234 CLAYS AND CLAY INDUSTRY. 



bricks are run in at the cooler end, and pushed along slowly to the 

 warmer end, where they are removed. This passage through the 

 tunnel requires commonly from 24 to 48 hours. If the bricks are 

 soft-mud, it is necessary that the cars be provided with pallet 

 racks, but if stiff-mud, they can usually be piled on top of each 

 other, a car holding about 350 brick. The tunnel dryers used at 

 different localities differ chiefly in the manner in which they are 

 heated, the following methods being employed. 



1. Parallel flues underneath and heated by fire placed at one 

 end. 



2. By steam heat, the pipes being laid on the floor or sides of 

 the tunnel or both. 



3. By hot air, the latter being supplied from cooling kilns and 

 drawn through the tunnel by natural draft or fan. If the air is 

 too hot, cooler air is mixed with it before it enters the dryer. The 

 temperature to which tunnels are heated varies, and in most cases 

 is not over ioo° C. (212 F.) 



Floor driers. — Floor driers are used at some brick works, al- 

 though their application is more extended at fire-brick works. 

 They are made of brick, and have flues passing underneath their 

 entire length, from the fireplace at one end to the chimney at the 

 other. Such floors are cheap to construct, but the distribution of 

 the heat under them is rather unequal, and a large amount of labor 

 is required to- handle the material dried on them. 



In rare cases, drying racks are set up on the top of the kiln, and 

 in at least one instance in the State, brick are dried by being placed 

 on steam pipes not enclosed in tunnels, but merely roofed over to 

 afford protection from the rain. 



BURNING. 



This stage of the process of manufacture is an important one, 

 and although the clay may have passed safely through the preced- 

 ing stages, much loss may occur at this very point. The imper- 

 fect bricks thus obtained may be due 1 ) to mistakes of the burner, 

 2) to the clay, 3) to the fuel, 4) to the construction of the kiln. 

 In burning, certain changes, partly physical and partly chemical, 

 take place in all clays, as a result of which the brick is converted 



