steam

Part 14 - Glass

Silicon is an abundant element in the earth's crust typically combined with oxygen as silicate (silicon dioxide) and usually mixed with impurities. The melting point of silica is 1700°C (3090°F) but pure silicon melts at 1410°C which allows glass to be worked at a lower temperature.

The first pure glass containers were made around 1500 BCE in Egypt and Mesopotamia but the long metal blowpipe that made production easier and cheaper was not invented until 30 BCE. Glass-blowers began inflating a blob of molten glass on the end of a blowpipe and then inserting it into a mold which increased the variety of containers and decreasing their cost to the point where they were less expensive than baked clay pots. By 1290 CE, Venetian glass-workers had perfected Cristallo glass made from almost pure silica sand produce from quartz pebbles ground into a fine sand. 

Lead Glass

By 1575, English glass-makers had discovered the secret of making Cristallo glass and, in 1674, George Ravenscroft patented a new type of glass containing lead oxide. This lead glass had a high refractive index and was well suited for deep cutting and engraving. In the 20th century leaded glass was used for windows in thick walled cells producing radio-active isotopes. The leaded glass absorbed the radiation permitting technicians to see and to remotely manipulate chemical processing equipment.

Plate Pouring Process

In 1688, French glass- makers developed a technique of making plate glass with good optical properties, particularly for mirrors. Molten glass was poured onto a table and rolled flat. When cool, the plate glass was ground on large round tables with rotating cast iron discs and increasingly fine abrasive sands and felt disks. Mirrors were then made by coating one side with a tin or other reflective metal having a low melting point. 

In the crown glass process, used up to the mid-19th century, the glassblower would spin about 9 pounds (4 kg) of molten glass at the end of a rod until it flattened into a disk approximately 5 feet (1.5 m) in diameter. The disk would then be cut into panes that were joined with lead strips to form windows. By the early 1800's, crown glass had become common for windows in Western Europe and in the USA. 

In 1798, P.L.Guinand, a Swiss bell founder, used a fire clay stirrer to more uniformly distribute the heavy lead oxide in glass and reduce the size of air bubbles. The denser glasses increase the range of refractivity. Working with Joseph von Fraunhofer, a spectroscopist at the Munich Optical Institute he produced large high quality lenses.

Between 1750 and 1850 most bottles were black or very dark green because of iron and other impurities in the glass. Drinking glasses were made with more expensive clear glass. Carefully controlled amounts of more elements were gradually introduced to make a larger range of colours for window glass. These included;- chromium, manganese, copper, sulphur, uranium and nickel in addition to the traditional cobalt for blue glass. By 1825, molten glass, blown into the shape of a cylinder was cooled, sliced down one side, reheated and opened up to form a large sheet of thin window glass. In 1851, Joseph Paxton built the Crystal Palace in London to house the Great Exhibition. The revolutionary building inspired the use of glass for many other architectural purposes. By 1886 Ernst Abbe and Otto Schott had joined Karl Zeiss at the Jena glassworks and by the end of the century they had listed about 80 different optical glasses using more than 29 elements not previously used in glass-making.

In 1887 Britain, the Ashley Bottle Company developed a process where two machines and two men could make 200 glass bottles per hour, making them many times cheaper than traditional bottles. Previously, 5 men produced only 150 bottles per hour.

In 1903, the American, Michael J. Owens, invented a fully automatic bottle-blowing machine capable of producing 2,500 bottles an hour. This continually sucked a blob of glass into a metal mold where it was blown into shape and cut off automatically. 

 In 1905 Belgium, Émile Fourcault (1862–1919) invented a process of vertically drawing a continuous sheet of glass from a tank of molten glass and about 1918 another Belgian engineer, Emil Bicheroux, poured molten glass from a pot through two rollers. Both processed produced plate glass with a more uniform thickness making polishing and grinding easier.

In 1910, French scientist Edouard Benedictus strengthened glass by laminating a celluloid material between two glass sheets. Bulletproof glass is still made by bonding many layers of glass and plastic together. Toughened glass, used in car windshields, is made by cooling molten glass very quickly making it much harder and less prone to shatter into sharp fragments like conventional glass. The float glass process was perfected in Britain by the Pilkington Brothers Ltd., in 1959. Molten glass was poured across the surface of a bath of molten tin where it flattened before it was drawn horizontally in a continuous ribbon into an annealing furnace.

Float glass combined the optical qualities of plate glass with the brilliant finish of sheet glass and, today, most flat glass is made by this process. 

Borosilicate glasses were developed by German glassmaker Otto Schott in the late 19th century in Jena. Borosilicate glasses have a very low coefficient of thermal expansion making them more resistant to thermal shock than any other common glass. They can withstand temperature differentials of about 165 °C (297 °F) without fracturing. It is typically used for chemical laboratory equipment such as reagent bottles and flasks, as well as lighting, electronics and cookware.

Borosilicate glass (trademark PYREX®) is typically made from 80% silica, 13% boric oxide, 4% sodium oxide and 2–3% aluminum oxide. Its superior durability, chemical and heat resistance makes it valuable despite a higher cost because its high melting temperature makes it more difficult to produce.