104 GEOLOGY OF OHIO. 



The slip clays used by Ohio stoneware potters are mainly as follows: 



1. The Albany Slip from Albany, N. Y. 



2. The Rowley Slip from Northern Michigan. 



3. The Brimfield Slip from Summit county, Ohio. 



4. The Kaolite Slip. 



These are named in the order of their importance. Albany slip is 

 the finest, single, natural clay glaze known in the country and, while its 

 qualities can be enhanced by mixture with other clays, it alone will make, 

 on clays which are refractory enough to stand the required heat, a won- 

 derfully fine and beautiful glaze. 



The peculiarity of this kind of a glaze is that it never cracks or 

 "crazes"; it is itself a clay; it is burned into another clay; the chemical 

 changes of burning affect glaze and body alike, and when the operation 

 is finished there are no artificial conditions of expansion and contraction 

 always at war with each other in the body of the ware. 



The fault of all these natural glazes is, that they require a good heat 

 to bring them to thorough fluidity and thus many clays which would 

 make a fair grade of ware are rejected as being too easily fusible for the 

 glazes. Potters have frequently tried to remedy this fault by use of the 

 common fluxes, lead oxide, or litharge, borax, spar, etc. It is a possible 

 thing to make a glaze of which slip clay is the base and which is fusible 

 at a low heat, but many of those who have tried to do it so far, have too 

 little technical knowledge to do so successfully. 



I The addition of lead or borax alone will reduce the temperature of 

 fusion but it destroys the beauty of the glaze; instead of being a clear 

 velvety black or reddish black color, it appears thin and washy in color 

 and irregularly spotted with specks of coloring matter. 



An attempt has been made to counteract this clearness by the use of 

 a little manganese in imitation of the Rockingham glazes, but while a 

 softened slip glaze thus treated is improved, it still lacks the original 

 quality. The cause of the failure of these attempts, is, in many cases, 

 the addition of a quantity of some base like lead, without any acid to 

 satisfy its affinities. The glaze has the desired qualities naturally except 

 as to fusibility. Then a satisfied compound which is fusible and color- 

 less or of the same color as the slip should be added, so that the mixture 

 will fuse easier without undergoing a radical rearrangement of its nature. 



The chemical constitution of these natural glazes has been inves- 

 tigated by the Survey to some extent and the results will be found incor- 

 porated in the following table, Table No. II. 



