CLAY WORKING INDUSTRIES. 107 



their oxygen ratio is about as follows: 



2.5 acid to 1.0 base and 1 alumina to 1 flux. 



Calling to mind the average analysis of the stoneware clays, which 

 constitute the body which these glazes are used to adorn, we see the wide 

 difference best expressed in the oxygen ratio : 



3 acid to 1 base and 10 alumina to 1 flux. 



The colors which these slip clays burn, varies entirely with the heat 

 to which they are burned. The Albany slip and the Brimfield slip are 

 both dark colored, normally. The Albany is in itself a perfect glaze, how- 

 ever, and becomes glossy and bright alone, while the Brimfield has a 

 muddy black color, without luster, when used alone. The Rowley slip 

 and the Springfield slip both burn to a yellowish green glass, due to the 

 high amounts of lime and magnesia in both. They are neither suitable 

 to use alone. The finest raw clay glaze in use, is a mixture of the Albany, 

 Rowley and Brimfield clays. The Rowley adds fluidity, the Brimfield 

 adds dark color to counteract the light color of the Rowley, and the 

 Albany gives its own fine tone to the whole. Ware glazed with this 

 imitation can be detected by an expert as soon as he sees it — it has a 

 lustre, a clearness and a brilliancy all its own. 



In addition to the natural glazes, three or four of the most enterpris- 

 ing of the stoneware potters are bringing a line of ware into the market, 

 which is glazed with a white translucent glaze of artificial preparation. 

 It is a simple white glaze, such as is familiar to every whiteware potter, 

 but it has to be modified to suit the stoneware body. 



In using the natural glazes, the stoneware men meet no problems of 

 any difficulty, but in adopting an artificial glaze to a natural clay, and 

 burning both together, there is much skill required; if the glaze fluxes 

 too ear ly, it will be bubbled by the escape of the gases of the clay through 

 it; if it be too hard to run, the ware will suffer. 



The results when this glaze is successfully used are beautiful, the stone 

 gray color of the ware is softened and reduced to an even tint. 



The same potters are turning out a fancy red glazed ware, which is 

 probably a compound of the three slip clays named before, mixed with 

 the other constituents of a clear artificial glaze and softened a little. The 

 colors attained are similar to the mixture of the natural slips, but clearer 

 and brighter and more uniform, a result which has been accomplished 

 by the use of machinery and expedients similar to those used in the 

 manufacture of yellow and white ware glazes. 



It is a note-worthy fact that nearly all these improvements and spec- 

 ifies are worked up and introduced by potters who have been or are at 

 present manufacturing some higher class pottery, usually yellow and 

 Rockingham wares. Their knowledge of the theory of artificial glazes 

 enables them to devise and work out these improvements, which would 

 never come except in some such way. 



