Suzanne Spinks, age 11, of Houston, Tex., for her question:
What ingredients are used to make glass?
Glass can be made from a few of the most plentiful minerals in the Earth's crust. But no one knows for surf how or when the recipe was discovered. One ancient story claims that it was discovered by sailors who made a campfire on a beach. Lumps of glass were formed by accident and found among the cooling sand and ashes.
Modern industry makes more different kinds of glass than you could count in a week. One large plant uses glass to manufacture 37,000 different products and keeps on hand more than 50,000 recipes for making different kinds of glass. You might start this long list with all the different colored glasses you have seen. Then add heat resistant and bulletproof glass, window glass and spectacle glass. In addition to these, scores of different recipes are used to make glass for special duties in laboratories and in electrical equipment.
The list of ingredients in all these recipes is very long. But the basic ingredients are the same, and they make up more than 95% of almost every glass making mixture. The main ingredient is silica, the most plentiful mineral in the Earth's crust. Silica minerals are compounds of the elements silicon and oxygen, and silica is the main ingredient in most sands. If silica rich sand is melted at a very high temperature, its materials will fuse together and form gobs of glass. In nature, glassy obsidian rock is formed from silica rich molten lava.
If about 15 pounds of alkali, such as soda ash or potash, are added to 72 pounds of silica, the mixture will melt and form glass at a much lower temperature. However, the furnace still must reach about 3,000 degrees Fahrenheit, which is hot enough to melt iron. Silica and soda produce water glass, which tends to melt in water and become runny glue. In order to make the product durable, about nine pounds of lime must be added to the mixture. The lime or calcium may be added in the form of crushed limestone. Silica, soda and lime are the three basic ingredients in all the thousands of recipes for making different types of glass.
One of the most useful is the heat resistant glass used to make baiting dishes. The recipe for this glass calls for a greater percentage of silica and less soda. A trace of alumina and a large helping of boric oxide are added to the brew. Most colored glass is created by traces of metals or metallic salts added to the basic three ingredients.
The sand or sandstone used to make glass must be free of iron or the finished product will be tinted with green. Green glass also may be created by adding a trace of chromium salts to the recipe. A trace of gold, selenium or copper oxide may be used to produce red glass. Copper oxide also produces blue or green glass, depending upon how the mixture is heated in the furnace.