278 



APPLIED SCIENCE 



Burnishing is the process of producing a smooth surface 

 by pressing down the inequalities or rough spots. It is, 

 therefore, best adapted to soft materials. 



317. Development of Grinding Stones. 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. Ex- 

 perience 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 

 FIG. 139.-Grindstone. ^^ and thftt 



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 parti- 

 cularly sharp. Corundum grain is sharp and transparent, 

 with distinct evidences of crystallization. Carbide of silicon 



