CLAYS OF KEW YORK 653 



ing quartz and feldspar to clay is rather expensive, and these two 

 could probably be replaced by the use of a feldspathic quartz sand. 



Experiments of this nature carried out by Btecht (Thonindus- 

 trie zeitung. 1894. p. 309) indicated that it is undesirable in most 

 cases to decrease the clay substance below 30^, and that if the clay 

 contain over 50^ most of the glazes adhere with difficulty. 



He found that colored glazes with a formula 



held on mixtures whose rational co^mposition was: 



Clay sub- 

 stance . ... 50 50 50 50 40 30 30 30 30 30 30 



Quartz .... 40 30 20 10 50 60 50 40 30 20 10 



Feldspar ... 5 15 25 35 5 5152535 45 55 



Lime car- 

 bonate ... 55555555555 



Hecht in his experiments used iron, and pink glazes, because 

 they represented extremes of composition. He found that the glazes 

 held best when the clay substance was 30^. As the clay substance 

 increased, the adhesion of tee clay decreased, and it did the same 

 with an increase of the feldspar, while it adhered better as the 

 quartz increased in amount. 



The tendency of the glaze to^ craz;e also becomes greater with 

 the size of the quartz grain, the reason being that the greater the 

 grain the more difficult it is for a thorough chemical action tO' take 

 place between the particles of the clay and those of the brick. 



Methods of manufacturing brick 



Bricks are usually made by one of the following four processes. 



Soft mud 



Stiff mud or wire-cut 

 Dry press ' 



Semi-dry press 



