CLAY WORKING INDUSTRIES. 169 



The operation of the progressive dryer offers us the best example 

 of the highest type of the art of artificial drying at the present time. 

 The bricks are brought to the rear end of the dryer on their car and 

 passing through double doors are placed in the rear of a line of similiar 

 cars. The heat and air are being introduced at the opposite end of the 

 dryer. By the time the air currents reach the rear end they have 

 absorbed in passing through the great volumes of brick ahead, about all 

 the vapor which they are capable of retaining, consequently, the temper- 

 ature is low, generally 80 to 100 degrees, and the air is filled with humid- 

 ity, almost to the dew-point. The new carload of bricks in an atmos- 

 phere like this, does not begin to dry at all, but it begins to warm 

 through, till the individual bricks are as hot as the surrounding atmos- 

 phere. After a time, the cars are shoved down the tunnel to make room 

 for other cars in the rear. The first car now begins to find itself in an 

 atmosphere a little warmer and not quite saturated with moisture. The 

 water now begins to dry on the surface of the bricks, and as they have 

 been previously brought to a warm steamy condition, the surface evapor- 

 ation is constantly replaced by moisture from the inside. Hence, there 

 is no tendency for the outside of the brick to contract faster than the 

 inside and therefore no tendency to cracking or breaking. As the bricks 

 proceed onward they yield up successive portions of their moisture and 

 finally emerge from the hot end of the dryer, ready for the kiln. There 

 are a few instances of clays which will not yield to this system of drying 

 as it is ordinarily carried on. These clays are of such a nature that they 

 are almost sure to crack even on exposure to the open air for a half hour 

 after they are made. Such clays as these can only be treated in any 

 form of dryer by the use of special conditions, and the special conditions 

 necessary, are a prolonged exposure to a wet steamy warm atmosphere, 

 so that the clay has an opportunity to become not only hot through and 

 through, but can also have an opportunity to absorb moisture rather 

 than lose it during this heating up. 



The theory of this plan of drying is as near perfection as it is pos- 

 sible to attain at the same time with rapid and effective work. Where 

 these styles of dryers are used, it is usually the intention to dry every 

 day the 1 entire quantity manufactured in the day's run. It is usually pos- 

 ible to do so with no harm to the bricks, and therefore to make the 

 investment as small as possible in proportion to the work accomplished, 

 the drying is always estimated on the basis of the shortest possible 

 exposure to the conditions. • 



It is the preliminary exposure to the steamy atmosphere of the rear 

 end of the dryer which makes the rapid and effectual drying without 

 loss by cracking and checking a "possibility." In the beginning of the 

 tunnel dryer system, the arrangements in rear comprised only one door 

 to keep out the outside air and make the draft arrangements do their 

 work. But great trouble was experienced from cracking of brick, and 



