84 GEOLOGY OF OHIO. 



The oxides of iron have proven themselves to be very sensitive to 

 these influences. It is a fact that oxide of iron, if it has once been 

 affected by a reducing influence, is far more susceptible to an oxydizing 

 influence than it was at first. A clay which has been burnt in a steadily 

 oxydizing atmosphere shows, for example, a steel gray color. Now burn 

 the same clay in a fire alternately reducing and oxydizing, if the sample 

 be withdrawn and suddenly cooled when the fire is reducing, or has been 

 so lately, the color will be still gray or blue. If the ware be left to cool 

 slowly in the hot air of the kiln, it will be strongly colored by the iron, 

 much more so than that which had never been reduced. Potters who are 

 engaged in producing stone ware, have to watch this point with the. 

 greatest care, and use all pains to prevent their fires from burning smoky, 

 for they know, practically, what has just been stated, that "smothered 

 ware is always dark colored." 



The influences which the burning of a clay have on its ultimate qual- 

 ities, are thus seen to be very great. While the composition of the clay 

 limits the qualities which can be obtained from it, while no amount of 

 skill, in burning, can make a bad clay into a good product, yet, unless the 

 burning be substantially right, the finest clay in the world is easily 

 injured or ruined, and unless these principles, which underlie the burning 

 of clay, be understood by those in charge, there is likely to be a constant 

 production of wares which are either inferior or a total loss. Now and 

 then a kiln will be burnt which happened to avoid the transgression of 

 any law, but, as a general thing, the loss in each kiln will be a noticeable 

 percentage. Part of this belongs to defects of the kilns employed, per- 

 haps, but the importance of manipulating the burning in accordance with 

 the natural laws is none the less sure. 



e. The testing of clays. As to the best means of testing a clay with 

 regard to its fitness for manufacturing any kind of ware, it cannot be 

 claimed that much that is new or valuable has been added to already 

 existing methods in the last decade. Those establishments which are 

 engaged in manufacturing clay products on a large scale and of a high 

 grade, usually have perfected a system of testing clays for their own pro- 

 cesses, which is, in effect, merely their regular process, carried out in a 

 small and rapid waj^. Small testing kilns fired by gas or oil, which can 

 be heated up and burned in a few hours, are sometimes employed. Some 

 few works, where the artistic effects sought are of the highest class, and 

 where the use of the highest skill of the potter's art is called in play, 

 employ skillful chemists, not so much in testing the nature of the crude 

 clays employed as in working out and keeping control of the delicate 

 problems of glazing and coloring. 



It is a fact, and one to be regretted, that there is no really valuable 

 way to find out what a clay can be made to do but to try it. It may be 

 tried in a crude and ignorant way, and rejected, or it may be tried in the 

 light of all the technical knowledge available on the subject and found 



