Letting in the Light

Before the 18th century window glass was a luxury. Only the rich could afford to glaze their windows. The poor were more likely to use shutters, strips of animal horn or oiled cloth. As factories began to produce cheaper glass by the crown and cylinder process, windows became bigger. Panes were still small owing to the limitations of the blowing methods but by Georgian times nearly everyone could afford to glaze their windows despite the increasingly heavy tax on glass, which continued to hamper the industry until 1845.

Plate Glass

The Romans made flat glass by rolling out hot glass on a smooth table. This produced a window glass that was uneven and not very transparent. The traditional method of overcoming such defects has been the use of ground and polished plate glass. Plate glass was first produced at St. Gobain, in France, in 1668, by the ‘broad glass’ method – blowing a long glass cylinder, slitting it lengthways and gently unrolling it to form a rectangle. After annealing, the plate was ground and polished on both sides. The quality of broad glass was not good and it was largely superseded by the crown glass process.

Crown Glass

Crown glass windowpanes were made by gathering molten glass on to the blowpipe and blowing it into a large sphere. An iron rod known as the pontil was attached to the sphere opposite the blowpipe, which was then cracked off. The glass sphere was then re-heated and spun rapidly to cause it to open out into a slightly concave disc, over a meter in diameter, known as the table. This was then separated from the pontil and cooled gradually in the annealing kiln. When cool, a ‘crown’ or ‘table’ of window glass was marked into panes and when cut, would produce about a square meter of glass. The glass often had a pale greenish tinge and had slight imperfections in it. The point where the pontil was attached, the bullion or ‘bull’, was regarded as a waste product but was sometimes used as a cheaper second. In course of time the bullion glass became fashionable as a period detail. The waste around the edge of the crown was either cut into smaller panes or recycled as cullet in a new batch.

The Crystal Palace

Manufacturers began to employ an improved method of making cylinder glass from which larger panes could be produced. This made possible the great Victorian extravaganza in glass, the Crystal Palace. Chance Brothers of Birmingham introduced the improved cylinder method of glass-making to England. They produced nearly a million square feet of hand blown cylinder glass for glazing the Crystal Palace.

The Improved Cylinder Method

This method was similar to that for making broad glass. A large cylinder was blown (very large cylinders were sometimes made by swinging the blowing iron in a pit) and glass was allowed to cool before being split with a diamond. It was then reheated in a special kiln and flattened on to a piece of polished glass which preserved its surface. In 1871, William Pilkington invented a machine which took over the functions of supporting, swinging and rotating the glass cylinder, thereby enabling even larger sheets of glass to be made.

All the hand-made methods gave way to mechanized sheet glass production in the 1900s. Here a ribbon of glass was drawn continuously from a tank furnace between cooled rollers. It produced cheaper window glass but was still not free from imperfections. Better quality plate glass was made by casting molten glass on an iron table and then grinding and polishing it to a high finish. All these processes were overtaken by the invention, in 1959, of the float glass process by Alistair Pilkington of the Pilkington Glass Company. In this process a continuous strip of molten glass at approximately 1000 degrees centigrade is poured continuously from the furnace onto a large shallow bath of molten metal, usually tin. It floats on the tin, spreads out and forms a level surface. Thickness is controlled by the speed at which the solidifying glass ribbon is drawn off the bath. After annealing the glass emerges as a ‘fire’ polished product with virtually parallel surfaces. The glass can be formed at high speeds and is much less expensive to produce than similar quality glass made by grinding and polishing. As a result, many glass manufacturers have converted to the float process, and today 90% of the world’s flat glass is made by this process.