Tools were originally shaped by chipping one stone against another until the stone which was to be the tool was made the desired shape. When man learned the use of metal, he continued to sharpen his tools on certain grinding stones or rocks. Experience taught him that the most effective way to grind his tools, was to make the stone circular, with a flat edge, and mounted on a shaft, and that to reduce the heat of friction the stone should be rotated through a water bath.
Originally grindstones were made of sandstone, composed of hard, sharp particles of sand or quartz. Since then better and harder forms of stones have been discovered and placed on the market. These modern grindstones (Fig. 139) are made of emery, alundum, corundum, and carbide of silicon. Emery has a rounded, opaque grain, while alundum grain is particularly sharp. Corundum grain is sharp and transparent, with distinct evidences of crystallization. Carbide of silicon grain presents a distinct crystalline structure with a sharp cutting edge. From these materials are made a wide variety of grinding wheels and sharpening stones.
Fig. 139. - Grindstone.