Manufacture of the Brick

After being molded, or after being re-pressed, the brick are placed on trucks or cars, and conveyed to the dry house. Thor ough drying greatly facilitates the burning of the brick.

Burning.

Paving brick are usually burned in down draft brick-ovens having fire pockets or furnaces built in their outer walls. The bottoms of the kilns are perforated to allow the gases to pass through the flues, which are beneath the floor, and which lead to the chimney. The fire passes up from the furnaces into the kilns, then down through the brick to be burned to the flues, and thence to the chimney. The burning is the most critical step in the manufacture of paving brick. At first the heat is applied slowly in order to drive off the water, without cracking the brick, which contain from 20 to 30 per cent after being dried. A low heat is continued until the smoke passing off shows no further signs of steam or "water-smoke," after which the fires are gradu ally increased until the temperature throughout the kiln is suffi cient to vitrify the brick. Most shales vitrify at from 1,500° to 2.000° F.; but impure fire-clays require from L.SOOs to F. From seven to ten days are required to raise the entire kiln to the vitrifying temperature.

There has been much as to the meaning of the term vitrification as applied to brick, maldng. Literally speaking. to vitrify means to render glassy, but as applied to day working. vitrification has come to mean incipient fusion of the particles of the clay into a new chemical compound. The degree of vitrification increases with the temperature, and the logical end of the is complete fusion. A clay is partially vitrified if its constituents have begun to unite by heat into a compound silicate, even though it may not have a glassy fracture. The physical peculiarities which mark vitrification in a burnt clay are the conchoidal frac ture, the absence of pores, and the blending of the ingredients into one mass. Cracks, fissures, and cavities may be found. but porosity must not exist in a well vitrified brick, and the original particles must have begun to cohere by the bond of heat instead of the bond of plasticity. Within limits the degree of vitrification in a burned clay is measured by its ability to absorb water. A lightly burned brick will greedily asborb water, and the greater the degree of vitrification the less the water absorbed, a perfectly vitrified brick absorbing absolutely no water.

After the bricks have been vitrified entirely through, the kiln is tightly closed and allowed to cool very slowly. Rapid cooling renders the brick brittle; but by slow cooling they are annealed and rendered tough. Slow cooling is the secret of toughness, and

the slower the cooling the tougher the brick. The annealing is frequently unduly hurried, much to the detriment of the toughness of the brick. The kiln is often cooled in. three to five days. when seven to ten would materially improve the product and usually would be worth the extra cost.

With the utmost care a considerable per cent of the contents of the kiln are unsuitable for paving purposes, because of some being under-burned and some over-burned. With shale 70 to 80 per cent of first-class paving brick is a high average, while with impure fire clay SO or 90 per cent may be produced.

Size of the Brick. Formerly there was considerable differ ence of opinion as to the best size for paving brick, some advocating 2" X 4" X 8", others 3" X 4" X 9", and a kw 4" X 5" x 12". The first is known as a brick and the last two as paving blocks. It was often claimed that one or the other size made the better pavement, but there is no material difference' in the quality of the pavement between the different ^izes, except that possibly the block may be a little more durable. • The advantages of the .;)uilding-brick size are: (1) being smaller they are more easily vitrified, and therefore a little cheaper to manufacture; and (2) brick unsuitable for use in the pavement can be more readily disposed of for building purposes, a fact which tends to cheapen the cost of the brick used in the pavement. The advantages of the block-size to the manufacturer are that there are fewer pieces to handle; and in the pavement the blocks chip or spall on the edges less than the bricks, particularly if the filler is not rigid (see § 773). The manufacturer of the block usually places building brick in that part of the kiln in which it is difficult to burn blocks thoroughly (the bottom of a down-draft kiln), a process which decreases the per cent of blocks unsuitable for paving pur poses, and at least partially eliminates the second advantage of the building-brick size as above. In the early history of brick paving, bricks were most in favor; but now the blocks are most common. Apparently the introduction of the blocks is due, at least in part, to the fact that in the ordinary method of testing the bricks or blocks by tumbling in a cylinder (§ 740), the bricks lose a greater per cent of their weight than do the blocks, and consequently manufacturers preferred to submit blocks rather than bricks for a competitive test, particularly as in the early history of testing clay paving-material specifications made no distinction between bricks and blocks in the loss permissible (§ 748).