652 NEW YOEK STATE MUSEUM 



The matter of glazing and enameling is tke most difficult part of 

 tlie whole process of manufacture, and as such is kept secret, by the 

 maker t^s far as the details are concerned. In general it may be 

 said, however, that the enamel is simply a mixture of clays similar 

 to that used in making good porcelain, which is applied tO' the sur-. 

 face of the brick in the condition of a thick liquid or slip. This 

 enamel, when dry, is coated with a fusible glaze, such as is used 

 for ordinary porcelain. 



Enamel brick are usually burned in. saggers, placed in a down- 

 draft kiln, or sometimes even in a muffle kiln. The double fire 

 method mentioned above greatly increases the cost of manufacture. 



Glazed brick. By this we understand a brick which is covered 

 by a transparent glaze, and not an opaque enamel. 



The best results are probably obtained by glazing vitrified bricks, 

 as porous ones are seldom able to^ resist the weather, specially in 

 severe climates. 



If the glaze is applied to green brick, which is by far the cheapest 

 method and the two burned together, the glaze will often show 

 crazing under certain conditions, and it is found that there should 

 be a certain amount of agreement between the composition of the 

 clays and that of the brick, so that they will not only have a simi- 

 lar expansion and contraction when burned, but will show the 

 proper relation between their fusing points, and the glaze will not 

 fuse much, or not at all, below the temperature at which the brick 

 vitrifies. 



In the first place it is of course necessary that the brick shall 

 not contain an excess of lime, so that it can be vitrified without 

 fusing; and having found a clay of the right kind, it then remains 

 to find a glaze which it will carry without causing it to craze. 



One way to arrive at this experimentally is to take a clay whose 

 rational composition is known, and by the addition to it of different 

 quantities of, say, quartz sand, feldspar and lime, make mixtures 

 which will show considerable range in a rational composition. Add- 



