CLAYS OF NEW YOEK 779 



for this purpose, only hard fire or -imderglazed colors .can be used 

 (D mmler Ziegel Fabrilcation. p. 95). In producing definite and 

 distinct colors the composition of the glaze exerts as much effect as 

 the character of the coloring material. It is therefore highly 

 essential that the colors used shall always be of the same compo- 

 sition, be ground to the same degree of fineness, and burned at the 

 same temperature. If new glazes or colors are used, they should- 

 first be tried experimentally. In the process of burning, the glaze 

 may or may not exert action on the underlying coloring material. 

 In some cases the color will dissolve in the glaze, in others it will 

 only become suspended in it, but in the latter cases the shades are 

 not often soft, and the change from one color to another will be 

 sudden. If on the other hand a color is easily soluble in the glaze, 

 there is the danger that it may run, causing the design to be 

 blurred. In order to prevent this, the coloring materials are gen- 

 erally combined with others, so that compounds of the spinel type 

 are obtained. "~ 



To prevent hard colors several devices are employed, such as 

 substituting for a portion of the coloring material a slight amount 

 of arsenic, or placing a little arsenic in the kiln. This method is 

 most advantageously employed when cobalt colors are used. 



In addition to underglazed painting, slip or engobe decoration is- 

 used. This consists in applying white or colored clay paste to the- 

 white or colored tile. 



The body can also be colored by dipping it into hydrochloric or 

 acetic acid solution of the coloring metallic oxids. In this case the 

 product is to be once more subjected to a slight ignition. 



In slip painting the slip is made of a mixture of thei powdered 

 tile glaze and metallic oxid or underglaze colors. These decorated 

 pieces are dried and burned at the temperature of the melting of the 

 glaze. The whole is then covered with a thin coating of glaze and 

 burned once more. If the slip contains 50^ or more of glazing ma- 

 terial, the second glazing operation is not necessary. 

 . Overglaze decoration requires strongly coloring oxids which are 

 mixed with some easily fused material, and rubbed together -with 



