2 ARCHAEOLOGICAL GLASS AND ITS DECAY

2.1 STRUCTURE AND MECHANISMS OF ATTACK

Archaeological glasses are all silicate glasses. The melting temperature of pure silica (1710�C) is too high to be reached using traditional technology, so fluxes such as alkali metal oxides had to be used to produce a glass of lower melting point. Figure 1 depicts the formation of glass from silica and an alkali metal oxide (M is an alkali metal such as lithium, sodium, or potassium). The alkali metal compounds act as fluxes that break up the silica network and so lower the viscosity and glass transition temperature of the resulting melt.

Aqueous attack on alkali metal silicate glasses has two stages. The first stage is ion exchange. Alkali metal ions are lost from the surface layer of the glass and replaced by hydrogen ions accompanied by water. The reaction in figure 2 shows the sodium ion migrating into the solution, which becomes progressively more alkaline. In effect, it becomes a solution of sodium hydroxide (NaOH). The rate of leaching of alkali metal out of the glass is governed by rates of diffusion. The nonbridging oxygens (i.e., those that do not form a link or bridge between two SiO4 tetrahedra in the glass but instead are bonded on one side to silicon and on the other to an alkali metal such as sodium or potassium) may be pictured as being more polarizable than the bridging oxygens and thus more susceptible to attack by a hydrogen ion. This depiction is in line with the observation that potassium silicate (potash) glasses are more susceptible to aqueous attack than sodium silicate glasses of the same molar composition, the potassium ions being larger, the center of positive charge being further away, and the corresponding nonbridging oxygens thus being more polarizable than their sodium silicate counterparts (Weyl and Marboe 1962–67).

The second stage in aqueous attack on silicate glasses involves hydrolysis and breakdown of the silica network. When the alkalinity of the attacking solution rises above a pH of 9, hydrolysis of the hydrated silicate network occurs, with hydroxyl ions attacking the silica network itself eventually forming Si(OH)62- ions, which dissolve in the alkaline solution. As the silica network breaks down, it becomes less of a barrier to diffusion, and the rate of leaching of alkali metal into the solution becomes limited by factors other than diffusion.

Both ion exchange and network dissolution occur to some extent at all pH levels, but ion exchange is the dominant mechanism of attack on glass below a pH of 9 and network dissolution predominates above a pH of 9.

The leached surface layer of a silicate glass being attacked by water is under tension because the H+ ions are far smaller than the alkali metal ions they replace (Metcalfe et al. 1971). The tension in the leached layer may cause it to crack or break away from the underlying unaltered glass, thus exposing a fresh glass surface for the cycle of leaching to start again. The surface ion–exchanged layer may also be to some extent hydrated and bulked out by water: such layers may lose water and crack on drying.

2.2 COMPOSITIONAL FACTORS INFLUENCING RATE OF DETERIORATION

The factors that influence rates of deterioration of glass may conveniently be considered in terms of the composition of the glass, the state of its surface, and the nature of the environment to which it is exposed. The fact noted above that potash glass is less durable than the corresponding sodium glass is of particular import in the conservation of medieval window glass, much of which has a high potash content owing to the use of wood ash as the source of alkali to flux the silica. The presence of moderate quantities of alkaline earths, aluminium, or phosphorus improves the durability of the glass.

Glass durability is not solely a function of bulk composition. Microheterogeneities in the glass structure, such as those arising from phase separation during cooling of the glass melt or poor mixing of the components of a melt, are likely to influence the durability of glass considerably. In the case of phase separation during cooling, one phase may be less durable than the corresponding homogeneous glass. Furthermore, the weathering of a less durable phase may produce a local environment that is detrimental to the stability of the other phase, even though the more durable phase might be quite stable with regard to the overall ambient environment. The tendency of a glass melt to undergo phase separation during cooling is a function of composition and also depends on whether the rate of cooling is slow enough to allow time for phase separation to occur. If phase separation produces droplets much greater than 180 nm in diameter and the refractive indices of the two phases are significantly different, the glass will be cloudy or white like milk, due to scattering of light by the droplets of one phase suspended in another.

