The green bricks, after being carefully dried, either in the sun or by artificial heat, are usually baked in a kiln with a suitable arrangement of fires and flues. Kilns are of many forms, and the time required for firing in them varies from 40 to 60 hours for common red and white bricks, while for some fire-bricks 150 hours are necessary. Where kilns are not used, bricks are burned in clamps, the clay requiring to be mixed up, in the process of tempering, with a quantity of ground coal sufficient to burn them. A good test of the character of a clay is obtained by the result of firing. The average contraction in the kiln for prepared clays is 7+ per cent. If a brick contracts much more than this, the clay is too fusible; If less, then it is likely to of an open porous body, which retains its shape well during the firing process.
All brick clays contain iron, and the color of a burned brick almost entirely depends on the amount of it which is present; thus clays containing less than 1 or 1+ per cent of iron, change in the kiln to various shades of cream color and buff, whilst those contain ing more than 2 per cent, range in color from yellowish-fawn to dark red. Blue bricks are made from the same clay as the red, by controlling in a peculiar way the supply of air in firing, and by carrying the heat slightly further. It is asserted by some that the
red is changed to the black oxide of iron in the process.
Fire-bricks are made from clay as free as possible from oxide of iron and alkaline substances, so that there may be no tendency to fuse in the kiln, however high the heat. Fire-clays are abundant in the coal-measures, some of fine quality being found about Newcastle and Glasgow, but the most celebrated is that of Stourbridge, which is ex ported to all parts of the world. See FIRE-CLAY.
Much attention has been paid of late years to the manufacture of fine bricks and terra cotta, which is only another name for ornamental bricks of various shapes, or architectural enrichments of the same material. The effect of some of the public build ings recently erected in London and elsewhere, in which terra cotta has been used, is really beautiful. Although it cannot be said to equal sandsone in appearance, it has .yet the advantage of giving a much greater variety of color, an is infinitely better and more enduring than a facing of stucco or cement.
The duties formerly levied on bricks were wholly repealed i 1830.