90 GEOLOGY OF OHIO. 



clay to a state of vitrification in burning, any clay of suitable plasticity 

 and color will fill the requirements. Even the purer grades of Drift 

 clays are occasionally utilized. The earthenware manufactures of Zanes- 

 ville have, however, been developing a specialty in cuspidors, which 

 they find it necessary to glaze inside. On account of this specialty 

 the clays suitable for use are narrowed down very greatly and it is nec- 

 essary to select a clay which has refractory qualities sufficient to make it 

 stand up well, and not vitrify to any extent at the heat required to melt 

 a glaze composed of about two-thirds Albany slip clay and one-third 

 oxide of lead. The Albany slip, the great stoneware glaze, will be 

 further described under that head; it requires more heat to melt the slip 

 alone than the clay body of earthernware will stand without vitrification 

 and hence the addition of lead oxide is necessary to cause the lower 

 melting point of the glaze. 



The choice of clays is also controlled by the color of ware which it is 

 desired to make. The staple articles manufactured are flower pots, jar- 

 diniers, cuspidors, vases and umbrella stands. . 



One manufacturer uses a body composed of two parts of an exces- 

 sively tough plastic clay, probably of glacial origin, not unlike the ball 

 clays used by whiteware potters, one part of a white hard refractory fire 

 clay and one part of red loam designed to add color. This body has 

 not much excess of refractory power, though it is successfully glazed. It 

 has a fine red color characteristic of typical earthenware. Another man- 

 ufacturer uses a fire clay obtained from the horizon of the Putnam Hill 

 Limestone. This clay, however, has too little heat resisting power 

 to be a profitable one and its color is not a clear yellow, but shades off 

 into brown by any excess of heat. 



The best clay used for this purpose is a very sandy stoneware clay, 

 which has so much silica and so little clay in its composition that it has 

 no tendency to vitrify at any heat necessary to use. 



The processes used in these factories are practically the same as in 

 the manufacture of stoneware and will be sufficiently described under 

 that head. The clay is prepared by washing, in two establishments, and 

 grinding, in the third. For the production of a mixed body like the hrst 

 one described, the washing process is necessary as a means of get- 

 ting uniformity of color. But where porosity and openness of structure 

 is desired in the ware, it would seem that washing would not be the best 

 method of preparation of the clay. The superior ease of using the washed 

 clay on the jolly wheels on which the production of these cheap articles 

 so largely depends is doubtless the reason why this method of prepara- 

 tion has been selected. 



The kilns used for the burning of earthernwares are not character- 

 istic of that business or specialty adapted to it. There are several kinds 

 in use. The Akron square, end-fired kiln, the up draft muffle, and the 

 old-fashioned updraft stoneware kiln are all used successfully. In 



