antimicrobial agent such as Dowicide A and add it to the first mixture of sucrose and water when it is initially prepared. This allows for the complete penetration and protection of the wood by the antimicrobial agent.
3. The incremental percentages of increase can be higher and more closely spaced if the wood is highly degraded. It is best to start with a low percentage increase, i.e., 1% to 5% until a concentration of 50% is reached. Then the solution can be increased in 10% increments. Again, if in doubt, the same incremental increases used at the start of the treatment can be used throughout the treatment. The treatment continues until sucrose concentration reaches 70% and the wood has equalized at this concentration.
4. If deemed necessary, select an additive that will discourage insect and rodent attacks on the treated wood. There are many pesticides that will work and selection depends on local availability. For thorough protection of wood, add the insecticide to the initial solution. If the wood is kept in a museum environment, problems with insects and rodents should be minimal and probably would be controlled by alternative means.
5. When the wood has reached equilibrium with the highest solution desired, air-dry it slowly under conditions of controlled high humidity. Humidity can be lowered slowly as the wood dries. Submitting the wood too quickly to conditions of low humidity will damage it. Slow, controlled drying and adjustment to the prevailing atmospheric conditions, as is the case in all the wood treatment described here, will maximize the success of the overall treatment.
6. Store the wood under conditions of less than 70% humidity if possible. The wood should not be subjected to humidity over 80% because of the possibility of condensation forming on the wood; this could leach out the sugar.

If sugar is selected as the treating medium, I have found that refined white sugar (pure sucrose) should be used. The brownish colored, coarse grained unrefined sugar (Type A sugar) should be avoided for wood treated in it is much more hygroscopic than wood treated in refined, white sugar. Every time the relative humidity rises, the surfaces of wood treated in unrefined sugar become wet. This hygroscopicity is analogous to that encountered when using the medium molecular weight PEG's. The type A sugar treated wood, however, remains dimensionally stable.

Maintaining artifacts treated by sugar in a controlled atmosphere will ensure the continued success of the conservation procedure. Artifacts thus treated require no more or no less care than those treated with other preservatives. This method constitutes an acceptable means of conserving waterlogged wood and is the least expensive of the methods currently available. Sucrose treated wood, however, has a dull muted color and it is very characteristic for small hair line cracks to form on the surface of the wood. The treatment will produce dimensionally stable wood, and is a viable alternative when the over-all cost is a major consideration. The required equipment is the same as discussed for PEG. The only difference being the cost of sucrose as opposed to PEG. Sucrose, from a producer, is in the range of 10 to 15^¢ as opposed to approximately $1.00 for PEG (when purchased in bulk) otherwise the price increases substantially for both.

The treatment consists of replacing the water in wood with a natural rosin, in this case pine rosin, also called colophony. This treatment was developed to conserve well preserved hardwoods such as oak where the higher molecular weight PEG could not penetrate (McKerrel and Varanyi 1972; Bryce, McKerrell, and Varanyi 1975).

The following procedure is recommended:

1. Wash object thoroughly, removing all dirt. It is usually necessary to store the wood in several rinses of fresh water.
2. Dehydrate the wood completely in 3 successive baths of acetone. Objects 5-10 cm. thick require about 4 days in each acetone bath. Objects less than 5 cm. thick require about 2 days in each acetone bath. It is important that all the water be remove, for the water is incompatible with the rosin.
3. Place the wood in a sealed container containing a saturated solution of rosin dissolved in acetone heated to 52^o C. Only lump, technical grade rosin should be used. Do not use powdered rosin, the dust is annoying and it usually has a powdered substance added to it to keep it from sealing together.
   
   In a sealed container at a thermostatically controlled 52^o C, a saturated solution of rosin in acetone is 67% rosin. To insure a saturated solution, an excess of rosin should be placed in the container so that there is a thick viscous layer of rosin in the bottom of the container. The object being treated should be suspended or supported above this thick undissolved rosin. Objects 5-10 cm. thick-leave in the solution for 4 weeks; objects less than 5 cm. thick-leave in the solution for 2 weeks. These times are only rough approximations and each piece of wood should be determined based on its own characteristics.
4. Remove from rosin solution and wipe off excess rosin with acetone wet rags. This treatment is feasible only for small objects, because of the cost and danger of using the organic solvent. The treatment has also been successfully applied at room temperatures and using solvents such as isopropanol and any of the other alcohols which are less flammable. The HCL pretreatment is optional, and is often eliminated because of the potential damage to the object.

