108 GEOLOGY OF OHIO. 



The foregoing remarks on the glazing of stoneware are all directed 

 to the slip glazing. At present the importance of slip glazing is on the 

 increase; some works produce little else but slip glazed ware. But this 

 state of affairs is of recent occurrence. The standard outside glaze of 

 stoneware in the past has been the salt glaze, so called because it is made 

 by throwing salt into the fires and allowing the vapors to ascend through 

 the kiln and attack the surface of the hot ware. The salt glaze is a sili- 

 cate of soda formed by the decomposition of the chloride of sodium and the 

 liberation of hpdrochloric acid gas or volatile chlorides. It has the merit 

 of being cheap and easy to apply, and it answers well enough for the out- 

 side surface of the ware, but it is usually much crazed and easily perme- 

 able to water. The glaze on the outside of the ware answers no very 

 useful effect except making it easy to dean and wash. 



The different facilities with which different clays will take a salt 

 glaze depends, probably, on the amount of free silica they contain and its 

 fineness of division. The difference is very marked. Some which will 

 receive the salt retain a granular appearance which is not sharp to the 

 feel, though it is rough; it is as if the glaze had gathered together in 

 minute pools like beads of prespiration, instead or spreading evenly. Occa- 

 sionally the glaze is perfectly smooth like an artificial glaze. 



There are chemicals which operate to interfere with the formation of 

 the salt glaze. Chief among these are the sulphate and carbonate of lime. 

 The nature of the action of these bodies has been much disputed. The view 

 which seems to be supported most closely by the facts, is that the inter- 

 ference is a mechanical one and not chemical as has been generally 

 assumed. The sulphate of lime aud the carbonate are soluble in water in 

 the order named and if these bodies exist in clay they are apt to be dis- 

 solved, in part or in whole, and as the water evaporates from the surface 

 of the ware in drying, these salts are left in a whitish deposit on this sur- 

 face. 



Now when the kiln is very hot and the salt vapors are produced in 

 the fire holes and ascend through the wares, this film- of lime protects the 

 real surface of the clay, so that there is but little free silica or even sili- 

 cate of alumina exposed to the action of the salt fumes. The tempera- 

 ture at which the limesalts are decomposed to form a lime silicate is a 

 little higher than the heat at which the salt is generally used, hence the 

 salt fails to do its work simply because it fails to meet the surface of the 

 clay. Now if the kiln be made hotter and the salting be kept up, a glaze 

 is finally produced, but it is greenish yellow in color, indicating clearly 

 that at the increased temperature the lime salts finally united with the 

 soda to produce a lime-soda-silicate glaze, which, at the lower heat, was 

 not formed at all. But this glaze is of a dirty color, and requires too 

 much heat for the safety of the ware. Hence its formation offers no 

 relief to the interference caused by soluble lime salts in the clay. 



The color of a piece of salt glazed ware depends on the amount of 

 iron in the clay and the kind of fire which was maintained in the burn- « 



