The great principle of "continuous" firing is the utilization of the waste heat from one kiln or section of a kiln in heating up another kiln or section, direct firing being applied only to finish the burning. In practice a number of kilns or firing-chambers, usually rectangular in plan, are built side by side in two parallel lines, which are connected at the ends by other kilns so as to make a complete circuit. The original form of the complete series was elliptical in plan, but the tendency in recent years has been to flatten the sides of the ellipse and bring them together, thus giving two parallel rows joined at the ends by a chamber or passage at right angles. Coal or gas is burnt in the chamber or section that is being fired-up, the air necessary for the combustion being heated on its passage through the kilns that are cooling down, and the products of combustion, before entering the chimney flue, are drawn through a number of other kilns or chambers containing unfired bricks, which are thus gradually heated up by the otherwise waste-heat from the sections being fired. Continuous kilns produce a more evenly fired product than the intermittent kilns usually do, and, of course, at much less cost for fuel.
Gas firing is now being extensively applied to continuous kilns, natural gas in some instances being used in the United States of America; and the methods of construction and of firing are carried out with greater care and intelligence, the prime objects being economy of fuel and perfect control of firing. Pyrometers are coming into use for the control of the firing temperature, with the result that a constant and trustworthy product is turned put. The introduction of machinery greatly helped the brickmaking industry in opening up new sources of supply of raw material in the shales and hardened clays of the sedimentary deposits of the older geologic formations, and, with the extended use of continuous firing plants, it has led to the establishment of large concerns where everything is co-ordinated for the production of enormous quantities of bricks at a minimum cost. In the United Kingdom, and still more in Germany and the United States of America, great improvements have been made in machinery, firing-plant and organization, so that the whole manufacture is now being conducted on more scientific lines, to the great advantage of the industry.
Blue Brick is a very strong vitreous brick of dark, slaty-blue colour, used in engineering works where great strength or impermeability is desirable. These bricks are made of clay containing front 7 to 10% of oxide of iron, and their manufacture is carried out in the ordinary way until the later stages of the firing process, when they are subjected to the strongly reducing action of a smoky atmosphere, which is produced by throwing small bituminous coal upon the fire-mouths and damping down the admission of air. The smoke thus produced reduces the red ferric oxide to blue-green ferrous oxide, or to metallic iron, which combines with the silica present to form a fusible ferrous silicate. This fusible "slag" partly combines with the other silicates present, and partly fills up the pores, and so produces a vitreous impermeable layer varying in thickness according to the duration and character of the smoking, the finishing temperature of the kiln and the texture of the brick. Particles of carbon penetrate the surface during the early stages of the smoking, and a small quantity of carbon probably enters into combination, tending to produce a harder surface and darker colour.
Floating Bricks were first mentioned by Strabo, the Greek geographer, and afterwards by Pliny as being made at Pitane in the Troad. The secret of their manufacture was lost for many centuries, but was rediscovered in 1791 by Fabroni, an Italian, who made them from the fossil meal (diatomaceous earth) found in Tuscany. These bricks are very light, fairly strong, and being poor conductors of heat, have been employed for the construction of powder-magazines on board ship, etc.
Mortar Bricks belong to the class of unburnt bricks, and are, strictly speaking, blocks of artificial stone made in brick moulds. These bricks have been made for many years by moulding a mixture of sand and slaked lime and allowing the blocks thus made to harden in the air. This hardening is brought about partly by evaporation of the water, but chiefly by the conversion of the calcium hydrate, or slaked lime, into calcium carbonate by the action of the carbonic acid in the atmosphere. A small proportion of the lime enters into combination with the silica and water present to form hydrated calcium silicate, and probably a little hydrated basic carbonate of lime is also formed, both of which substances are in the nature of cement. This process of natural hardening by exposure to the air was a very long one, occupying from six to eighteen months, and many improvements were introduced during the latter half of the 19th century to improve the strength of the bricks and to hasten the hardening. Mixtures of sand, lime and cement (and of certain ground blast-furnace slags and lime) were introduced; the moulding was done under hydraulic presses and the bricks afterwards treated with carbon dioxide under pressure, with or without the application of mild heat.
Some of these mixtures and methods are still in use, but a new type of mortar brick has come into use during recent years which has practically superseded the old mortar brick.