286 CLAYS AND CLAY INDUSTRY. 



pointed out by Langenbeck, 1 soluble lime salts may come from 

 the coloring materials used. Thus the manganese and umber 

 used for chocolates, browns or black is seldom free from gypsum. 



Floor tile when white are commonly made of a mixture of 

 white-burning clays, flint and feldspar. Buff-colored tiles and 

 artificially colored ones are usually made from fire clays, while red 

 tiles are often made from a red-burning clay or shale. A certain 

 amount of flint and feldspar is usually added to the clay to> regu- 

 late the shrinkage or degree of vitrification in burning. 



Method of manufacture. — Floor tiles are always molded by 

 the dry-press process in hand-power machines, the raw materials 

 being first carefully ground and mixed. In making encaustic tile, 

 the design is produced by using a framework of brass strips, ar- 

 ranged so ! as to- form the outline of the colors making the pattern. 

 This framework is placed in the mold and the colored clays 

 sifted into their proper divisions to the depth of about one- fourth 

 inch. The brass framework is then removed and the rest of the 

 mold filled up with a buff-burning clay which forms the "backing." 

 It is necessary that the backing should be as dense as the clays 

 forming the upper surface or face of the tile, otherwise the latter 

 may split in freezing weather due to the expansion of the water 

 absorbed. 



In burning the tile they are placed in saggers and burned in 

 down-draft kilns. Those made in New Jersey are burned at from 

 cone 9 to 12, depending on the character of the body and degree 

 of vitrification to be obtained. The burning is regulated in some 

 factories by cones and in others by trial pieces. 



Character of product. — Owing to- the conditions under which 

 they are used, floor tile should possess sufficient hardness to resist 

 abrasive action, sufficient transverse strength to resist knocks, and 

 sufficient density to prevent excessive absorption of water. Many 

 floor tiles, especially the white ones, show little or no absorption, 

 but most of the other colors soak up from 1 per cent, to; 5 per 

 cent, of moisture or perhaps even more. 



The absorption tests were made of a number of single-colored 



1 Chemistry of Pottery, p. 155. 



