r92 NEW YORK STATE MUSEUM 



parting " sand " is usually dry brick dust, dry powdered cinders 

 or some similar dry and fine material. 



In some of the commoner classes of work, where a smooth sur- 

 faced casting is not required, the above operations are all that 

 are necessary to prepare the mold. In finer work, however, after 

 the mold has been made, its surface is dusted with oak charcoal 

 dust or with finely powdered coal. When the molten metal is 

 poured in, these materials will ignite, causing a thin layer of gas 

 to form between the metal and the sand. This results in giving 

 a smooth surface to the casting, as it prevents the metal from 

 entering the minute interstices between the sand grains. Of the 

 two materials above noted as being used for this purpose, char- 

 coal is commonly employed for light castings, being dusted over 

 the mold as described. In heavier work, coal is used, and is 

 often not only dusted over the mold but also mixed with the 

 sand before the mold is prepared. 



In English foundry practice, and in practice on the continent 

 of Europe, artificial molding sands are often employed, and such 

 natural sands as are available often require mechanical treat- 

 ment before being fit for use. The New York deposits, however, 

 yield a high grade of natural molding sand, and such different 

 degrees of fineness can be obtained from these sand beds that 

 little or no artificially prepared sand is used in this country. 



Requisite properties of molding sand 



The principal properties which should be possessed by a sand 

 to be of service as a molding sand, are refractoriness, porosity, 

 tenacity and fineness. As will be seen later, several of these 

 properties are, to a certain extent, incompatible. For example, 

 a highly refractory sand can not well be a sand possessing a 

 very good bond. In such cases the particular use to which the 

 sand is to be put will determine which of the properties is the 

 more important. 



Refractoriness. The refractoriness of a sand depends largely 

 on the amount of silica it contains, being greatest in the sands 

 highest in silica. The presence of any considerable amount of 

 lime or alkalis, on the other hand, decreases the refractoriness 