In some cases, when conserving very well preserved hardwood, the conservator might consider submerging the wood in a 10% muriatic acid (HCL) bath after washing the object and before dehydrating the wood in step 3. To make the bath, mix 1 vol. HCL to 9 vol. water, in conservation terms, this is a 10% solution. It is very variable as to how lone any given piece needs to stay in the acid, but as a rough guide, objects 5-10 cm. thick, leave in the acid for 4 days, objects less then 5 cm., leave in 2 days. Treatment with HCL is supposed to bleach the wood to a more natural or original color, but I have found that the bleaching is only temporary and rarely affects the final color of the treated piece. The HCL treatment improves the penetration of the resin into the wood by breaking down the organic acids in the wood. Caution must be exercised for the HCL treatment can cause the treated piece to have a checked surface and be more subject to cracking after the conservation treatment is completed. Treatment with HCL can also be used to improve the penetration of PEG into wood. Caution is advised for it has been noticed that the surface of the wood tend to checks (have numerous small cracks on the surface) more often and shrink more when treated with HCL. If a HCL pretreatment is used, it is necessary to rinse the wood in running water for 3-5 days to thoroughly remove all traces of the HCL before start step # 2 above.

The advantages of the acetone/rosin treatment include the fact that treated wood is light in weight, it is dry, it can be glued and repaired easily, it is strong, and it can be used on compound wood and metal objects, for the rosin does not react with any of the metals. Thus, it is considered by many to be the treatment of choice for all composite wood/metal artifacts. Disadvantages include high costs, because of the organic solvents, and the flammability of the acetone. In cases where it is necessary to reconstruct a composite piece, and where it may be necessary to flex a piece of wood, acetone/rosin would not be an ideal choice because the treated wood will splinter and break if it is flexed too much.

Over the years, I have had the best success rate with the acetone/rosin treatment. Generally the only problems that have resulted from using acetone/rosin have occurred when an old solution was used and the acetone in the solution had adsorbed a lot of water from the atmosphere. It is important that dry acetone or alcohol be used. Despite the inherent dangers of the treatment and the relative expense, the acetone/rosin treatment should be used more, especially for small, important pieces. This treatment has one of the better success records and produces the most dimensionally stabilized wood after the PEG 400 and 540 Blend treatments, but without the hygroscopic problems of the PEG (Grattan 1982b).

In practice, ethanol is use as often as a solvent for the rosin as acetone (especially when treatment is carried out in a PVC pipe. Also room temperature treatments, both in acetone and isopropanol are commonly employed. If room temperature treatments are used, the treatment time is increased considerably to insure complete saturation with the rosin solution.

Because solvents are used, this technique is employed mainly for small wood artifacts, although a few laboratories have the facilities to treat objects several feet long. Colophony cost $108.45 for 25 pounds or $4.34 a pound. Acetone and other alcohols, all run about $2.90 a gallon when purchased in bulk.

This method is similar to the process used for drying out biological specimens. If necessary, the wood should be cleaned. The waterlogged object is first immersed in successive baths of alcohol until all the water has been replace by the alcohol. Isopropanol or ethanol is usually used. This is followed by successive baths of acetone. If deemed necessary, the dehydration progress can be monitored by measuring the specific gravity of each bath. When all water has been replaced by acetone, the object is immersed in successive baths of dimethyl ether to replace all the acetone with ether. When this has been accomplished the object is dried very quickly by placing it in vacuum so that the ether volatilizes rapidly. Ether is used because it has a very low surface tension of 0.17 dyne/cm compared to 0.72 dyne/cm for water. This means that when the ether evaporates, the surface tension forces are so low that there is no appreciable collapse of the weakened cell wall. If necessary, 10-20% dammar resin, colophony rosin or a mixture of the two can be dissolved in the final bath of ether so that the resin is deposited in the pores of the wood to act as a consolidant. Alternatively PVA can be used on some pieces. The resins consolidate the wood, but more importantly, they seal it off so that it is not as subject to warping due to changes in the relative humidity.

This method has proved to very successful, producing a very natural looking, light (weight & color) wood. It is only economically feasible for the treatment of small objects. The alcohols and especially the ether are highly flammable and extreme caution should taken. The dehydration process can be very effective, but the alcohols and ether must be water free. For many objects, a dehydration of only alcohol and acetone is effective.

This treatment is analogous to the alcohol-ether method. The success of this treatment depends on replacing all the water with a water miscible alcohol, and then replacing the alcohol with camphor, which results in all the cavities and cell walls being filled with camphor. The camphor then slowly sublimates (goes directly from a solid state to a gas) without exerting any surface tension on the cell walls. Consequently, the wood does not collapse, shrink, or distort. The treatment results in a very aromatic, light weight and light colored wood. The camphor can be dissolved in any of the alcohols. In essence, this treatment is a dehydration method, as describe above, but with a temporary bulking agent added until it sublimates.

