148 GEOLOGY OF OHIO. 



The other self-emptying device is that employed on the Carlin pans 

 made in Pittsburgh. It consists of a movable section in the side of the 

 pan rim, which is hinged solidly against the frame of the pan. In this 

 case the bottom of the pan revolves but the rim is stationary. When the 

 pan is grinding the movable section is out in its place as part of the rim 

 When the charge is ready to empty, the section is moved inwards slowly 

 by a screw worked by a hand wheel, and all clay striking it is swept out 

 of the.pan into a belt or table. The action is that of a movable plow, 

 entering from one side and plowing the contents of the pan out into the 

 receptacle beyond. 



Neither of these devices make any great difference in the expense of 

 tempering; the only difference is in the slightly increased out put. The 

 character of the tempering given by a wet pan is the most thorough and 

 efficient it is possible to give clay. The grinding action is a very impor- 

 tant adjunct to the tempering. When a hard gritty shale is being used, 

 and it fails to develop the proper plasticity in being treated by pug mills 

 it is necessary to add a plastic clay to assist it. But a wet pan will make 

 any clay plastic that has the elements of plasticity in it. Also in mixing 

 clays, a pug mill merely stirs the particles together. A wet pan grinds 

 them together so that it often happens that the result cannot be detected 

 as a mixture. 



In mixing two clays of different fire qualities together by pug mill, 

 the ability of the mixture to keep its shape depends practically on the 

 character of the mineral in excess. Thus a soft vitrifiable shale, would 

 allow a brick to bend under heat if there were fifty per cent of the shale 

 used with fifty per cent of the refractory fire clay, but if the ingredients 

 of the two clays were worked together until practically united, it is pro- 

 bable that twenty-five per cent of fire clay would sufficiently elevate the 

 melting point of the shale. 



The defects of the wet pan as a tempering machine are 1st. Its cost, 

 which is two to three times that of the pug mill. 2nd. The fact that it has 

 to be put on the bottom floor of the building on account of its weight 

 and vibration and thus generally necessitates a second elevation of the 

 clay. 3rd. The clay is tempered in batches and even with the utmost 

 fa thfulness it is impossible to always get each batch of an even tem- 

 per. 4th. The clay will sometimes absorb water so fast that it will 

 become quite dry and hard in temper in two minutes after a copious 

 addition of water. In treating a clay of this nature, the first portions 

 removed from the pan are likely to be more soft than the last portions of 

 the same batch. 



As to capacity, one good wet mill, well handled, will temper suffici- 

 ently for brickmaking all the clay that one dry pan will grind. For the 

 additional plasticity, required by sewer pipe machinery, an additional pan 

 will be needed to keep one dry pan working constantly. Instances are 

 on record of one wet pan taking all the clay from two dry pans but this 

 is exceptional. 



