Peter Walsh, Jr., age 72, of Westbrook, Me., for his question:
HOW IS GLASS MADE?
Man first made glass about 3000 B.C. in the form of glaze for his ceramic pots. Then about 1500 B.C. the Egyptians and Mesopotamians took it one step further and came up with glass vessels. By 30 B.C. a first blowpipe for glass blowing was invented and then in 50 A.D. man came up with the first piece of manufactured window glass.
Glass is certainly one of man's most useful materials. More than 700,000 different kinds of glass items are produced, and you'll find the products touch every part of your daily life.
To make glass, glassmakers combine a batch of materials that includes 72 percent sand, which is called silica; 75 percent sodium oxide, or soda, and 9 percent calcium oxide, which is lime. Four percent of the batch is made up of alumina, white oxide or arsenic, saltpeter and lead oxide.
About 90 percent of all glass being made today is based on this soda lime formula. It is from this mixture that glassmakers produce plate and window glass, electric light bulbs and almost all glass containers.
Glass made with a soda lead formula substitutes lead oxide for calcium oxide and often uses a smaller percentage of silica. This type of glass has important optical properties and is used for making the finest tableware and art objects.
A heat shock resistant glass is made by Corning Glass Works under the name of Pyrex. Here 80 percent silica is mixed with 4 percent alkali, 2 percent alumina and about 13 percent boric oxide.
There are four ways to shape glass: blowing, pressing, drawing and casting. In glass blowing, the job can be done by either having a workman actually blow into a pipe or by using complicated machinery. In pressing, a hot gob of glass is put into a mold and then pressed with a plunger until it spreads to fill the desired area. Drawing produces sheet and plate glass, tubing and fiberglass and is made by flowing molten glass through special water¬cooled rollers.
Special glass blowing machines, called ribbon machines, can turn out 2,000 light bulbs in one minute from a ribbon of hot, flexible glass.
Glass was made by hand before 1890, but then at the turn of the century large manufacturing plants were built. At first they were all located near forests since wood was used as fuel. Coal then was used for a time with natural gas finally winning out as the best source of heat.
Here's an interesting footnote on glass: glassmakers are able to draw a fiber 90 miles long from one glass marble that is less than one inch in diameter.