164 Chapter 7
be based, for example, on the ratio of selected major compounds (e.g. high
lime/low alkali for 16th century German glass, in contrast to Roman glasses
with a low-lime content), on the presence of certain trace elements (e.g. Ni, Zn)
or on the isotope distribution in lead glasses.
A number of physical properties are characteristic for each type of glass
composition: the viscosity of the molten glass determines the working range
during production, the thermal expansion connected to the transition point (Tg)
marks the transition of the liquid and the glassy state, the optical properties in
relation to the refractive index characterise the transparency for light. Besides
density, hardness and brittleness are characteristic physical properties. The most
important chemical property for glass is its chemical durability, which is the
key to degradation and described in the next section.
2.3 Degradation Mechanisms: Basic Reactions in Water
The chemical degradation of glass is initiated by water attack on the surface.
The principal reactions can be explained by investigating the alteration of
freshly-prepared glass in aqueous solutions. For modern glass compositions, a
large number of studies have been carried out to explore the chemical dur-
ability of container glass or nuclear waste encapsulations. Their major con-
clusions are also relevant for historic glass. The term corrosion, originally
referring to the oxidation of metals, is frequently used as a synonym for the
degradation of glass.
For glass in contact with water, the degradation mechanism is clearly dom-
inated by the pH value of the liquid. In acidic media, water and hydronium
ions (H 3 O) migrate into the glass to replace positively-charged ions of alkaline
or alkaline earth elements, which are leached out of the glass. This leads to
the formation of a hydrated layer, also called “gel layer” or “depleted layer”,
with a composition significantly different from the bulk glass (high content
of Si, low content of residual Ca, Na, K) (see Figure 4).
At high pH values (in general above pH 9), another type of mechanism
controls the degradation of glass: hydroxide ions attack the bridging SiOSi
bonds, which leads to dissolution of the glass network.
Both reaction mechanisms, the ion exchange and the network dissolution,
compete with each other in natural conditions. In the laboratory, those condi-
tions can be manipulated to clearly favour one mechanism over the other.
●Glass damaged by leaching develops a depleted layer, which may not be
visible under the microscope if it is limited to a few nanometers in thick-
ness. If the leaching increases and the thickness of the hydrated layer
reaches several micrometers, a characteristic pattern of interconnected
cracks is developed (craquellée).