684 GEOLOGY OF OHIO. 
silica. On heating, the mixture becomes a clear glass, which is called 
“fret.” This fret is ground up and mixed with fresh proportions of 
Paris-white, carbonate of lead, flint and spar, and is put into a grinding 
pan or vat lined with French buhrstone. In this it is ground up in a 
thin slip of pure, white clay, as something of this sort is necessary to 
incorporate the heavy parts of the mixture and keep them from setling 
out. This way seems a cumbrous programme to go through to get a 
liquid which shall vitrify the outside of a clay vessel, but the use of 
more direct agents, as the alkalies, is prohibited by the method of using 
it, which brings in the services of men whose hands and arms are in the 
glaze all day, which would be impossible if the glaze were alkaline. 
The most common difficulty met in glazing is the tendency of the 
polished surface of the ware to crack or “craze.”’ This is due to a lack of 
adjustment between the coefficients of contraction of the body and glaze ; 
and in the proper adjustment of this point lies the hardest problem of 
the potter. 
Other troubles are also known; if a sagger leaks or admits air while 
hot, it causes the ruin of everything in it, for the sulphurous gases of the 
kiln immediately attack the hot lead silicate of the glaze, causing a 
sulphide film to form on it, which is black and unsightly. Even a piece 
of Rockingham ware shows immediately if air reached it while hot. 
The grading of white-ware is made as follows: Ist, Ironstone 
china; 2nd, Majolica; 3rd, C. C. an abbreviation for cream-colored — 
or for common clay, as variously explained, but in any case it 
is a cheaper grade of white-ware made from inferior clays. The dis- 
tinction between china and majolica is much the same as that between 
yellow and Rockingham, a distinction of finish and glazing only; the 
body is the same. There is no reason why a C. C. ware should not be 
treated with a majolica glaze also, but so far it is not done. 
The glazes used in majolica are applied, after the first or ‘‘ body” 
glaze, in a soft, pasty state, and in dabs, which would presage a very 
rough appearance when finished ; but on heating they melt and flow over 
the ware, making an effective play of color. The colors used are in the 
glaze, and differ from all other styles in being neither beneath or above it. 
The decoration of pottery is a very large and complex subject. 
The theory of the art is not at all difficult to understand, but the longest 
experience is necessary to attain much real skill in its application. 
The coloring agents employed in glass, pottery, enamels, etc., are 
