Dating glass by color archaeology
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.” 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 microporosity of the surface layer, which in turn might cause the weathering layers.
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 fragile to be handled.” 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.
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.
It has also been suggested that the layering is caused by periodic or cyclic changes, such as seasonal variations in temperature and rainfall.
The porosity of the silica gel “provides a matrix in which subsequent precipitation and crystallization reactions can occur.” The parameters that influence the formation of silica gel and the reactions that occur within it are still being studied.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.” and has a sugary appearance which is sometimes difficult to identify as glass.