The following procedure is recommended:

1. Wash object thoroughly and with care.
2. Dehydrate the specimen in a series of alcohol baths. Start with a 50% alcohol, 50% water bath (50%/50%), then 75%/25%, 90%/10%, and finally 100% alcohol. This is the most conservative procedure and, in practice, the exact strength of the alcohol baths can vary. The dehydration process depends on the condition of the object to be treated. As originally described, the wood was dehydrated in methanol (cumulative poison) so either ethanol or isopropanol can be substituted.
3. Immerse the object in a 95% alcohol/5% camphor solution. Accurately weigh the object after it has been dehydrated. Leave the specimen in the 5% camphor solution until the piece stops gaining weight. Check the progress by weighing daily. Each time the weight levels off, add 5% camphor to the solution. Make 5% increments until a concentration of 75-80% camphor is achieved. The process may take several weeks or even months. Throughout the treatment the solution is kept heated to 52^o C and the level of the solution is kept constant by the addition of more alcohol. In practice, the treatment is carried out to completion with little monitoring.
4. After the object is removed from the bath, the alcohol will evaporate over a period of weeks leaving the crystallized camphor in the cell walls. Over a period of months the camphor will vaporize by sublimation, exerting no surface tension on the cell walls. Varnish, wax, polyurethane, dammar resin, colophony and even PVA can be applied to the surface of the wood to reduce the evaporation of the camphor. This method comes very highly recommended, but like the alcohol-ether method, it is only economically feasible for treating small specimens. Also, it is highly flammable.

Freeze drying is used with some regularity of small pieces of wood, but the only limitation is access to the proper size freeze drying container Ambrose 1970, 1975; Rosenquist 1975, Watson 1982; McCawley, Grattan and Cook 1982). In the past the main problems that presented themselves was the tendency for the surface of the wood to check and crack. This is caused by the water freezing, and the ice crystals expand and expand and damage the cell walls. Ambrose (1970) found that if the wood was pre-treated by soaking it in a 10% solution of PEG 400, until it was saturated, then it the formation of ice crystals is essentially eliminated.. In more recent years, pre-treating in 10% PEG 400 before freezing the wood has become a standard part of freeze drying wood, just as it has become for leather also. More recently, Watson (1987:274-275) soaking in 20% PEG 400, "rather than the 10% recommended by Ambrose in order to prevent bacterial slime from forming in the soaking bath. At concentrations above 20% micro-organisms are dehydrated by osmosis and cannot survive." If you use a PEG solution of less than 20%, a fungicide such as 1% borax/boric acid or Dowicide 1, or other fungicide should be mixed with the PEG solution to stop any slime or mold from growing in the solution during the soaking.

Although the freezing can be done in a chest freezer, like biological specimens, a quick freeze is best. This can be achieved by immersing the wood in acetone with dry ice (frozen CO₂) in it. Some acceptable results has been had using non-vacuum freeze drying in a domestic freezer chest, especially the frost free freezers. When a domestic freezer is used, the wood is place in the freezer, after pre-treating in the appropriate PEG solution and left there until it is dried. In this non-vacuum process, the treatment time is much longer, in terms of months, as opposed to weeks in the vacuum freezer driers (McCawley, Grattan, and Cook 1982).

Of the treatments discussed here, freeze-drying is the most expensive. The cheapest freeze drier on the market is right at $5000.00 and with that you have the capacity to treat a hand size object. The only chemical costs is for the PEG 400 used to pretreat the wood and the amount required to treat a hand-size artifacts is negligible. Because of the size limitations, and the substantially higher costs when investing in equipment capable of treating larger objects, freeze-drying is restricted to small objects in most laboratories.

For the past three years, Dr. C. Wayne Smith of the Conservation Research Laboratory at Texas A&M University has been conducting research in the use of polymer media for the stabilization and conservation of organic materials. We have successfully conserved waterlogged wood, glass, leather, woven basketry, cork and notably, artifacts such as corn cobs, which have been nearly impossible to conserve maintaining the diagnostic features of the samples. We have also had great success in conserving animal hides, biological tissues, archaeological and histological bone samples, as well as a range of applications that are useful for medical and forensic investigations. Electron microscopic and chemical analysis of organic samples that have been stabilized by the displacement of free water and air with silicone polymers, exhibit some unique qualities over water-stored and air-dried specimens. An informal survey of university laboratories and departments has indicated that there are numerous areas where silicone bulking and related technologies would have almost immediate beneficial impact. The same holds true for other universities as well as in museum and artifact conservation, archival work and in industrial applications.

