THE CONSERVATION GLOSSARY

Glass

- known to man since at least 4000BC, the basic ingredients are sand and a flux to reduce the melting point, such as soda. Ideal ingredients are basically, soda ash, lime and pure white silica sand, which are then fused together at high temperature. The Romans used glass in windows, after which it became rare until the 16th century by which time taxes were in place which limited its use. Glass was taxed by weight, not area, and until the tax was finally lifted in 1845 light crown glass and astragled windows persisted. After 1845, the Victorians were able to make great advances in glass manufacture which saw an increasing use in architecture (Crystal Palace 1851). Technology continued to improve, the major breakthrough occuring in 1959 when Pilkingtons invented the float glass process where a continuous sheet of molten glass is floated on a bed of molten tin before entering an annealing chamber, leaving the glass perfectly clear, eliminating expensive grinding and polishing.The 20th century has bought fibre-glass, heat resistant glass, laminated glass, one way glass etc.

Glossary
Annealing - the process of cooling glass slowly and evenly, to eliminate stresses, which build up during manufacture.
Bullion or bullseye - the centre knob of a table of crown glass where the pontil was attached. Originally recycled or used at the rear of buildings, their use for decoration is a comparatively recent trend.
Cylinder glass - also muff or broad glass, a blown sphere which was then swung or twisted into a sausage shape. The ends were removed and the resulting cylinder was split and flattened and cooled in the annealing oven. This process produced quite large sheets, approximately 2ft by 1ft, characterised by parallel lines of deformity and a rather dull finish from its contact with the annealing table. Seldom completely flat, the convex face was always pointed outwards to deflect prevailing wind.
Crizzling - a network of fine cracks caused by an imbalance in the materials used.
Crown glass - a method of producing glass in which a globe is transferred from the blowpipe to a pontil or punty, reheated and then spun until it opened up into a large flat plate - a table of crown glass. It often has a greenish tinge, curves slightly and can have small radial imperfections, all of which can give the glass a wonderful reflective quality. While more costly to produce through the wastage associated with its circular shape, because it does not come into contact with any surface during manufacture, it is more transparent than cylinder glass.
Cullet - recycled glass. (Can be quite difficult to re-use because of colour mix and impurities etc, has been used as aggregate in road construction)
Glasshouse cone - feature of early 19th century glassworks. A giant furnace chimney, big enough for the glassworkers to work inside. Only one left in Scotland at Alloa (Four in Britain).
Ground glass - obscure glass formed by grinding one face, usually with sand.
Plate glass - made the same way as cylinder glass, it was thicker, and was then ground and polished to produce quality glass. Now, rolled plate glass is a fully mechanised process.
Pontil - an iron rod used for gathering glass.
Stained glass - there are two basic types of stained glass, pot metal glass in which the glass is given colour in its molten state, and flashed glass in which colour is applied.