CLAY WORKING INDUSTRIES. 81 



of sulphuric acid still continues to come after no more steam can be 

 detected by the eye. Sometimes the gases from sulphurous clays are not 

 blue as have been described, but brown or reddish in color. This may 

 be due to the simultaneous expulsion of carbonaceous matters and sul- 

 phuric acid, and the reaction between these two. The production of all 

 these gases and their expulsion from the. mass of the clay is easily 

 effected if the temperature of the clay is kept nearly stationary while the 

 expulsion is in progress. The heat must not be allowed to fall below the 

 initial point, and it must not be allowed to go much above, for as fast as 

 the clay loses its combined water, it undergoes a great physical change, a 

 closing up of its pores, a general shrinking and settling together of its 

 molecules, so that it becomes more and more difficult for the gases of the 

 interior portions of the clay to find their way out through the dense 

 layer of the exterior. 



It is by a failure to recognize and obey this simple law that 90% of 

 the losses in burning clay wares occur. All clay workers appreciate the 

 importance of burning slowly at first, but very few realize that the real 

 time to go slow is midway in the process of burning, or more explicitly 

 while the combined water or "water smoke" is being driven off. After a 

 kiln of clay ware is lit and the moisture is dried out, the heat may safely 

 increase as fast as convenient till the water smoke begins to appear. At 

 ■ this stage the heat must be simply maintained until the steam ceases to 

 issue from the stack or until every part of the kiln has attained the dull 

 red color in daylight and been held at that point for a few hours. 



If this precaution is not observed and the heat is allowed to increase 

 on the ware before the water is all expelled, the escaping steam from the 

 inside of the ware being confined by the denser layers surrounding it, 

 causes the clay to swell or puff up like a loaf of bread under the action 

 of yeast. This comparison is not an idle one. A piece of bloated clay 

 ware and a loaf of light bread have both been made porous and spongy 

 by the efforts of an imprisoned gas to escape from confinement. 



This action, variously named as puffing, bloating, blowing, swelling, 

 blistering, blubbering, manifests itself in very many ways in the different 

 kinds of wares which are made out of clay. As might be expected from 

 the principles involved, the larger the mass of the clay the greater dan- 

 ger of bloating it, and the longer the heat must be held stationary to 

 allow the escape of the gases. Large objects like fire bricks, glass pots 

 and glass furnace blocks and retorts contain a notable quantity of calcined 

 clay or ground brickbats from which all water has been expelled. This 

 produces an open structure and allows the escape of the gases from the 

 interior. Great care and long time has still to be taken in burning these 

 objects. But in the case of large paving blocks, where no calcined clay is 

 used and where the body clays are generally somewhat plastic and easily 

 fusible, the difficulty is much increased, as the clay tends to begin to 



6 G. O. 