The use of polymerized silicone oils for the stabilization and consolidation of anatomical and biological specimens has nearly unlimited possibilities. Organic samples conserved using siloxane polymers suffer less cellular damage and structural disfigurement than samples stored in formalin, formaldehyde and alcohol. These same samples are more durable, and the absence of noxious chemicals makes them more attractive and safe for hands-on study. Acquiring microtome sections of silicone bulked specimens for microscopic analysis is easier for most organic specimens since the stabile nature of polymer treated specimens ensures that there is less stress and distortion caused by the sectioning process.

The goals of research at CRL have expanded to include continuing research on the development of areas of application for silicone bulking as well as defining new areas of application and research. Currently, Our silicone bulking processes have immediate applications in laboratories and facilities at Texas A&M University as well as other major educational and scientific institutions.

In all cases, the long-term best interest of an artifact should be the main concern when considering a conservation strategy. Too often, however, the diagnostic attributes of artifacts are compromised and lost because a conservator is forced to choose a less-than satisfactory process, due to cost constraints or lack of experience. More artifacts are lost due to these short-falls than all other reasons. Neglect also claims many artifacts. Failure to design adequate conservation strategies takes its toll amongst inexperienced conservators and adherence to dogmatic ideals can also place stresses on the well being of an artifact assemblage. In recent years, I have seen situations where conservators have insisted on using traditional processes to conserve glass beads that yielded poor results, simply because they believed that the issue of reversibility was more important than successfully conserving what has become a nearly depleted assemblage. Common sense appears to be one of the missing ingredients from the conservator's tool kit.

Ideally, reversibility is a desirable aspect of any conservation process. In reality however, the issues of reversibility have been grossly overstated and in many cases, misrepresented. To the best of my knowledge, it is absolutely impossible to remove all of the polyethylene glycol from a conserved piece of waterlogged and badly deteriorated wood. During the process of treatment, some of the PEG is chemically bonded to the remaining lignin and cellular structures of the wood, making complete removal of this polymer impossible. In addition to chemically bonding, additional PEG will simply be trapped in cellular voids, and therefore, remain in the wood. Clearly, even the best of processes in treating wood with PEG causes intracellular damage during treatment. In essence, the process of conserving the wood can undermine the structural integrity of the wood. The process of removing PEG causes additional damage to the already weakened physical structure of the wood. More times than not, the process of retreating heavily waterlogged damaged timbers causes more damage than should be desirable. Too often, the theoretical state of reversibility of an artifact outweighs important issues including the real potential for successful 100% reversibility, degree of degradation, effects of attempted treatment reversal and best interest of the artifact. In contrast, our new processes for treating even heavily waterlogged damaged wood do not cause the cellular distortion that has been associated with PEG treatments. Notably, after treatment, thin sections of polymer treated wood samples are so well preserved that in most cases, post-treatment genus and species identification are possible.

To date, we have shown that to some degree or another, silicone and polymer treatment processes are reversible. In reality however, we consider the potential for loss of diagnostic attributes to be too high, suggesting that a great deal of research needs to be completed in the development and of reversible processes. Expected longevity, short time frame for conservation and ease of curation however, are invaluable aspects of silicone and polymer processes that make them a serious consideration for the treatment of many artifacts.

The greater issue which conservators need to addresses is the long-term well being of an artifact assemblage. Many of the conventional processes which are routinely used for the conservation of artifacts have a relatively short life expectancy. This is why reversibility has always been an issue. It has become common knowledge that over time, PEG treated timbers and other artifacts tend to warp and become unstable as the water miscibility of PEG allows the intra-cellular flow of PEG to distort and eventually, escape to the surfaces of the artifact. Simply put, even in very controlled environment, PEG breaks down. Our research has helped us understand new ways to remove a portion of the unstable PEG and re-constitute the remaining bonded PEG into a more stable bulking material. The technology we use for this processing is polymer technology - without the polymer.

Addressing the issues of life expectancy of treated artifacts is another area in which the benefits of conventional processes have been overstated. In the case of PEG treated artifacts, permanent curation in climate and temperature controlled environments only prolongs, to some degree, the life expectancy of the artifact. Water miscibility and chemical changes within this bulking media inevitably cause slow degeneration within the artifact.

