808 NEW YOKK STATE MUSEUM 



total shrinkage of 14^. Incipient vitrification occurred at 1700° 

 F., complete at 1850° F., and viscosity above 2000° F. . 



From the analysis made by H. H. Griffon it is ,<>en that the 

 clay approaches closely to the formula : 



IRO, .7KA, 4:Si02, 

 which is similar to that of an alkaline glaze, but with an excess of 

 Rg Os* The addition of lead increases this excess of bases, and it 

 is necessary to add silica also. 



For many years the slip has been used as a glaze without the 

 addition of any artificial fluxes, for attempts in this direction had 

 always been without success. A number of experiments were 

 made by Mr Griffen, to determine in what manner it was possible 

 to lower the fusibility of the slip clay, and make it run more easily 

 without destroying its richness of color. The addition of lead 

 alone gave a transparent and greenish colored glaze, which showed 

 a tendency to blister; alkalis added alone gave the same result. 

 It is, therefore, necessary to add other materials with the lead. 

 Good results were obtained by adding iron alone, but the combina- 

 tion of chromium, manganese and iron produced the best effect. 

 The chromium, Mr Griffen finds, takes from the iron its tendency 

 to run into greenish and yellowish tints. The best form in which 

 to introduce the chromium is as chromate of lead, this giving the 

 finest color effect ; but, as an excess of this sort also has a tendency 

 to cause blistering, it is well to add some of the chromium in the 

 form of chromate of iron. 



The following recipe is for a moderately low heat glaze, the 

 variation being for different conditions. 



Albany slip clay 63 . 30 to 70 



White lead 25.30 to 17 



Flint 6.30 to 7 



Oxid of iron 72 to .79 



Oxid of manganese 56 to .61 



Chromate of lead 1 . 27 to 1 . 40 



Chromate of iron 67 to .73 



Oxid of zinc 1.88 to 2.07 



