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Conservation of Glass

Conservation

Glass is usually the most stable of archaeological materials, but glass artifacts, and 17th-century glass in particular, can undergo complex disintegration. Ideally, glass should consist of 70-74 percent silica, 16-22 percent alkali or soda ash ( sodium carbonate ) or potash ( potassium carbonate, usually derived from wood ash ), and 5-10 percent flux ( lime [calcium oxide] ). Soda-lime glass has been the most common glass throughout the history of glass-making, and the modern equivalent is 74 percent SiO2, 16 percent Na2CO3, and 55 percent 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 interior 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 are 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. 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 are present in the mixture, for the glass will be especially susceptible to attack by moisture. If old glass contains 20-30 percent sodium or potassium, it may have 'glass disease, ' where the glass weeps and begins to break down.

In all glass, the sodium and potassium oxides are hygroscopic; therefore, the surface of the glass absorbs moisture from the air. The absorbed moisture and exposure to carbon dioxide causes the NaO2or 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 percent and above ( and in some cases as low as 20 percent RH ), drops of moisture appear on the glass surface. In water, especially salt water, the Na and K carbonates in unstable glass may leach out, leaving only a fragile, porous hydrated silica ( SiO2 ) network. This causes the glass to craze, crack, flake, and pit, and gives the surface of the glass a frosty appearance. In some cases, there is an actual separation of layers of glass from the body. Fortunately, these problems are not commonly encountered in glass manufactured in the 18th century and later. Pearson ( 1987b, 1987d ) discusses glass deterioration 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 and 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 has an iridescent film, which is formed by the leached silica layers. The alkali which leached 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. 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. Usually 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 professionals advise that the only treatment should be to keep the glass in low RH environments so the glass does not have any moisture to react with. While a RH range of 40-55 percent 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.

Most, if not all, of the glass manufactured from the 18th century on has been produced from a stable glass formulation, and there are not likely to be any considerable problems presented to the conservator other than normal devitrification. 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 expected. The main problems will be related to gluing pieces together. All the problems likely to be encountered are discussed thoroughly Newton and Davidson ( 1989 ).


TREATMENT OF UNSTABLE GLASS

Glass that is susceptible to weeping because of unstable glass formulations can be treated in different ways; the technique described by Plenderleith and Werner ( 1971:345 ) is representative of the treatments often recommended. It is presented below.

  1. Wash the glass thoroughly in running tap water and then soak it in distilled water.
  2. Dry the glass in two baths of alcohol. This treatment will retard the disintegration and also improves the appearance of the glass. It does not, however, always stop the breakdown of the glass.
  3. If applicable, apply an organic lacquer ( PVA or Acryloid B-72 ) to impede disintegration.
  4. For assurance, store the glass in a dry environment with the relative humidity no higher than 40 percent; other professionals say that an RH of 20-30 percent is ideal. The Corning Glass Museum keeps incipient crizzled glass stored at 45-55 percent RH. RH 42 percent is the critical point at which KCO3 becomes moist.

The above treatment does not attempt to remove any of the glass corrosion products, which often result in layers of opaque glass that may be removed with various acid treatments. The decision to remove surface corrosion products, which often mask the color of the glass, must be made on a case-by-case basis. Removal of corrosion products may also significantly reduce the thickness of the walls and weaken the piece significantly. Indiscriminate removal of surface corrosion products can weaken, blur, or alter surface details. The corrosion layers of a glass object may be deemed a part of the history of the object, and thus a diagnostic attribute, and should not be removed without good reason.


DEVITRIFICATION

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, especially soda glass, usually becomes hydrated through time and so will eventually devitrify. Devitrification occurs when the surface of the glass becomes partly crystalline as it adsorbs moisture from the atmosphere or from a submerged environment. 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 surfaces, a coating of PVA or Acryloid B-72 should be applied to the piece. Either of these surface adhesives will smooth out the irregularities in the pitted, crazed surface of the glass by filling in the small cracks and forming optical bridges, making the glass appear more transparent. Merely wetting glass will cause it to be appear clearer for the same reason.


