476 ries. FOUNDRY SANDS [Ch. 26 



The most common clay mineral in foundry sand is illite, but kaolin- 

 ite and montmorillonite have also been reported. Some sands are 

 practically free from clay, and the foundryman often refers to these 

 as sharp sands. Those sands which contain clay mixed with the sand 

 grains are referred to as naturally bonded sands, and those free from 

 clay, to which the latter is added before use, are called synthetic 

 sands. Bentonite and fire clay are the materials most commonly added 

 to form the bond. Practically all steel foundries now use synthetic 

 sands, and their use is extending to the casting of iron and other metals. 



PROPERTIES OF SAND GRAINS 



Shape of sand grains. Foundry sand grains vary in shape, size, and 

 assemblage. In shape they may be angular, subangular, or round. 

 They may also consist of aggregates of smaller particles cemented 

 together by silica, iron, or calcium carbonate. For illustration of the 

 separate sizes of a number of different sands see Ries and Conant 

 (1931, p. 353). The degree of angularity varies in different sands, 

 but most grains are either angular or subangular. Round grains are 

 rare and occur usually only in sizes larger than 40 mesh (0.4 milli- 

 meter) . 



Most sand grains contain minute fractures, which may cause the 

 grains to crack on heating, thus developing fines when the sand is ex- 

 posed in the mold to the heat of the molten metal. The foundryman 

 often refers to these cracks as cleavage. 



The surface of sand grains may be either smooth or rough, and the 

 smooth surfaces are sometimes frosted. In addition, the surface may 

 be clean, or it may be coated with a film of foreign matter such as 

 clay or iron oxide. It is probable that rough or coated surfaces offer 

 a better attachment for the bond. 



Fineness of sands. The size distribution of the grains affects the 

 type of casting for which the sands are used. In making a fineness 

 test to determine the percentage of different grain sizes, a sample of 

 50 grams of dried sand is usually taken. Clay if present is first sepa- 

 rated by stirring the sand in a special jar in water containing sodium 

 hydroxide. 



The suspension is then allowed to stand, the grains larger than 20 

 microns settling in 5 minutes. The material still in suspension is re- 

 moved by decantation, the settled material stirred again, and the 

 process repeated until the supernatant liquid is clear. The grains up 

 to 20 microns are known as AFS clay, which includes both true clay and 

 fine silt. 