Longevity of silicone and polymer processes is not an issue. Through extensive testing and nearly twenty-five years of data collected by the silicone and polymer industries, we know that the minimum half life of the polymers we use in conservation is at least two hundred years. Ease of treatment in using these new technologies, too, is another consideration. Actual treatment times for the conservation of very delicate glass beads recovered from excavations of the Uluburun site (1300BC) is approximately twenty minutes. Once completed, the beads require only a few hours of uninterrupted curation before they can be handled.

The last consideration to bring to the whole issue of reversibility is that strict adherence to traditional technologies is a good way to never discover new, and hopefully better technologies. Our current ability to rework PEG in waterlogged wood is one such example. Research into the use of silicones and polymers for the preservation of archaeological materials is a reality. So too are the new technologies that we can foresee in the near future. We are not suggesting that silicones are the panacea for all conservation needs. We are suggesting that these new technologies will, and are, having an impact in archaeological conservation, simply because reversibility has never been an absolute fact using traditional processes. Silicone and polymer processes are simply an additional set of tools in the conservator's tool kit. Our own research has indicated that the following decades will hold exciting new advances in the science of archaeological conservation. Conservation sciences we feel, have a responsibility to seek out, define and refine tomorrow's technologies.

A simplified version of the silicone bulking process that is applicable for the treatment of small wood artifacts and other organic material is as follows:

1. Take waterlogged wood and place directly in a bath of ethanol and hold under a vacuum for approximately one hour.
2. Place the dried wood into a bath of acetone and hold under vacuum for approximately one hour.
3. Remove wood and place it in SFD-1 silicone oil that has had 4% Isobutyltrimethoxysilane added to it. The isobutyltrimethoxysilane is a cross-linker that sets the silicone oil up for curing in the nest steps. Keep wood submerged in this mixture under vacuum over night.
4. Remove wood, and pat dry with a dry rag to remove excess silicone oil on surface
5. Place the wood in a closed container over a small dish containing a small volume of FASCAT Catalyst 4200 in it. Place everything in a furnace heated to 52^o C. The heat of the furnace vaporizes the FASCAST and the vapors causes the silicone oil to cure in the wood, stabilizing it.

This silicone oil treatment results in a very naturally colored wood that undergoes little to no dimensional changes. The wood is stable and does not require the close environmental controls that some other treated woods do. Still, it must be kept in mind that this treatment is not reversible, but for that matter most of the other treatment are not either.

There are several other treatments that can be used for treating waterlogged wood, such as bulking with paraffin in a solution of hexane, but they are not extensively used. What is important to know, is that the problems of conserving waterlogged wood can be overcome with a number of treatments. The decision on what treatment is selected might be entirely a matter of aesthetics, in that the treatment results in a certain color wood, a light colored wood, it enhances the wood grain, it is glueable, it is flexible, it is rigid, it is part of a compound wood-metal artifact, it is not sensitive to fluctuations of humidity and can stand storage in adverse conditions. All of these are considerations, and there are ways of treating waterlogged wood that results in each of these considerations. All the treatments are applicable in given situations and all are acceptable alternatives.

LEATHER CONSERVATION

PRELIMINARY CLEANING

When conserving leather it is often safer to select a treatment that least affects the leather. As for all porous material, it is necessary to remove the bulk of the soluble salts which will be resent. The procedure is the same as described for bone and ceramics.

All archaeological leather conservation is preceded by washing to remove any ingrained dirt. First try washing in water alone. If this is not successful, try alternative methods. Leather may require a variety of mechanical cleaning techniques, depending on the condition of the leather and the particular cleaning problem. Soft brushes, water jets, ultrasonic cleaners and a Cavitron, an ultrasonic dental tool, may be required. If chemical cleaning is necessary to remove ingrained dirt, a small amount of non-ionic detergent (about a 1% solution) or sodium hexametaphosphate can be used. If Calgon, a commercial water softener, is used, make sure the pH is 3-5; the addition of additives may make it unsafe to use on leather. Rinse well after washing. Do not use any chemicals that will damage the collagen fibers of the leather.

A safe storage solution for waterlogged leather, or any organic material for that matter, can be made by taking 90% of a stock solution of 50% water/50% ethanol. To the 90% solution add 10% glycerin and 2-3 drops of formaldehyde. A 50% mix of ethanol and water is also an effective storage medium for many materials.

