Celia Junghans, age 13, of Barrington, Ill., for her question:
How can they make glass from sand?
The main ingredient is a special sand with a high content of a min¬eral called silica. The earth uses a recipe that changes silica into shiny black obsidian, which is a natural glass. The secret is enough heat to melt and fuse silica to form a tacky substance. When this cools, it is smooth, hard glass. The manufacturers of glass use nature's basic recipe, plus many improvements and refinements.
Different types of sand contain various minerals, but almost all of them contain silica. Only sands that contain very high percentages of silica are suitable for making glass. Silica is the most plentiful and in many ways the most wonderful of the earth's rocky ingredients. It forms opals, agates and a long list of other semi precious stones, plus a multitude of ordinary rocks, including quartz, which is the hardest common mineral.
This wonder mineral is a simple chemical compound of the plentiful elements oxygen and silicon. And, as so often happens, what nature can do, mankind can do better. Nature uses the heat from fiery volcanoes to change silica into obsidian, the natural glass mineral found among the lavas of our western mountains. Thousands of years ago, people discover¬ed how to change silica sands into glass and even how to tint it to re¬semble precious gems.
Nowadays a manufacturing plant may have 50,000 recipes for making different kinds of glass. In all of them the main ingredient is silica, usually in the form of hard grains found in sand or sandstones. All of them require very high temperatures to melt and fuse the gritty grains to form the tacky substance that cools to become glass.
Other ingredients in the basic recipe are alkali and lime. The al¬kali causes the silica to melt with less heat, but the finished glass tends to dissolve in water. The lime is added to make the glass durable and waterproof.
The basic recipe for ordinary window glass requires 72 pounds of silica to about 15 pounds of alkali and about nine pounds of lime. The silica comes in sand or crushed sandstone, the alkali usually is in potash or soda ash, the lime is added with crushed limestone. This basic recipe must be heated to 3000 degrees Fahrenheit, which is as hot as a blast furnace used to spelt iron.
This seething heat separates the molecules in the sturdy crystal structure of the silica. As the mixture melts, the particles flow around and fuse together in a different structure. The hot, molten material is viscous, or thick and sticky. In this state, it is taken from the fur¬nace and rolled, molded or blown into its final shape. The hot viscous material sets hard quite quickly, so the shaping must be done in a hurry.
There is no sign of sandy grains in the finished glass because their silica molecules have been fused and remodeled. Alumina and boric oxide are added to heat resistant recipes, but they are not visible in glass baking dishes. Traces of iron or chromium in the mixture are remodeled to add a greenish tint to glass bottles. Traces of gold, selenium or cop¬per oxide also seem to disappear, though actually they add ruby red tints to the finished glass.