REMOVAL OF SULFIDE STAINS FROM LEAD CRYSTAL

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 may be recovered from marine and/or anaerobic sites with a very dense black film on its surface. A 10-15 percent hydrogen peroxide solution 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.


RECONSTRUCTION

Glass can be repaired and reconstructed with the same glues as described for pottery. Optically clear epoxy resins are generally preferred as they adhere to the smooth, non-porous glass more readily. They also dry clearer and shrink less than the solvent resins. The resulting bonds, therefore, are less noticeable and stronger than with other glues. 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, ' which are 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 artist's brush to permanently bond 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 of these problems are adequately discussed in greater detail in Newton and Davison ( 1989 ).

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


SUMMARY

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


Conservation of Pottery & Stone

Conservation

In general, pottery survives well in marine environments and requires only minimal treatment after recovery ( 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 temperatures that they are impervious to liquids and thus do not absorb soluble salts from their archaeological environment; therefore, it is not necessary to take them through long rinses to remove soluble salts. However, with certain kinds of stoneware and porcelain, glazes are applied in subsequent firings, and sometimes salts may be deposited between the glaze and the body. If these salts are not removed, the glaze may flake off. So, even caution must be exercised with stoneware and porcelain. Well-fired pottery need only be washed in a mild detergent and the edges and surfaces scrubbed with a soft brush. Care should be taken not to remove traces of food, paint, pigments, or 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, poorly 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.


SOLUBLE SALTS

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, particularly iron objects, will be enclosed by the encrustation forming around the metal. Soluble salts ( chlorides, phosphates, and nitrates ) are potentially most dangerous to the integrity of pottery, and they must be removed in order for the object 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 may 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 method 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 cascades into a series of additional vats. This minimizes water waste, especially if using de-ionized water. Very simple rinsing procedures exist, such as putting soluble, salt-laden shards in a mesh bag and placing the bag in the reservoir of a toilet. Innumerable volunteers assist you each day in changing the water, and the salt content in the shards quickly equalize with that contained in the supply water. Then, if necessary, the rinsing can then be continued in several baths of de-ionized 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 shards or pottery are too fragile to withstand the rinsing process, surfaces may be consolidated first with Acryloid B-72 then rinsed. Since Acryloid B-72 is somewhat water-permeable, it will allow the salts to diffuse out, albeit significantly more slowly than in non-consolidated material.


REMOVAL OF INSOLUBLE SALTS

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 appliance. Dental burrs and pneumatic air chisels are also quite useful.

The insoluble salts may also be removed chemically, but it is important to pre-wet the shard. Nitric acid, hydrochloric acid, and oxalic acid are most commonly used. Before using any acid on pottery, however, make sure that the paste is thoroughly wetted so that the acid will not be absorbed. Although 10-20 percent nitric acid can be used to remove calcareous concretion, it is potentially the most damaging acid of the three. More care should be exercised in its use, as dilute nitric acid will dissolve lead glazes. In most cases, 10-20 percent hydrochloric acid is safer than nitric acid to clean glazed pottery. The shards are left immersed in the acid until all gas evolution ceases ( usually less than an hour ); this process may be repeated if necessary. Care must be exercised, since hydrochloric acid can discolor glazes, especially lead glazes, which will turn milky. The samples are then washed thoroughly in tap water and, if necessary, immersed in 10 percent oxalic acid for 10-20 hours to remove iron stains. A thorough rinsing should follow, and the shards should be then dried. It is imperative that pottery with a carbonate temper ( shell, calcium carbonate ) not be in immersed in hydrochloric or nitric acid because 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 shards should always be pre-wetted by soaking in water and then by applying the acid locally on the surfaces with a cotton swab or by drops. The excess acid is immediately removed when the effervescing action stops, either by wiping the area or rinsing the object( s ) under running water to remove the acid. Earthenware and terra cotta often contain iron oxides, are more porous, and thus more prone to deteriorate when treated with these acids; acid treatments should be used on such materials with some discretion.

A useful chemical for removing calcareous deposits from ceramics is ethylene-diaminetetraacetic acid ( EDTA ). A 5 percent 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 shards 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. It is a slow but effective treatment.

