Dating glass by color archaeology
“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.” 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 p H of the leaching solution.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.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.The deterioration of the glass surface is generally known as .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.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.
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.
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.