80 GEOLOGY CF OHIO. 



But in attempting to develop these qualities which its compositions 

 indicates, it is necessary to understand the chemical and physical changes, 

 which -take place in all clays when subjected to heat. 



Clay when heated, first loses the water which has been mechanically 

 added to it, to render it plastic and soft. This water having been mechan- 

 ically added to the clay, has no chemical ties to break in coming out again, 

 and therefore evaporates at all temperatures and at the temperature of 212° 

 F. is converted into steam. Therefore it is necessary to raise the heat so 

 slowly as to allow all of this moisture to escape as vapor before the clay 

 becomes heated above the boiling point of water, for the generation of 

 steam would of course tend to destroy the structure which has been im- 

 parted to the clay and which it is important to keep. After the moisture 

 has been thoroughly driven out, the temperature may be increased with 

 much more speed. For purposes of investigation, if the heat be raised at 

 a slow Fate and the clay weighed at frequent intervals, it will be found 

 that no change in weight occurs (after the moisture is expelled) while the 

 clay is successively raised past the melting point of tin, lead, zinc and an- 

 timony, and that it is considerably above this point, probably not far from 

 1,000° F. or a low red heat, hardly perceptible in daylight but clearly per- 

 ceptible in the dark, when the clay next begins to lose weight. When this 

 heat is attained, the combined water, which has been a chemical part of 

 the clay heretofore, is expelled and the clay loses weight to this extent. 

 A pure kaolin loses about 14%, and any other clay less, in proportion 

 to the amount of real clay it contains. If there is any organic matter in 

 the clay it will probably be driven out at about this stage in the burning. 

 It depends on the kind of organic matter; if it were sawdust mechanically 

 added to the clay to induce porosity of structure, it would require further 

 heat to remove the charcoal produced; if it were hydrocarbons and coaly 

 materials existing in the clay naturally, they would probably be driven 

 out at this heat. If there is any sulphide of iron present in the clay it 

 begins to decompose at this temperature, giving off free sulphur, which 

 oxydizes at once to sulphurous acid, and usually in the presence. of the 

 superheated steam and air to sulphuric acid. 



These reactions are beautifully illustrated in the successive changes 

 of vapor which discharge from -the stack of a kiln which is burning 

 wares made from a sulphurous clay. First of all comes a copious cloud of 

 steam while the moisture is being driven off; then a period during which 

 no gases are seen to emerge except black smoke after each fire is replen- 

 ished with coal, then the combined water beginns to appear in the form 

 of steam, and soon the steam is reinforced by a blue permanent smoke 

 which still floats away into the atmosphere after the steam disappears. 

 When occasionally this blue smoke is brought down to the surface of the 

 earth it is found to be intensely irritating to the lungs of man and beast, 

 and fatal to vegetation. The production of the first sulphuric acid is 

 accomplished while the water of hydration is still passing off, but the cloud 