Soaking calcareous-encrusted shards in a 5 percent 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 shard 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 three drops of 1 percent barium chloride solution. A white precipitate indicates the presence of sulfates ( Plenderleith and Werner 1971 ). These can be dissolved slowly by immersing in 20 percent nitric acid. As the sulfates dissolve, sulfuric acid is produced, which cancels out the reaction of the nitric acid. The nitric acid must be changed often. This technique is not generally recommended, however, and 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.


STAIN REMOVAL

Iron oxide stains can be removed with 10 percent oxalic acid applied locally with cotton swabs on the surface of pre-wetted pottery. This is a generally successful method for removing iron stains from stoneware and earthenware ceramics, although a small amount of the iron in the paste may be removed. A 5 percent 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 and Pearson 1975; Pearson 1987d ). The disodium 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 percent by volume hydrogen peroxide solution until the stains disappear. The time required to remove the stains ranges from a few seconds to several hours. No rinsing is required after treatment with hydrogen peroxide. Hydrogen peroxide can be applied directly to shards that have been treated with nylon, as the hydrogen peroxide will permeate 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 also 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 glues' ) are very handy. These can be dissolved slowly in acetone and toluene after setting. In most instances, it is necessary to consolidate earthenware shards 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 shards in a dilute solution of the resin.


GENERAL STEPS FOR the REMOVAL OF INSOLUBLE SALTS AND STAINS

  1. Thoroughly wet the pottery.
  2. For sturdy pottery, immerse in 10-20 percent nitric or hydrochloric acid until effervescing ceases. Hydrochloric acid is preferable for glazed pottery. Glazed, friable, or carbonate-tempered surfaces should be cleaned with cotton swabs or by applying concentrated acid, drop by drop. Immediately wipe off the excess acid or rinse in running water when effervescing stops. Continue the process from spot to spot or area by area.
  3. Thoroughly rinse the pottery in running water to remove excess acid.
  4. Remove iron oxide stains with 10 percent oxalic acid or 5 percent EDTA and rinse thoroughly.
  5. Remove iron sulfide and organic stains by immersing in 10-25 percent by volume hydrogen peroxide.
  6. For marine-recovered earthenware, it is advisable to thoroughly consolidate the material in a dilute solution of PVA or Acryloid B-72. This is especially important if the artifact will be reconstructed.

STONE CONSERVATION

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 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 percent. Do not use any acids on any of the sedimentary rocks ( e.g., limestone, marble, sandstone, etc. ), as these can be quickly destroyed by acid treatments. The acids can be used effectively on metamorphic and igneous rock.


SUMMARY

The conservation of ceramics recovered from a marine site is not complicated. When pieces are found encrusted, the most difficult part of the conservation process is the removal of the adhering material without damaging the paste or glaze. For this reason, mechanical cleaning techniques are preferred, but hydrochloric acid is used with some regularity to remove calcareous encrustation. The soluble salts that are invariably present in any porous material recovered from a marine site are removed by rinsing in water. In most instances, tap water is all that is needed, but to the use of de-ionized 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 without adversely affecting the material. If the decision is made to remove the more difficult stains, the material should be thoroughly wetted with water before immersing or applying the appropriate chemicals. Monitor the process carefully and rinse thoroughly in water after using any chemicals. After treatment, allow the pottery to air dry. Solvent drying is not required, but it may be used if desired. After drying, consolidate by completely immersing the material in a dilute solution of PVA or Acryloid B-72. Pottery vessels can be reconstructed after the consolidated shards have dried. Equipment required to conserve ceramics includes appropriately sized vats, tap water, de-ionized water, acetone, ethanol, PVA, Acryloid B-72, hydrogen peroxide, hydrochloric acid, EDTA, dental picks, and pneumatic chisels.


Donny L. Hamilton
1998. Methods of Conserving Underwater Archaeological Material Culture. Conservation Files: ANTH 605, Conservation of Cultural Resources I. Nautical Archaeology Program, Texas A&M University, World Wide Web, http://nautarch.tamu.edu/class/ANTH605/.

Copyright 2000 Donny L. Hamilton, Conservation Research Laboratory, Texas A&M University


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