"Hacks" of partially dried soft-mud brick. The men are placing freshly molded brick on the drying floor. The Sayre & Fisher Company, Sayreville.

DRYING.

Bricks made by either the stiff-mud or soft-mud process have to be freed from most of their water of tempering before they can be burned. This is done by drying them in 1) open yards, 2) covered yards, 3) on pallet racks, 4) tunnel driers, or 5) floors.

Open yards. These are used at most soft-mud brick plants, and are simply smooth flat floors of earth or brick, on which the bricks are dumped as soon as molded, and allowed to dry in the sunlight. (Pl. XXVII, Fig. 1.) They are cheap, but require much space, and in case of a sudden shower the green bricks are washed from lack of cover. After being spread out for a day the bricks are generally piled in double rows several courses high along the sides of the yard. These rows, called "hacks," are often covered with planking as protection from rain (Plate XXVII, Fig. 2).

Covered yards.-Covered yards differ from the preceding in having a sectioned roof that can be opened in fair weather. They are not used to any great extent, for when some form of protection against weather is desired, the type of drier next mentioned is more commonly used.

Pallet driers. These are covered frames for holding the boards or "pallets," on which the bricks are dumped from the mold at the machine. They are used at many soft-mud yards and even some stiff-mud plants, and possess the advantage of cheapness, large capacity, economy of space and protection against rain.

One disadvantage common to all three of the above methods is that they cannot be used in cold weather. Dampness in summer may also interfere with them, and therefore sunlight and wind are the most favorable weather conditions in most cases. Some clays are quite susceptible to air currents, however, and crack easily when exposed to them.

Drying tunnels.-Many brickyards dry their product by this method, especially if they continue in operation throughout the year. With this system the bricks, after molding, are piled on cars, which are run into a tunnel heated artificially. Several of these tunnels are generally constructed side by side, and the green

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 100° C. (212° F.)

Floor driers.-Floor driers are used at some brick works, although 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 preceding stages, much loss may occur at this very point. The imperfect 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

into a solid mass, which is hard and rock-like when cool. Other changes, due to the presence of certain ingredients or certain physical characteristics of the clay, occur in specific cases.

The amount of heat required for burning brick will vary with the clay, and the color, density and degree of hardness desired, the same clay giving different results, when burned at different temperatures. Common bricks are rarely burned higher than cone 05 or 03, while pressed brick are frequently fired to cone 7 or 8, because the clays generally used have to be burned to that point to render them hard.

General effects. In burning, the last traces of moisture are driven off. This vapor, which is termed water-smoke or steam by the brickmaker, is simply the moisture which has been retained in the pores of the clay. Its expulsion results in a slight loss of weight. With further heating to very dull redness the chemically combined water disappears.

If the clay contains considerable carbonaceous matter, this will burn off at a low red heat, provided in the first place sufficient air is present to insure an oxidizing atmosphere. In this case carbon in the clay uniting with the oxygen of the kiln atmosphere, burns off as carbon dioxide. If the heat is raised too rapidly the clay contracts before all the carbonaceous matter has burned off, and the result is a black centre to the brick, which may also be accompanied by a swelling of the clay. In calcareous clays the carbonate of lime present also loses its carbon dioxide. The driving off of all these substances will, therefore, tend to make the brick very porous. Further heating, however, after the volatilization of these substances, causes a drawing together of the clay particles, or shrinkage, and this is accompanied by an increase in density and hardness, the maximum density and shrinkage being reached when the brick is vitrified.

These effects of heating a clay can be summarized as follows: I. Loss of volatile substances present, such as water, carbon dioxide and sulphur trioxide, the volatilization of these leaving the clay more or less porous.

2. A shrinkage of the mass, by further heating.

3. Hardening of the clay due to fusion, of some at least, of the particles.