What is it?
The Principal ingredient is SILICA, which- occurs naturally in great abundance. The main varieties of silica are SAND, FLINT AND QUARTZ. When heated to a high enough temperature, they melt to form glass. When the first atomic bomb was exploded in the desert at the White Sands proving ground, New Mexico, the heat wave from the bomb turned the surface of the desert to glass. In the crater, the desert sand, which is largely made of silica melted and became a mildly, radioactive, light green glass, which was named Trinitite.The crater was filled in soon after the test. Also, a dark natural substance known as OBSIDIAN, is often formed by volcanic eruptions, when silica is present.
The temperature required to melt silica (about 1800°C) is higher than could be obtained in the primitive wood-burning furnaces used by the earl glassmakers. Man-made glass only became possible when it was discovered that the addition of soda ash or potash (obtained from burnt vegetable material) caused the silica to melt at 900-1100 °C. There is a story that this was discovered accidentally, by nomads lighting fires on sand. The ash from the burned vegetation, combined with the sand to make glass.
The Mediterranean area favoured soda ash and Europe potash. Natural tints were removed by the addition of nitre, manganese or arsenic. This was not a precise thing because the additions produced tints of their own, explaining why there was a wide range of colours in early samples. Modern pure additives mean that a consistently clear and colourless product can be produced.
The molten silica is usually referred to as the METAL. Additions of COPPER, COBALT, IRON, TIN and other materials produce BLUE, RED, GREEN, AMBER, WHITE OR PURPLE colours.
Methods of manufacture.
Early glassmakers used MOULDING techniques. This lasted from about the 15th Century BC – 1St Century BC. Blowing was discovered during the 1st Century BC and by the 1st Century AD virtually every technique of manipulating and decorating glass had been discovered – moulding, free blowing, blowing into a mould, cutting, engraving, enameling, gilding, overlaying with coloured layers, enclosing the decoration between layers, millefiori and glass made to look like natural stone. All these were practiced 2000 years ago and have reappeared at various times down to the present day.
The tools used in glassmaking have remained unchanged for many centuries, mainly because they are so simple that they allow for little improvement.
The main tools are as follows:
BLOWPIPE – a long hollow tube made of steel on which molten blob is gathered on the end and by blowing down the pipe a shape is produced.
PONTIL IRON – a solid metal rod. The end is heated and applied to the base of the vessel being blown, to support it when the blowing pipe is cut away.(when the pontil is removed, it produces a characteristic mark known as the PONTIL MARK on the base)
MARVER – a flat iron plate on which the vessel is rolled during the blowing process to smooth and shape it.
PUCELLAS – metal tongs with a curved surface on the outside and a cutting edge on the inside.-, used to shape anf trim the molten glass bowl of a drinking glass.
CLAPPERS – a pair of wooden paddles used to shape the foot of a drinking glass.
CHAIR – the glass-blowers workplace – a bench with flat arms along which the blowpipe can be rolled to shape the vessel. Confusingly, in a glass-works making hand-blown glass, it also refers to a team of three or four men. Thus the size of the factory is reckoned by the number of chairs working in it.
If you have found this interesting, look out for the next part of the series, looking at the history of glassmaking.