2.3 SURFACE STATE FACTORS INFLUENCING RATE OF DETERIORATION

Apart from composition, a second significant factor controlling the deterioration of silicate glass is the state of its surface, since that is where the deterioration begins. One of the most important parameters governing the rate of glass deterioration in a specific instance is the ratio of the glass surface area (SA) to the effective volume (V) of the attacking solution (SA/V ratio). This parameter controls the time that must pass before, through the mechanism of ion exchange, the attacking solution reaches a pH of 9 and network breakdown starts. pH is a measure of the concentration of hydrogen ions in aqueous solution, so the change in pH brought about by a given number of alkali metal ions replacing hydrogen ions in solution is greater for small solution volumes. Surface roughness increases the surface area of the glass available to undergo ion exchange with the aqueous medium. Cracks, scratches, pores, and indentations contain very small volumes of solution surrounded by glass surface. Very little alkali has to diffuse into the solutions held within them to raise the pH above 9.

Surface defects promote rapid attack, so conservation procedures should, as far as possible, avoid roughening or scratching the glass surface. High SA/V ratios should be avoided when packing samples. Flat pieces of wet glass should not be left lying against each other for long periods with a thin film of water between them. If glass needs to be packed wet, there should be lots of water rather than a little, and alkali-resistant materials should be used. The use of flat or flexible sheet materials in a way that keeps a small volume of water next to the glass should also be avoided. Changing the holding solution periodically helps to avoid the development of elevated pH and may also limit the development of organic growths.

Stress in the surface of the glass is another factor that is detrimental to its stability. Stress causes disproportionation of bonding forces and regions of greater polarizability and greater chemical vulnerability than are present in the absence of stress (Weyl and Marboe 1962–67). Stresses arise during cooling of glass after manufacture and are due to ion exchange. Grinding causes plastic deformation and a region of residual stress below the tracks of the abrasive particles, in addition to the detrimental effects of scratches, surface roughness, and cracks noted above. Inhomogeneities such as regions of phase separation, bubbles, and crystals cause stress because of their different mechanical and thermal properties.

2.4 ENVIRONMENTAL FACTORS INFLUENCING RATE OF DETERIORATION

Apart from composition and surface state, the third class of factor affecting the durability of glass is the environment with which the glass is in contact. The most damaging aspect of the ambient environment is water vapor. A thin layer of water condenses on glass at all relative humidities above zero. Leaching of alkali occurs in the ordinary air, and not only when the surface of the glass is palpably wet. For example, sodium ions are leached out of soda-silica glass in air and react with water and carbon dioxide to form crystals on the surface of the glass. Potassium salts are particularly deliquescent, so crystals of potassium salts forming on the surface of a potassiumsilica glass might result in the repeated re-creation of aggressive droplets of solution on the glass surface during commonplace cycles of relative humidity.

A number of tests on the effects of humidity have been published, and the results are not wholly clear-cut in their implications. It may safely be said, however, that the worst attack occurs at above 50% RH. It is not entirely clear whether condensation is more or less deleterious than high humidity, but it is clear that both are very deleterious to the condition of susceptible glass. Heavy condensation may be less harmful than slight condensation because droplets running off the surface may wash away the ions leached out of the glass and so prevent the buildup of high pH in the droplets. Cycles of condensation and drying would cause the droplets to have an increasingly high pH as they dried out and would leave a precipitate of deliquescent salts. Upon a subsequent increase in relative humidity, these might again be the points at which condensation would first occur. This idea is speculative at present, but experiments are being conducted to test the model. Such a mechanism might explain some of the deterioration phenomena observed on ancient glass, such as the development of pits.

The importance of pH as an environmental factor influencing glass deterioration has already been noted. Frequent replacement of the attacking solution (in the case of glass in wet storage) or washing of glass in windows or on display might lower the mean pH of the attacking solution. Buffers control pH in solution but they may also have other effects on the leached surface layers of the glass surface, either promoting or impeding the passage of particular constituents. Similarly most ions in solution have some effect on the transport properties of the leached layers of the glass, but understanding of these processes is not yet at a level where solutions of a particular composition can be recommended for uses as holding solutions or to impede or prevent deterioration. Although it has been attractive to implicate gaseous atmospheric pollution as contributing to the deterioration of glass exposed to urban environments, there seems to be little evidence to support such a case.

