WEATHERED ARCHAEOLOGICAL GLASS
Weathered Archaeological Glass By Astrid van Giffen The Corning Museum of Glass
Glass is found at archaeological excavations in a variety of conditions. The glass condition can range from pristine, where no deterioration is visible, to so heavily degraded that practically all the glass has been transformed into corrosion products. The deterioration of the glass surface is generally known as weathering and the deteriorated area as a weathering crust.
The corrosion process
The chemical and physical properties of the burial environment and the composition of the glass itself are the main factors that determine the rate of deterioration of glass in the ground. Too little silica and more or less than the optimum 10 % of lime are especially detrimental for the stability of a glass. Soda glass is almost twice as stable as potash glass. However, under the right conditions any glass can show signs of deterioration.
In general, glass found in dry soils is in better condition than glass found in moist soils. This is because water is the primary cause of deterioration of glass. The exposure of glass to moisture causes alkali ions in the glass network to be slowly leached out and replaced by hydrogen ions from the water. This leached layer is referred to by several different terms: alkali-deficient layer, silica-rich layer, or hydrogen glass. It usually occurs within a few years of burial. Interestingly, it re-occurs in a cyclic manner, with additional layers being formed again, every few years. Distinct layers can often be seen, and as they build up, the weathering crust gets thicker and thicker. The final crust can vary in thickness from microscopically thin to so thick that it can easily be seen without a microscope. Frequently the leached crust is found to have a laminar structure with individual parallel layers ranging in thickness from less than 1 μm to about 25 μm. “The laminated structure can cover all the fragment homogeneously or it may start at one single point on the surface, which leads to circular patterns.”1 In some cases the alkali-deficient layers protect the remaining glass from further deterioration, or slow down the access of water to the glass and thus slow down the formation of new layers. Whether the crust is protective or not depends primarily on the composition of the glass and the pH of the leaching solution. In alkaline environments the silica network is attacked, eventually causing the total dissolution of the glass.
Although the chemical processes of glass deterioration have been extensively studied, they are not yet entirely understood and cannot be predicted. It is not clear why glass often decomposes in layers.
One theory for the laminar structure of weathering crusts is related to the glass’ contact with moisture. After the initial stages of attack, the leached layer is believed to partially transform into a new structure called silica gel, which is more porous than the leached layer. The porosity of the silica gel “provides a matrix in which subsequent precipitation and crystallization reactions can occur.”2 The parameters that influence the formation of silica gel and the reactions that occur within it are still being studied.
Another theory states “that as large sodium or even larger potassium ions are replaced by protons the physical stress on the structure causes the surface layer to split.”3 This allows water to get through to the fresh glass underneath and the process is repeated. The decrease in volume caused by the leaching of ions can lead to micro porosity of the surface layer, which in turn might cause the weathering layers.
It has also been suggested that the layering is caused by periodic or cyclic changes, such as seasonal variations in temperature and rainfall.4 Because such changes occur in yearly cycles the number of layers should be an indication of how long the degradation has been in process, similar to counting growth rings in a tree trunk to indicate the age of the tree. Several examples of glass with a known burial date support this theory. However, for the majority of objects the number of layers does not correlate to the estimated number of years of burial. In addition, layered weathering crusts have been produced in burial environments as well as in controlled unvarying laboratory conditions, in some cases in as little as 6 weeks. If the theory can be applied to date archaeological glass, it can only be used on a small fraction of them, most likely “those ranging from the early eighteenth century back through the medieval period. Roman and Byzantine glasses are generally too resistant to become heavily weathered; Egyptian glasses are most often found in arid environments, and therefore have not suffered much from corrosion; Mesopotamian glasses are often so heavily weathered that no glass remains, and what does is too
Visual appearance of deterioration
The deterioration of weathered glass can have an extensive variation of appearances. The visual effects of degradation most commonly found on excavated glass are dulling, iridescence, opaque weathering, a total loss of glassy nature, pitting, cracking of the surface, and discoloration.
Dulling refers to a loss of original clarity and transparency that is quite distinct from haziness caused by scratches or stains. It is closely related to iridescence, which is a rainbow-like effect on the surface of the glass similar to a thin layer of oil on a water surface. Both are caused by changes in the composition of the surface of the glass altering the refractive index. The weathering crust is made up of many thin layers leading to the iridescence, which is caused by “the interference between rays of light reflected from thin alternating layers of air and weathered glass crusts.”
Opaque weathering also has a laminar structure, but has a much larger number of layers. “The layers may be adhering to one another and may penetrate the entire surface or they may be laminating and superficial.” This type of weathering is characterized by opaque areas, usually white, on the surface gradually eating deeper into the glass, and is generally referred to as opalescent weathering. At more advanced stages the color can be black or brown or even a mottled polychrome. The incipient stage is sometimes referred to as milky weathering because of the small spots or streaks of white. At the most extreme stage it is termed enamel-like weathering and is present as a thick covering varying in color.
Pitting can occur when the corrosion “eats” its way into the glass from a starting point either on or just below the surface, sometimes creating concentric circles around the starting point. When the weathering is lost, a hole or pit is left in the surface of the otherwise undamaged glass. Pitting often occurs simultaneously at individual sites throughout the surface of a fragment. Weathering from individual starting points can later grow into one another.
Shrinkage of the alkali-deficient layer, due to temperature and humidity changes, can cause cracking of the surface and within the weathering crust itself. Often the cracking does not become visible until some-time after the glass has been excavated. This is especially true for glasses buried in wet soils.
Discoloration of the glass can be found in combination with any of the above mentioned types of weathering and is caused by the migration or alteration of coloring ions and other trace elements. The ions can be leached out of the glass network or be taken up by glass from the environment. Iron and manganese cause the weathering crusts to blacken and contact with copper corrosion products can cause green staining. Certain ions, most notably manganese and copper, may change color through oxidation.
Combinations of several of these manifestations of glass deterioration are usually found on a single object. The extent of the degradation can also differ from one area on an object to another.
The thickness of the weathering crust can vary greatly, depending on the chemical stability of the glass and the aggressiveness of the burial conditions. In extreme cases corrosion products may have completely replaced the original glass. Underneath the weathering crust the so-called glass core retains the original composition and color of the glass.
Importance of preserving the weathered surface
Preservation of surface details is not the only reason to ensure that the weathering remains whole. The removal of weathering often reveals a very irregular and often pitted surface because air bubbles and debris trapped in the glass during production are exposed as a result of the degradation. The exposure of such a surface can result in misinterpretation of the original appearance of the object and is aesthetically unappealing.
Finally, there is the aesthetic appeal of the weathering itself, especially iridescence, which has become so associated with archaeological glass and is much valued for its beauty.
By Astrid van Giffen for the complete text with picture illustrating the different appearances please follow this link http://www.cmog.org/article/weathered-archaeological-glass