Roman glass beaker
The recent introduction of new adhesives and synthetic materials has significantly improved glass conservation and repair. Epoxy, polyester and acrylic resins can be made to match colour, shape and even refractive index so that repairs are hardly noticeable.
The new materials must be strong, but not stronger than the glass, so that the glass itself is not broken. They weigh about the same as glass. They have the same refractive index, so that joins do not affect light differently and become obvious. They have the correct viscosity to make a good bond.
The processes must be reversible so that joints can be undone and, since all objects are different, the conservation of every piece has to be worked out individually.
The science of glass
Glass is really a liquid, a super cooled liquid. Its atoms are not arranged in any regular pattern to form crystals and it has no definite melting point, like most other solids. Despite the fact that glass is hard and shatters easily, it flows very slowly, which produces distortions in very old windows. Glass occurs naturally as the result of high temperature events such as volcanoes or meteorite impact.
Sand or quartz (Silicon dioxide) is the largest ingredient (70 - 80%), with around 17% Soda (Sodium bicarbonate) and 7% Lime (Calcium carbonate) in typical glass.
Colours come from various metals added either in an oxygen rich or oxygen-less atmosphere. Iron gives pale green or aqua blue in oxygen rich surroundings or amber in a reducing atmosphere; manganese for purple (oxidising) and clear (reducing); copper for aqua blue to green (oxidising) and opaque red (reducing), copper and cobalt for blue. Antimony in oxidising conditions gives white, whilst antimony and lead together give bright yellow.
Stone Age people used the sharp edges of broken natural glass as tools. Around 3500 BC the first glass objects, beads, were made in Egypt and Mesopotamia.
Around 1500 BC pots with decorations were being made in Egypt and Mesopotamia. Phoenician merchants spread glass technology.
Between 27 BC and 14 AD glass blowing was developed by Syrian craftsmen.
0 BC - the Romans were using patterned, blow-in moulds.
100 AD Romans began to make glass windows
In the middles ages Venice became the undoubted European centre of expertise.
1674 - George Ravenscroft, an English glassmaker, patented lead crystal, a brilliant glass for cutting and optical use.
1959 - float glass process invented by Pilkington, St Helens. Large sheets of high quality, very clear glass become readily available at reasonable cost.