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Brick Pavements the

clay, lime, burning, soda, potash and alumina

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THE requisites for a good paving-brick are that it shall be hard, tough, and impervious, as well as capable of enduring against the disintegrating influences of the weather.

The bricks in most common use are made from fire clay of an inferior quality, or from an indurated clay or shale of somewhat similar composition.

These clays consist essentially of silicate of alumina, with usually small percentages of lime, magnesia, iron, potash, soda, and sometimes other elements. The range of composition for clays in common use is approximately as follows: Per cent Silica 6o to 75 Alumina . Io to 25 Iron oxide 3 to 8 Lime . o to 4 Magnesia o to 3 Potash 0.5 to 3 Soda ... o to 2 In a few cases the quantity of lime is greater, vary ing from 8 to 12 per cent.

When the clay is very nearly pure silicate of alumina, it is ` capable of withstanding a high degree of heat without fusing, and is known as fire-clay. As the per centages of other ingredients increase, it becomes more fusible. The lime, magnesia, potash, and soda act as fluxing agents, and the readiness with which the clay can be melted depends upon the relative quantities of refractory and fluxing materials that it may contain.

Silica in excess tends to make the brick weak and brittle, while too great quantity of alumina causes the brick to crack and warp in the shrinking which occurs during burning. The proper adjustment of the rela tions between these elements is necessary to good results.

The quantity of lime in the clay is an important matter, as the presence of lime in an uncombined state in the brick may be productive of disintegration when the brick is exposed to the weather. A large percent age of lime in a clay is therefore to be regarded with suspicion, although not necessarily as cause for con demnation, as its effect depends upon the state of com bination of the various ingredients of the brick. Mag nesia probably acts in much the same manner as lime. Potash and soda are considered desirable elements in quantities to properly flux the clay in burning.

The fineness of a clay is also a matter of importance, both because a fine clay will fuse at a lower tempera ture than a coarse one, and because fineness is neces sary to the production of even and close grained brick, and therefore conduces to make them tough and impervious.

To produce a good paving-brick, a clay is required which will vitrify at a high heat. A very refractory clay will make a porous brick, while if it melts at too low a temperature it cannot be burned sufficiently to become hard and tough.

The methods of manufacturing paving-brick vary in different localities according to the character of the material to be worked. They are quite similar to those in use for common brick, only more thoroughly executed.

The clay is commonly reduced to a fine powder, tempered with water and passed through a machine that molds the•bricks, which are then dried and after ward burned. Repressed bricks are those which are compressed in 'a mold after coming from the brick machine and before drying.

The process of burning occupies usually from io to 15 days.

The heat is at first slowly applied to expel the water, then raised to a high temperature for several days, after which the bricks are very slowly cooled.

There is considerable difference of opinion among engineers and manufacturers as to the exact amount of burning necessary. It is usually stated that the brick should be burned to the point of vitrification, but not completely vitrified. The burning must be thoroughly done to produce a strong and impervious brick, but there is undoubtedly a point beyond which, for some brick, further burning causes brittleness. Very gradual cooling is also necessary in order to toughen the brick. Smoothness and uniformity of texture in a paving brick is an important consideration as affecting its re sistance both to crushing and to abrasion. The broken surface of the brick should present a uniform appear ance both in texture and in color.

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