One must always remember, that it is often better to leave stains on the leather in order to prevent the damage caused by trying to remove them. Some metal stains are stable. For stain removal, particularly iron staining, 3-5% ammonium citrate or ethylene-diamine-tetraacetic acid (disodium EDTA) is used. Commercial trades names for EDTA are Titriplex III and Disodium Deterate. Soak for 2-3 hours, monitor closely. Rinse in running water or standing tap water until all chemical residues are removed. Check the pH of a standing bath of water containing the leather to determine complete removal of the chemicals. Always keep in mind that chemicals used to clean rusts and mineral concretions (oxalic acid, EDTA) may produce further hydrolysis of the proteinaceous collagen fibers, leather's main constituent, and that they can remove tanning, coloring agents, painted decorations and other attributes that are part of the diagnostic attributes of the leather object. Diagnostic attributes should never be removed. Caution should be exercised when using any of these chemicals on leather.

For waterlogged leather, freeze drying and solvent dehydration are often selected, quite often without adding any additional lubricant. Very good success have been achieved conserving bog bodies with freeze drying using 15% PEG 400 as a lubricant to minimize skin and bone shrinkage during drying. The drying behavior of any piece of leather is dependent upon its condition at deposition, the burial environment, the genus, species, health and sex of the individual -- man or beast, the location of the skin on the body, manufacture or tanning method used, and finally, the leather object's history.

The following treatments, involving the addition of lubricants, have been used successfully on brittle and/or desiccated leather. Glycerol, which is soluble in water and alcohol, acts as a humicant for the leather..

59% glycerin (glycerol)
39% water
1% formaldehyde or 1% Dowicide 1

OR

25% glycerin
75% alcohol

Immerse the artifact in the solution until the leather is pliable. When an alcohol solution is used, it is difficult to determine when the leather is pliable because the alcohol makes the leather stiff. Treatment may require several weeks. The treatment restores flexibility, but glycerine is hygroscopic and can support mold growth. In spite of this the treatment has been widely used in the past successfully. The process went out of favor for a while, but recently (Jenssen, in Pearson 1987) interest in the treatment has been revived. As in the case with all organic conservation methods, this treatment should be used cautiously.

Waterlogged leather recovered from excavations by The Museum of London is conserved by placing it in a solution of 30% glycerol, 70% Alcohol (ethanol) for 2 weeks. The leather is then dried by 3 baths of acetone, 3 hours long each. Glycerol is not soluble in acetone. Basic success can be achieved using a glycerol treatment by using 10-40% glycerol mixed in 90-60% alcohol or water. Avoid using concentrated glycerol. While the solutions in alcohol can remove tanning agents, alcohol speeds up the process. In alcohol solutions, the leather has more overall strength during treatment. In other words, the alcohol allows the leather to support itself better than does a water solution.

The treatment has also been applied to basketry, matting, sandals, etc. to restore pliability; quite often with disastrous results. Always remember, there is no reason to make something flexible or pliable, if it was not particularly pliable in the first place. The glycerin treatment can be used in combination with PEG. To retreat any object that was conserved with glycerol, such as basketry, remove the glycerol with successive changes of alcohol baths.

200 gm. (7 oz) anhydrous lanolin
30 ml. (1 oz) cedarwood oil (acts as a fungicide)
15 gm. (½ oz) beeswax (optional)
350 ml di-ethyl ether, B.P. 60-80 F or 330 ml of hexane

Mix the first three by heating and then pour the molten liquid into the ether or hexane. Allow to cool while constantly stirring. Take extreme precautions!!! Ether and hexane are flammable liquids. Rub mixture well into the leather, but use sparingly; a little bit goes a long way. Apply, wait two days, then polish the treated leather with a soft cloth. Beeswax can be omitted; its function is to act as a polish. Very hard leather can be soaked in BML diluted to 1 part BML, 3 Parts Stoddards Solvent. BML darkens the leather, but it is a treatment with a good success record.

Dry leather can be saturated with water or alcohol and treated with PEG 1450, PEG 540 blend, PEG 600 or PEG 400. In the past, leather was treated in PEG which was heated to a temperature of 40-50^o C. Presently, most leather treatments are carried out at room temperature because heat in any form is generally detrimental to leather.

The PEG treatment consists of immersing leather in a dilute solution of PEG, i.e., 10%, and increasing the consolidant concentration in 10% increments as the leather absorbs the PEG. A final concentration of 30% is adequate. Keep the artifact immersed for several days until the leather is flexible. Once pliable, remove from the PEG and clean off excess PEG with toluene or water. Allow treated leather to dry slowly under controlled conditions.

