The semi-plastic method has many advantages where shales are used, although the bricks are not as strong nor as perfect as the best "plastic" bricks. The method, however, enables the brickmaker to make use of certain kinds of clay-rock, or shale, that would be impracticable for plastic bricks; and the weathering, tempering and "ageing" may be largely or entirely dispensed with. The plant required is heavier and more costly, but the brickyard becomes more compact, and the processes are simpler than with the "plastic" method.
The drying of bricks, which was formerly done in the open, is now, in most cases, conducted in a special shed heated by flues along which the heated gases from the kilns pass on their way to the chimney. It is important that the atmosphere of the drying-shed should be fairly dry, to which end suitable means of ventilation must be arranged (by fans or otherwise). If the atmosphere is too moist the surface of the brick remains damp for a considerable time, and the moisture from the interior passes to the surface as water, carrying with it the soluble salts, which are deposited on the surface as the water slowly evaporates. This deposit produces the "scum" already referred to. When the drying is done in a dry atmosphere the surface quickly dries and hardens, and the moisture from the interior passes to the surface as vapour, the soluble salts being left distributed through the whole mass, and consequently no "scum" is produced. Plastic bricks take much longer to dry than semi-plastic; they shrink more and have a greater tendency to warp or twist.
The burning or firing of bricks is the most important factor in their production; for their strength and durability depend very largely on the character and degree of the firing to which they have been subjected. The action of the heat brings about certain chemical decompositions and re-combinations which entirely alter the physical character of the dry clay. It is important, therefore, that the firing should be carefully conducted and that it should be under proper control. For ordinary bricks the firing atmosphere should be oxidizing, and the finishing temperature should be adjusted to the nature of the clay, the object being to produce a hard strong brick, of good shape, that will not be too porous and will withstand the action of frost. The finishing temperature ranges from 900° C. to 1250° C., the usual temperature being about 1050° C. for ordinary bricks. As before mentioned, lime-clays require a higher firing temperature (usually about 1150° C. to 1200° C.) in order to bring the lime into chemical combination with the other substances present.
It is evident that the best method of firing bricks is to place them in permanent kilns, but although such kilns were used by the Romans some 2000 years ago, the older method of firing in "clamps" is still employed in the smaller brickfields, in every country where bricks are made. These clamps are formed by arranging the unfired bricks in a series of rows or walls, placed fairly closely together, so as to form a rectangular stack. A certain number of channels, or firemouths, are formed in the bottom of the clamp; and fine coal is spread in horizontal layers between the bricks during the building up of the stack. Fires are kindled in the fire-mouths, and the clamp is allowed to go on burning until the fuel is consumed throughout. The clamp is then allowed to cool, after which it is taken down, and the bricks sorted; those that are under-fired being built up again in the next clamp for refiring. Sometimes the clamp takes the form of a temporary kiln, the outside being built of burnt bricks which are plastered over with clay, and the fire-mouths being larger and more carefully formed. There are many other local modifications in the manner of building up the clamps, all with the object of producing a large percentage of well-fired bricks.
Clamp-firing is slow, and also uneconomical, because irregular and not sufficiently under control; and it is now only employed where bricks are made on a small scale.
Brick-kilns are of many forms, but they can all be grouped under two main types - Intermittent kilns and Continuous kilns. The intermittent kiln is usually circular in plan, being in the form of a vertical cylinder with a domed top. It consists of a single firing-chamber in which the unfired bricks are placed, and in the walls of which are contrived a number of fire-mouths where wood or coal is burned. In the older forms known as up-draught kilns, the products of combustion pass from the fire-mouth, through flues, into the bottom of the firing-chamber, and thence directly upwards and out at the top. The modern plan is to introduce the products of combustion near the top, or crown, of the kiln, and to draw them downwards through holes in the bottom which lead to flues connected with an independent chimney. These down-draught kilns have short chimneys or "bags" built round the inside wall in connexion with the fire-mouths, which conduct the flames to the upper part of the firing-chamber, where they are reverberated and passed down through the bricks in obedience to the pull of the chimney.
The "bags" may be joined together, forming an inner circular wall entirely round the firing-chamber, except at the doorway; and a number of kilns may be built in a row or group having their bottom flues connected with the same tall chimney. Down-draught kilns usually give a more regular fire and a higher percentage of well-fired bricks; and they are more economical in fuel consumption than up-draught kilns, while the hot gases, as they pass from the kiln, may be utilized for drying purposes, being conducted through flues under the floor of the drying-shed, on their way to the chimney. The method of using one tall chimney to work a group of down-draught kilns naturally led to the invention of the "continuous" kiln, which is really made up of a number of separate kilns or firing-chambers, built in series and connected up to the main flue of the chimney in such a manner that the products of combustion from one kiln may be made to pass through a number of other kilns before entering the flue. The earliest form of continuous kiln was invented by Friedrich Hoffman, and all kilns of this type are built on the Hoffman principle, although there are a great number of modifications of the original Hoffman construction.