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, both of 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 may 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 (UNESCO 1968; Dowman 1970) are:
Polyvinyl acetate (PVA) is the most commonly used thermoplastic polymer resin for organic material recovered from archaeological excavations (UNESCO 1968; Ashley-Smith 1983b). This is true in the field as well as in 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; the lower the molecular weight, the greater the penetration capability of the consolidant. The lower viscosity PVAs, however, have less bonding strength than those that are more viscous. In addition, 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 most commonly used PVA viscosities in conservation. PVA V7, with its smaller molecules, is frequently used on denser material, such as well-preserved bone and ivory; PVA V15 is a general-purpose resin; PVA V25 is used as a glue. PVA is also heat-sealable; for example, two pieces of cloth treated with PVA can be bound by ironing them together.
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, may be used as a surface consolidant or as a glue. Many conservators prefer to use PVA V25 as a glue, especially for pottery reconstructions, with good results. Ceramic vessels glued with PVA V25 have, however, occasionally fallen apart because of excessive cold flow of the resin in hot, humid 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 and 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 a PVA solvent. During 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 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.
Slow-Drying PVA Formula
Fast-Drying PVA Formula
Amyl acetate can be added to either to retard evaporation. Acetone can be added to ethanol to speed up evaporation, or ether can be added to either to considerably speed up the setting time.
If cellulose nitrate is used instead of PVA (not recommended) then either 2 percent Triacetin by volume or 5 percent castor oil by volume of cellulose nitrate must be added to plasticize the cellulose nitrate in order to retard, but not prevent, shrinkage, and brittleness.
PVA can also be purchased as an emulsion, such as CM Bond M2. Emulsions are stabilized dispersions of finely divided particles of the resin in water. The resin is merely suspended (rather than 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 non-emulsified 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, CM Bond M2 has approximately 0.6 grams of resin per 1 gram of stock mixture. PVA V25 and even V15 are often used as a glue. When used as a glue, it only necessary that the PVA be thick enough. One acceptable procedure for making glue is provided by Koob (1996). The procedure described by Koob uses Acryloid B-72, but the process works equally well with any PVA formulation.
Acryloid B-72 (referred to as Paraloid B-72 in Europe) 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. Durable and non-yellowing, Acryloid B-72 dries to a clear transparency, with less gloss than PVA, and is resistant to discoloration even at high temperatures. It is very durable and has excellent resistance to water, alcohol, alkalis, acid, mineral oil, vegetable oils, and grease, and it retains excellent flexibility. Acryloid B-72 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. Furthermore, it is compatible with other film-forming materials, such as PVA and cellulose nitrate, and can be used in combination with them to produce stable, transparent coatings with a wide variety of characteristics. In stronger concentrations, Acryloid B-72 can be used as a glue (see Koob 1996 for details). This glue formulation is the standard glue used at the Conservation Research Laboratory at Texas A&M University.
Acryloid B-72 is unique in possessing a high tolerance for ethanol, e.g., after being dissolved in acetone or toluene, up to 40 percent 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 upon drying. 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 percent Acryloid B-66 in non-water miscible toluene 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 and can be used in place of it in most instances.
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 are most PVAs. Therefore, cellulose nitrate has a much greater tendency than PVA to become brittle, crack, and peel off.
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 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 that is 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 has been used extensively, with varying success, in pottery reconstructions and general artifact mending. Duco is easy to use and is effective in the short run, but over the years, the glue may yellow and become brittle, resulting in the breakup of glued items. It is not recommended for use in archaeological conservation (Moyer 1988b; Feller and Witt 1990).
Cellulose nitrate is discussed here because of its availability and general misuse in many conservation projects. In a few given cases, it may be necessary to use several resins with mutually exclusive solvents to consolidate some complex object. The use of cellulose nitrate is in 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 can be used to stabilize material such as bone by impregnation. For dilution purposes, Duco has approximately 0.8 grams of resin per one gram of stock mix in the tube.
There are a large number of polymethylmethacrylate (PMM) resins that are easily obtained world wide under different trade names such as Perspex and Lucite (formerly called Plexiglass). There are many different formulations for PMM resin glue, which is commonly made from sheets of Lucite, although good results are also achieved with Elvacite 20/3. Even safety mask shields and motorcycle windshields can be dissolved in solvents. The toxicity of the solvents necessary to dissolve PMM resins restrict their wider use by conservators.
A typical formulation for a 'plexiglass' glue is as follows:
Grind, cut or drill a sheet of Lucite to get a cup of shavings. Place in a jar and add approximately the same volume of solvent, which should consist of 50 percent chloroform and 50 percent toluene. Caution: heat is generated. Add acetone to thin to the correct viscosity.
The PMM resins have similar properties to PVA. PMM resins are stronger but have fewer solvents. 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. 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 and have viscosities that ranged 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 percent 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 the addition of 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 PVAls to cross link in 3 to 5 years if exposed to strong light, dryness, and heat; especially temperatures over 100°C. If cross linking occurs, the resin becomes less soluble but most likely never becomes completely insoluble. Some conservators recommend that objects treated with PVAl be re-treated 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°C. 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 50-65°C.
ELMER'S GLUE ALL
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 20 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 0.9 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.
There are innumerable thermosetting epoxy resins on the market with many varied properties and special characteristics. Each conservator, through experience, has his or her 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; however, epoxies are occasionally required by conservators because nothing else has the necessary strength. They are excellent when a very strong, permanent bond is required. Epoxies 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 to follow the directions of the manufacturer on the recommended hardeners, mixtures, and thicknesses. If mishandled, a considerable amount of exothermic heat may be generated.
These are just a few of the most common adhesives/consolidants used in conservation. They have a long, successful track record and are, therefore, widely used for an extensive variety of conservation purposes. Specific applications are discussed in the following files.