170 GEOLOGY OF OHIO. 



the draft was found to be taking the lighter part of the atmosphere and 

 the steam out of the top part of the tunnel and the bricks below were in 

 an atmosphere where they were warmed or dried very slowly. Hence the 

 top part of each car was often cracked and the bottom part sound, hav- 

 ing been raised to the dryer heat before meeting the dr> current of air. 

 Sometimes the reverse of this was found, and was due to the lower parts 

 of the cars going down into the lower parts of the dryer before they 

 have been properly prepared for it. 



A vertical sliding door was then arranged inside of the outer door 

 and in front of the stack, so that the entire flow of heated air and steam 

 had to go down nearly to the floor of the tunnel before it would escape 

 into the draft. Thus the whole car was subjected to the steamy atmos- 

 phere and the drying was equally good at the bottom and top. 



The importance of the preliminary heating up of the bricks in a 

 steamy atmosphere, or one so moist that no drying can take place, cannot 

 be overstated. In any form of dryer where the operations of nature are 

 sensibly hastened, or rather where air currents other than natural circu- 

 lation are used, it is the key to the success of the operation. 



In the chamber dryer, this condition is best produced by shutting 

 off all air flow and producing the moist atmosphere by gentle heat alone 

 until the steaming is sufficiently performed; then the heat and air may 

 be both increased. In the progressive dryer, the effort must be directed 

 to continuously maintaining the atmosphere of the rear end of the dryer 

 in this condition. 



The action of air in drying brick has been indicated briefly. The 

 mere use of heat, as is done on the dry floor, is the crudest and most 

 expensive plan ; water is literally baked out or driven out of the clay. 



The atmosphere does its cbying by absorption or evaporation of the 

 water from the surface of the wet clay. The ability of the air to absorb 

 the moisture of the clay depends on its temperature and its previous 

 condition of humidity. T>xy air absorbs water at all temperatures, but 

 the amount which it will absorb increases enormously as its heat arises. 

 So that in drying a large quantity of' brick, in a small space, in a short 

 time, the use of the air as a vehicle for the moisture in the bricks is 

 most important. A strong current of air will of itself dry the clay quite 

 rapidly, even at a low temperature, and in tunnel dryers the production 

 and control of the air currents is of equal importance with the production 

 of heat. The methods b3^ which air currents are produced in tunnel or 

 compartment dryers are two: 



First by stacks and second by fans. 



The stacks employed are usually tall wooden chimmeys, of large 

 internal area, in order to secure the movement of a large volume of air 

 with a comparatively small difference in temperature between the inside 

 and outside of the stack. 