As mentioned above there are several types of PEG and each has its own characteristics. PEG 540 Blend (equal parts of PEG 1450 and 300) is slightly hygroscopic and becomes moist at high humidity; for this reason the surface of the leather is sometimes sealed with a hard wax, i.e., a mixture of 100 gm. micro wax and 25 gm. polyethylene wax. PEG 3250 is very hard and is not very hygroscopic. Its main disadvantage (in some cases an advantage) is that the treated leather is rigid. When using PEG 3250, form the treated specimen to its final shape while the wax is still warm and then allow the artifact to cool. PEG 1450 seems to give consistently good results. The various PEG treatments are more commonly used for the conservation of dry leather. 15% PEG 400 is commonly used as a pretreatment when the leather is to be freeze dried.

PEG treated leather can be hygroscopic, greasy, dark and there is the danger of the PEG migrating out of the leather.

Bavon ASAK 5205 is an emulsion that is water soluble. Bavon ASAK ABP is solvent soluble. The exact chemistry of Bavon is unknown. In some sources it is described as being an alkylated succinic acid, mineral oil blend. Bavon ASAK-ABP is described as being a copolymer of polyhydric alcohol and a partial ester of an unsaturated hydrocarbon. When applied as a leather dressing it lubricates the leather, resulting in a pliable, natural brown leather.

The emulsion form of Bavon (ASAK 5205) can be used in the same way as described below except that there is no need to dehydrate the leather since the treatment starts with waterlogged leather. Many conservators report unsatisfactory results with the emulsion form of Bavon. Some conservators report too much shrinkage while others report good results. Personally, I have had better results with solvent soluble Bavon.

When it was first introduced, Bavon, was widely accepted and used. Many conservators are now abandoning treatments using Bavon. Glycerol and PEG treatments are used.

Very hard, desiccated leather has been successfully softened by soaking in a concentrated Bavon leather dressing consisting of 6 parts Bavon ASAK ABP to 4 parts 1:1:1 trichloroethane (Bavon is described below). Soak until satisfactory pliability is reached; then place the leather between blotters and glass and allow it to dry.

Leather, like a lot of organic material from a marine environment, undergoes some complex changes in a marine environment (Florian 1987) and the difficulties in conserving it so that it looks natural and will not suffer from the chemical added has long been known (Jenssen 1983). The best review of the current treatment commonly in use is presented by Jenssen (1987). The most relevant treatments are discussed. below.

Prior to Treatment, waterlogged leather should be stored in water with 0.1% Dowicide 1. If the leather is to be treated in an organic solvent, the leather can be stored in 50% H₂0/50% ethanol or straight ethanol and a fungicide is not required. Treated leather should not be stored at a relative humidity higher than 63%.

Treat with PEG 400, 540 Blend, 600, 1450 and 3350 and proceed as described above for desiccated leather. It is recommended that you start with a dilute solution (1-10%) of PEG and gradually increase the concentration through evaporation of the solvent or by adding PEG up to 30%-100%. This allows the water to evaporate as equal amounts of PEG replaces the water.

Aqueous solutions of PEG are slower processes, but less expensive. Solvent solutions are much faster and considerably more costly. They however, produce a lighter leather with more uniform shrinkage. Some conservators prefer alcohol treatments, while others think that alcohol treatments cause the leather to shrink more than comparable aqueous treatments. All the PEG treatments can be satisfactory by themselves, but the treatments are considerably enhanced if taken through a freeze drying as the final step. The steps are identical to those described under wood. A commercial freeze drying vacuum chamber works the best; however, very good results have been obtained using domestic chest freezers. The former takes only a week or so, while the latter may take several weeks. Progress can be determined by regular weighing of the object to determine weight loss as the leather loses moisture.

1. Wash leather in a 1% solution of Lissapol; never use commercial detergents on leather as they may extract tanning materials. Use castile soap, soft soap or saddle soap.
2. To remove iron stains place leather in 3-10% Disodium EDTA (pH 4) or ammonium citrate (pH 5) for a maximum of an hour; shorter if possible. Tap water can be used. The H₂O may yellow from the EDTA dissolving iron tannate. Note: EDTA has been reported to damage fibers of leather, but is relatively safe if used selectively and cautiously. A 3-5% ammonium citrate (pH 5) can also be used and, in fact, is recommended over Disodium EDTA by the Canadian Conservation Institute. This step is necessary only if iron stains are present.
3. If necessary, place in 2% HCL for up to one hour to dissolve calcareous material.
4. Rinse very thoroughly in running water for 30 minutes to lower the pH to 3-6 or to the pH of the rinse water.
5. Dehydrate in acetone. Use two or more baths; for example, four successive baths of one hour each.
6. Air dry until the "leather feels like leather", then place between absorbent tissue and glass to dry for 24 hours.
7. Apply Bavon Leather dressing with a brush. Flex and manipulate leather during the application of the Bavon.

