110 GEOLOGY OF OHIO. 



allow the heaviest sections of cl y to have the longest time to season 

 under the heat before the vitrification begins. 



Setting stoneware, like burning it, is an art which can only be 

 learned by actual practice, and which is likely to be costly practice, un- 

 less it is learned by slow degrees under the tutelage of one who knows 

 how it should be done. 



As in all other clay working processes the burning is the most im- 

 portant stage. Imperfections in the steps already described are imper- 

 fections at the end of the process and as such, affect the value and 

 quality of the ware. They grow no worse nor better; but a little 

 negligence or mismanagement in burning may ruin a large quantity of 

 ware, good and bad alike, and make it not only imperfect but valueless. 



The care, the skill and the knowledge of all clay working enter- 

 prises are bound, in the nature of the case, to center around the burning. 



In regard to the theory of the burning it is not necessary to make 

 any detailed statements; it is no way different from the general theories 

 already laid down, except that due consideration must be given to the 

 extremely thin section of the cla}^ in the most of the ware. A great 

 deal of it does not exceed one-fourth inch in thickness and the heaviest 

 sections in the bottoms and corners of any large crocks is hardly more 

 than seven-eighths of one inch in thickness. 



In sach a thin section it is obvious that the chemical changes of 

 burning are likely t ) progress very rapidly. The duration of a burn 

 varies with the size of the kiln, etc., but ranges from twenty-four to 

 ninety hours. Fifty-five hours is the average figure for the Akron dis- 

 trict, thirty for the Zanesville district. 



As the process is usually carried out, about half this time is spent in 

 warming the kiln up and getting the ware up to a black heat. This is 

 the critical point. When the ware is s . en to be dull red throughout then 

 the finishing can be rapidty brought forward. The temperature used on 

 stoneware kilns was the subject of some attention. The following meas- 

 urements were made by the Lunette pyrometer. 



