Page 42 - Curriculum Visions Dynamic Book

4: Machine-made glass
Glass is an extremely versatile material, and its uses are almost endless. However, in each application the glass has to stand up to a particular set of physical and chemical demands. Here you will see some modern examples of this.
The development of standard
machine-made glass
Making glass by hand can result in exquisite objects, but it is both time consuming and extremely skilled work. Making glass by hand also has severe limitations. Glass objects cannot be produced cheaply, they cannot be of uniform size (that is, you cannot have standardisation), and certain items, for example, large sheets of flat window glass, cannot be made at all.
Imagine trying to use glassware for chemical experiments when every glass object was of a different size. Each one would have to be measured separately. Furthermore, the chemist could not rely on having glassware that behaved the same way each time. This gives you some idea of the need to find methods for standardizing the making of glass.
Even if handmade glass could be standardized, the cost of labor would
still make its use very limited. That is
why mechanized ways of making glass were sought. The pioneering work for this was done in America at the start of the 20th century.
If you are going to mechanise glassmaking, you have to reproduce
The modern glass furnace
A modern glass furnace uses everything that has been learned from the past and the best available science.
The material going into the furnace (the charge) is a mixture of basic new ingredients and between 25 and 60% cullet, or crushed rejected glass of the same composition as the glass being made. The cullet not only recycles material, but it melts sooner in the furnace, which helps bring the fresh mineral particles together and so speeds up the glassmaking process.
Temperatures in a glass-melting chamber of a furnace depend on
the glass being made, but would typically be 1,475°C for a soda lime- silicate glass.
While the glass is forming, large amounts of gas are given off from the reaction of the raw materials. These gases, together with the air already
in the furnace, can cause problems because they can form bubbles in
the glass.
Although large bubbles generally float to the surface, small bubbles are harder to remove. They have to be taken out in the next part of the furnace, called the conditioning chamber.
In the conditioning chamber the glass is slightly cooled (to perhaps 1,300°C for soda-lime silicate glass). The small bubbles disappear as
they dissolve back into the glass. Mechanical mixers stir the melt to ensure it is all of the same quality.
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