1 liter of stabilized 1:1:1 trichloroethane
1 gram Dowicide 1
50 grams anhydrous lanolin
20 grams Bavon ASAK-ABP

Waterlogged leather recovered from excavations by The Museum of London is conserved by placing it in a solution of 30% glycerol, 70% Alcohol (ethanol) for 2 weeks. The leather is then dried by 3 baths of acetone, 3 hours long each. Glycerol is not miscible in acetone and is thus not removed in the acetone baths. Good results can be achieved using a glycerol treatment by using 10-40% glycerol mixed in 90-60% alcohol or water. Avoid using concentrated glycerol. While the solutions in alcohol can remove tanning agents, alcohol speeds up the process. In alcohol solutions, the leather has more overall strength during treatment. In other words, the alcohol allows the leather to support itself better than does a water solution, because the alcohol keeps the leather fibers stiff. removed by air drying and sometimes with the use of vacuum.

Freeze drying works the best as a means of conserving leather. In freeze dying the leather is first immersed in a 15% solution of PEG 400 which acts as a humicant, preventing excessive shrinkage. A fungicide, such as 1% borax/boric acid, or other fungicide should be mixed with the PEG solution. After immersion, the object is then frozen at -20 to -30^oC. Like biological specimens, a quick freeze is best. This can be achieved by immersing the leather in acetone with dry ice (frozen CO₂) The piece is then placed in a freeze dry chamber under vacuum for a period of 2-4 weeks. Some acceptable results has been had using non-vacuum freeze drying in a freezer chest --the frost free freezers work best and are faster.

This treatment involves the replacement of water in leather with a water miscible organic solvent. In most cases a sequence of solvents with decreasing polarity are used, e.g., a series of baths of x% H²O - x% of isopropanol, a bath of 100% isopropanol, a bath of 100% ethanol or methanol followed by 100% methyl ethyl ketone, then 100% acetone and finally 100% ether. Slow desiccation of glutinous collagen fibers allows their surfaces to become less sticky and less brittle and thus more flexible. This example is a very conservative method of treatment. In most instances fewer baths are used and for some leather, drying only through acetone is necessary.

Waterlogged Leather Treatment described by Plenderleith & Werner (1971:34)

1. Remove iron stains with 5-10% disodium EDTA (pH 4).
2. Rinse in clean water, brush lightly.
3. Remove excess water by soaking in MethylEthylKetone (MEK) or acetone.
4. Immerse in carbon tetra-chloride with a fungicide such as oxide of naphthenate.
5. Dry between blotting paper and glass plates.
6. Apply leather dressing.
7. Work into shape, if necessary.

In this treatment method, leather drys out in a flexible condition without undue shrinkage

Personally, I recommend the solvent drying treatments followed by the application of a leather dressing. Controlled air drying from an aqueous state never works. The contractile forces of the escaping water draw the protein fibers together causing the leather to harden and shrink. Others prefer freeze-drying with a pretreatment of PEG 400. The major costs involved are the organic solvents for drying and the cost of the freeze-drier for freeze-drying. These are the two most common treatments, and both give acceptable results.

The term "textiles" is applied to woven objects and also to those fabrics which are products of other kinds of interlacing of yarns of comparable structures such as braiding, looping, knitting, lace making, netting. The textile category also includes materials such as felts and non-woven materials in which the fibers gain coherence by a process other than spinning. The term "textile" is used to describe the fibers and also yarns, twines, cords, and ropes produced by spinning, twining and the rope-making process. The words "ancient textiles" refers to those materials that are made of natural fibers and are found in archaeological sites from past centuries.

This short discussion of textile conservation is limited to the natural fibers of animal and plant origin: wool, hair, silk, cotton, flax, jute, hemp, nettle, grass, etc. The animal fibers are primarily made of protein and are more resistant to decay than the vegetable fibers which are composed primarily of cellulose. For instance flax and cotton are very susceptible to attack by bacteria under humid conditions and seldom survive in archaeological environments. All textiles are deteriorated by light, insects, microorganisms and air pollution which singularly or together cause considerable loss of tensile strength and pliability. The oxygen in the atmosphere affects all organic substances to varying degrees. Textiles are very prone to aging and deterioration from exposure to the atmosphere. Prolonged exposure to normal atmospheric conditions will cause textiles to weaken and disintegrate. The speed of the deterioration varies according to the nature of the fibers and existing local conditions. The main factors that promote the decay of textiles can be categorized into three groups:

1. Organic - Because textiles are organic, they are subject to attack by molds and bacteria. Decomposition is greatest in situations that favor the growth of these organisms such as damp heat, stagnant air and the contact of the materi