Page 13 - Curriculum Visions Dynamic Book

Glass made only from silica changes size very little when it is heated and cooled. It also has a high fracture stress value. It could be an ideal material to use if it were not so expensive and difficult to make.
The first attempts to make a heat-proof yet economical glass happened because it was hard to use lanterns in rainy conditions. If the lantern glass got hot from the candle or oil lamp inside and the lamp was taken outside, the rain falling on the hot glass shattered it.
As a result of experimentation, it was found that if borax (boric oxide B2O3) was added to the glass, it would not crack when cooled quickly. However, the borax made the glass very prone to corrosion with water. Just as soda glass is very weak without lime, so a stabilizer was needed for the new borax glass.
The stabilizer was found in 1912. It was alumina (aluminium oxide, Al2O3). The stabilizer also had
the added, and unexpected, advantage of making the glass inert (unreactive) in the presence of food acids. This was discovered in 1914 when Jesse Littleton of the Corning Corporation was told of an emergency at home. His wife’s ceramic baking dish had broken. Littleton sawed the top off a battery jar and gave it to his wife. In the end, Bessie Littleton cooked cakes in sawn-off battery jars and made custards in lamp chimneys. As a result, new heat-proof glass was manufactured for bakeware and sold under the trade name Pyrex®.
Going one step further, experiments showed that the new borosilicate glass was also inert in the presence of most industrial acids and alkalis. As a result, this new glass was also produced for laboratory glassware and is still used today. Glass fibre, ceramic glaze, and vitreous enamel are also borosilicates.
Borosilicate glass is made with 5 to 20% boric oxide (B2O3) replacing most of the sodium oxide (Na2O), and with alumina (aluminium oxide (Al2O3)) replacing most of the lime (CaO).
(Below) The glass used to make light bulbs does not have to resist the rapid changes in temperature you might expect, even though the filament inside the bulb can reach a temperature of several thousand degrees in just a fraction of a second. As a result, it is made from soda lime-silica glass. That is a major factor in allowing light bulbs to be cheap. The glass bulb for a lamp is known in bulb making
as a “shell.”
13