Kevin Edwards, age 14, of Chattanooga, Tenn., for his question:
JUST WHAT IS AN ABRASIVE?
An abrasive is a material that is used to polish, grind or cut other materials. Abrasives are one of man's most widely used and simplest tools. Some abrasives are man made while others are made from materials that we can find in the earth. The most plentiful are the silica minerals which are a combination of silicon and oxygen.
Examples of the silica abrasives are quartz, sand, sandstone, tripoli and diatomite. Combinations of metallic elements with silicon and oxygen form silicate minerals such as pumice and garnet.
An abrasive mineral made of aluminum and oxygen is called corundum. And a mixture of corundum and magnetite or hematite is called emery.
The best of all abrasives, the industrial diamond, is a form of carbon.
Manufactured abrasives include synthetic diamonds, corundum, artificial corundum, steel shot, steel grit and glass.
Carborundum was accidentally discovered in 1891 by a man named Edward Acheson. He was hoping to find a way to make artificial diamonds. He mixed coke, sand, salt and sawdust together and heated the combination in a furnace. An electric current was then passed through the mixture and crystallized silicon carbide was formed.
Within 10 years after Acheson's discovery, a process was found for changing bauxite into artificial corundum.
The diamond is the hardest abrasive, but it is very expensive. Abrasive wheels that can cut very hard materials are made with diamond dust. Steel bits studded with industrial diamonds are used for drilling in bed rock.
The hardness, resistance to crumbling and the size of the particles are what determine how good an abrasive is. A German mineralogist named Friedrich Mohs set up a scale of hardness for minerals in 1812. He selected 10 minerals to act as the standards and numbered them 1 for the softest, to 10 as the hardest.
Abrasives may be used as loose particles while polishing, grinding or scouring, or they may be bonded with glue to paper or cloth. Examples of this are sandpaper and emery cloth.
Abrasives can be bonded together and used for making grinding wheels or oil stones.
Large blocks of abrasive may actually be cut into grindstones, millstones, whetstones and pumice blocks.
The size of the abrasive particles largely determines how an abrasive is used. Coarse particles will cut or grind while fine particles scour or polish.
Artificial corundum and carborundum are used for cutting machine parts and grinding lenses for microscopes and other instruments. Garnet is used in garnet paper or cloth and in grinding stones. Quartz is used in sandpaper, polishing soaps and sandblasting.