During the seventeenth century, the period of the scientific revolution, the invention of the telescope and the microscope made much greater demands on optical glass, but glass of satisfactory quality for lenses was not consistently made until Guinand’s invention in 1805 of a porous fireclay stirrer to bring about a proper mixing of the glass melt and eliminate gas bubbles. Another use of glass with a long history is for windows. In Roman times, only small pieces of flat glass could be produced, by casting in a mould. From the Middle Ages until the present century window glass was formed by blowing, following one of two processes. The crown glass method involved the blowing of a cylinder which was opened at the bottom; after heating the open end at the furnace mouth, or ‘glory hole’, the blowing iron was rotated rapidly until by centrifugal force the bottle suddenly flared out to form a flat disc. This was then cut into rectangular pieces measuring up to about 50 cm (20 in). The glass at the centre or crown of the disc (hence the name crown glass), where the iron was attached, was too thick to be used in windows except in lights above doors where light was to be admitted but transparency not required.
The other process also entailed blowing a cylinder, but this was then slit down the side and the glass gently flattened while still in a plastic state. In the nineteenth century very large cylinders could be blown and these were the source of the glass for such structures as the Crystal Palace and the large railway station roofs that were such a feature of Victorian structural engineering. Later, in the 1920s the drawn cylinder process was developed whereby a circular plate was dipped into molten glass, then slowly drawn up. The flat glass produced by these methods retained a fire-polished finish but was never perfectly flat. To achieve that, the cast plate process was invented in seventeenthcentury France, particularly for the large windows and mirrors for the Palace of Versailles. In the 17805 the process was established in England at the Ravenhead works near St Helens in Lancashire. Some forty years later the firm was rescued from the low ebb into which it had sunk by a Dr Pilkington, one of the most illustrious names in glass-making history.
Cast plate glass was certainly flat, but removing it from the casting tray destroyed the fire finish and this had to be restored by grinding and polishing. The age-old dilemma, between nearly-flat glass with a fire finish and flat glass without it, was eventually resolved in the 1950s by perhaps the most notable advance in glass technology this century, the invention of the float glass process. Patents for float glass date from the early years of the century but came to nothing. It was the invention by Sir Alastair Pilkington FRS that succeeded, working at Pilkington Bros (he is a namesake, not a relative of the family). In 1952 he conceived the idea of floating a layer of molten glass on a bath of molten tin in a closed container, in an inert atmosphere, to prevent oxidation of the tin. The product is flat glass that retains a polished fire finish. After seven years of development work, the new product was announced and became a commercial success.
During the nineteenth century the increased wealth generated by the Industrial Revolution led to an increased demand for glassware of all kinds and in 1845 the repeal of the excise duty that had been hampering the British industry since 1745 stimulated growth still further. The old furnaces with their small pots for making glass were outpaced and outmoded. They were replaced by the large-scale, continuous operation tank furnaces, developed by Siemens and others. The pot furnace survived only for small-scale handmade glassworking.
The glass bottle had begun to replace stoneware to contain wine and beer around the middle of the seventeenth century. The earliest wine bottles were curiously bulbous in shape but as the practice grew of ‘laying down’ wine, the bottles had to take on their familiar parallel-sided form, by about 1750. The use of glass as a container for food and drink grew considerably from the middle of the nineteenth century and improvements were made in form and process. Codd in 1871 invented an ingenious device for closing bottles of mineral water, by means of a marble stopper in a constricted neck. So far bottles were hand blown into moulds but in 1886 Ashley brought out a machine that partially mechanized the process. The first fully automatic bottlemaking machine appeared in the USA in 1903, invented by Michael Owens of Toledo, Ohio. Further development came with the IS (Individual Section) machine from 1925, in which a measured amount or ‘gob’ of molten glass was channelled to the bottle moulds.