Robert Pierce, age 10, of Montreal, Quebec, Canada, for his question:
HOW DO THEY MAKE GLASS?
Our ancestors discovered how to make glass many thousands of years ago. The recipe was lost or forgotten many times but somebody always discovered it again, often by accident. In the past the work was very hard and very costly. For example, in ancient times, colored glass beads were more expensive than real rubies and emeralds.
The three main glass making ingredients are plentiful minerals found in the earth's crust. But a very hot furnace is needed to melt them together to form a clear, tacky substance. When this substance cools, it becomes hard, solid glass.
The main ingredient is a special sand that contains a lot of silica. The silica molecules are packages of oxygen and silicon atoms. They form quartz, which is the hardest of the earth's ordinary stones. For glass making, this hard silica material may be gritty grains of sand or crushed sandstone.
Enormous heat is needed to melt this material, but there are ways to make it melt at lower temperatures. This is why a couple of other ingredients are added to the basic recipe. One is an alkali chemical, such as potash or soda ash. Another is lime, which also makes the finished glass more durable.
The limy ingredient is added in the form of crushed limestone. The alkali may be crushed rock or some ashy material. The basic recipe for ordinary window glass requires 72 pounds of the silica material, 15 pounds of the alkali and nine pounds of the limy ingredient.
The ingredients are assembled in large quantities and shoveled into a furnace. Then comes the hard part. The furnace must be able to heat the rocky mixture to a temperature of 3,000 degrees Fahrenheit. This is as hot as a steel making furnace.
At this temperature the rocky ingredients melt and fuse together. While still hot, the tacky substance can be shaped, rolled and polished. When it cools it is shiny solid glass. Actually, a glass making factory has thousands of slightly different recipes for making different kinds of glass. Traces of other ingredients are added to the basic three. The tacky material is rolled and pressed, shaped, polished and cut to make different kinds of glass for a whole multitude of different uses.