162 Chapter 7
Around 50 BC glassblowing was invented, which is considered a technological
milestone in the production of glass.
Throughout the Roman Empire, new centres for the manufacturing of vessel
glass were established. Glass was no longer considered as an item of luxury,
being used to produce containers for storage of goods. After the decline of the
Roman Empire, glassmaking experienced a slump, both in quality and quantity.
Natron, as a source for soda, an important raw material for the production of glass
previously supplied through Roman trade routes, became difficult to obtain. By
the end of the first millennium, potash, derived from the ashes of burnt trees,
replaced soda as fluxing agent in northern Europe. This change in raw materials
defines an important shift in chemical composition, from soda-lime silicate
glasses to potash-lime silicate glasses. The difference in durability of the two
types of glasses and the consequences of conservation will be discussed later.
The use of glass for windows probably has its origin in Roman times. From
the 9th century onwards, however, it gained rapidly in importance and reached
its perfection in the Gothic period. Pieces of coloured glass, often stained,
etched, engraved or decorated with black paint (trace lines and half tones)
were incorporated in a framework of lead clamps to facilitate artistic expres-
sion. Stained glass windows are among the most precious art objects of medieval
times. Many examples are still to be found in their original setting, in eccle-
siastical cathedrals, for example, in Cologne, Reims, Chartres, York or
Canterbury. Another possibility of using glass as an architectural element is
to embed small square glass pieces in cement or mortar to create pictures on
the wall or on the floor. Famous examples of these mosaics from the Roman
and Byzantine period are found in Rome, Ravenna, Venice or Constantinople.
This short overview cannot consider other important applications of glass in
the creation of art objects, such as mirrors, reverse paintings on glass, enamels
(glass fused on copper, silver or gold) and jewellery.
2.2 Chemical Composition, Structure, and Physical Properties
The structure of glass is a consequence of its chemical composition and thus
the raw materials used for its production. This chapter is limited to the
discussion of glass types relevant to conservation, neglecting specific develop-
ments in modern applications.
Crystalline silica (SiO 2 , quartz) consists of a strong network of silicon and
oxygen. Each silicon atom is surrounded regularly by four oxygen atoms. Two
oxygen atoms are shared by each of the adjacent tetrahedral units. In contrast
to the regular lattice of pure silica, glass has no regular structure with a long-
range order.
Apart from SiO 2 , other inorganic oxides, such as phosphorus oxide, can act
as network formers. Alkaline or alkaline earth oxides have a different effect,