2.5 TYPES OF DETERIORATION

The interaction of glass with the environment gives rise to a number of fairly common visible signs of deterioration. The leached layer is a hydrated layer of alkali-depleted hydrogen glass. It is not usually visible to the naked eye, although the change in refractive index between the leached and the bulk glass can be seen in section under the microscope. The behavior of the leached layer is crucial in governing glass deterioration (Hench and Clark 1978).

Fracture is rendered more likely by internal stress (from cooling, inhomogeneities, ion exchange, etc.), since less additional stress has to be added to the preexisting internal stress to exceed the strength of the material and result in fracture (Metcalfe et al. 1971). If thin layers crack off the surface of the glass but remain in place, iridescence may be observed due to constructive interference of light of certain wavelengths reflected from successive glass-air interfaces. Drying corroded glass reveals existing cracks because light is reflected from the air-glass interface to a far greater extent than from the previous water-glass interface (due to greater differences in refractive indices). In addition to revealing existing cracks, drying probably creates more cracks from shrinkage associated with dehydration of a hydrated glass layer. Cracks and stresses are not purely mechanical phenomena; they are accompanied by chemical changes associated with changes in the polarizability of oxygen ions (Weyl and Marboe 1962–67).

Crizzling and weeping are generally observed in glass with less than 5 mol% alkaline earth metal oxide, this being too low to stabilize the amount of alkali present (Brill 1975). In such glasses a very thick hydrated layer forms, which may crack into a crazy paving pattern (crizzle) in any case and will crack if stored at low ambient relative humidity. Where low alkaline earth glasses have a high potash content, potassium salts deposited on the glass surface as a result of ion exchange with water adsorbed on the surface may deliquesce if the ambient relative humidity becomes too high (weeping).

Blackening is quite commonly found on the surface of glass, particularly excavated glass, and it may exhibit a dendritic invasion into the interior, perhaps along microcracks. The staining might be due to lead sulphide, ferric ions, or manganese dioxide. Our preliminary electron beam analyses on excavated glass indicated that at least in the few samples studied so far, there was a markedly raised manganese concentration in the blackened areas. A similar phenomenon was noted by Alten (1988). The blackening may be the result of bacteria that use manganese in their metabolic processes and thereby concentrate it on and in the glass. Further work will be undertaken on the cause and possible remedy of this blackening.

A feature found particularly on medieval window glass is pitting. The pits may contain crystalline deposits or be empty. They may occur in clusters or lines but are not well understood. Pit distribution may be correlated negatively or positively with devitrification, organic growths, leading, painting, staining, surface deposits, or scratching, possibly by the effect of these factors on condensation patterns and the repeated occurrence of alkaline solutions at particular points on the surface. Under the microscope, pits can often be seen to be comprised of concentric hemispheres of glass originating from a point on the surface, sometimes with those hemispheres nearest the surface having disintegrated into a crystalline mass. The hemispheres are under tension, and if pressed with the point of a pin will sometimes detach themselves from the underlying glass and leap into the air. Further research is being undertaken to understand this phenomenon more fully.

Crusts on the surface of poorly durable window glass may range from off-white to black, depending perhaps on pollution and organic growths. The crusts are primarily composed of gypsum (CaSO4.2H2O) or syngenite (CaSO4.KSO4.H2O). They occur on glass that has too much lime and too little silica: the calcium leaches out and forms salts. There are sharp compositional boundaries between crusting and noncrusting glasses (Newton 1969).

Weathering layers are lamellar structures within the glass structure. They have high crystallinity, and elemental concentrations rise and fall in phase with the visible layers. There is often staining around fissures and milkiness. Weathering layers may be formed by leaching followed by the leached layer breaking away from the substrate and restarting the cycle at the new surface. With continued deterioration these layers may be transformed into a crystalline end product. It had been suggested that these layers might reflect annual rhythms and so provide a dating system akin to tree-ring or varve dating. The reliability of this method has been clearly refuted by Raw (1955) and others, but the processes of formation still retain their fascination.