The colour, which is very diverse according to the nature of the clays, varies from yellowish white through yellow, yellowish red, bright red, violet, and grey, to dark brown. For bricks intended for fagades the colour should be uniform over the whole surface. This condition is not necessary for inside masonry, but bricks of a good make usually present a single tint; those which are variegated are made of insufficiently prepared material or contain many impurities. In fact, the foreign substances enter during firing into combination with the clay, and, if the paste is not very homogeneous, produce various discolorations.
The gases of combustion also have an effect on the colour of bricks; this colour will be different according as the gases are reducing or oxidising (see p. 227).
Red is the commonest and the most characteristic colour of bricks; it is the one which, in architecture, gives those fine shades so pleasing to the eye. In order to break the monotony of one single colour, white bricks are frequently mingled with red ones, and designs are executed with bricks whose heads are black. We have seen how these bricks are produced in wood firing (p. 200).
A bluish black colouring may also be given to bricks in the following manner. They are fired in intermittent kilns; this is finished, the furnaces are filled with bundles of birchwood having the leaves still on, all openings are closed, and a copious smoke is given out which is deposited on the still red-hot products, giving them a black colour with a bluish lustre.
Instead of fagots, oils loaded with petroleum may be burnt. The arrangement of the kiln varies, but one of the most convenient consists of placing funnels in the arched roof .of a tunnelled intermittent kiln, and pouring through them on to the candescent mass tar or oil loaded with petroleum. Care is taken to close all openings beforehand. As the colouring is effected by a deposit of carbon, it is indispensable that all entrance of air should be prevented as long as the temperature is above dark red heat, otherwise the carbon deposit would disappear by combustion.
To obtain a black colouring with bluish metallic lustre, which is the one most esteemed, we must work with strongly ferruginous pastes. For 1000 kilog. of fired products, 3 to 8 kilog. of oil are required, and must be poured in several times at intervals of one or two hours.
At West Bromwich in Staffordshire blue bricks are obtained by the use of a special purplish clay containing a large proportion of oxide of iron.
The white bricks, which are always slightly yellow, are made of clays containing no oxide of iron; but they are not for that reason necessarily refractory.
Besides the differently shaped bricks of which we have already spoken, bricks with ornaments are also made, which are intended for cornices or entablatures. This result is easily obtained by stamping with a press like the one represented in Fig. 167.
Figs. 258 to 265 show some ornamented bricks of different shapes and sizes according to the uses for which they are intended. Clay by its plasticity lends itself admirably to the reproduction of all kinds of designs, and as, by suitable moulds, the same type may be repeated in numerous examples, this kind of decoration is comparatively cheap.
Figs. 258 to 265. Various Ornamental Bricks.
The brick represented in Fig. 263 is intended for a ventilator; in this way the use of those unsightly iron gratings frequently placed at the orifice of ventilating shafts is avoided in facades.
Another kind of decoration which, besides a pleasant appearance, gives valuable qualities to the bricks, is varnishing and enamelling; but the manufacture of such products comes under the head of Faiences, and we will postpone further details as to enamelled bricks until we come to the chapter referring to faiences..