An abrasive is a substance used for grinding, cutting, scroping or polishing the materials. There are two types of abrasives: natural and artificial abrasives. Natural abrasives include quartz, sandstone, pumice, diamond and corundum; artificial abrasives include rouge, whiting and carborundum.
Abrasives are available mainly in two forms: paper and grinding wheels. The abrasive paper is made by coating ordinary paper with glue and adding the abrasive material to it. The sandpaper, emery paper, and carborundum paper are made in this way. To make a grinding wheel, abrasive material such as quartz is mixed with clay and water. This mixture is then pressed into the desired size and shape and fired in a furnace. The heat inside the furnance makes a strong bond among the materials put inside the furnance.
The fineness or coarseness of the particles used in an abrasive material is described in terms of its ‘grit number’. The abrasive materials with a grit number of 60 are much finer than those with a grit number of 30.
The hardness of an abrasive is also an important factor. It is measured on the Mohs’ scale. The Mohs’ scale ranges from 1 to 10. An abrasive is chosen according to the material to be ground. It should be harder than the material that is to be polished.
The most widely used abrasives are fused aluminium oxide and silicon carbide. The aluminium oxide is known as alumina. It is used to grind and polish metals like steel, wrought iron and hard bronze. The silicon carbide is known as carborundum. It is made by fusing sand and coke in an electric furnace. Carborundum is used to grind and polish brass, copper, aluminium, stone, glass and ceramics.
Many varieties of quartz are also important abrasives. Pumice, a volcanic rock, when ground to a fine powder, can be used in scouring powder and soaps. Crystalline iron oxide is used to polish jewellery and glass. It is known as rouge because of its red colour.
The synthetic diamonds, diamond powders and diamond pastes are also used as abrasives. They are used to make drill bits and cutting wheels. Tungsten carbide is used in the machine tool industry for drilling, cutting and polishing metals. Boron carbide is another important abrasive. It is valuable because it is almost as hard as diamond. It is also used in nuclear reactor as a moderator and also as an abrasive.