The best materials from which to make white bricks are a refractory clay, which will naturally burn to pale yellow or white, and a fine white or yellow sand, which vitrifies slightly under a strong heat.
In the absence of such material, however, every clay which does not contain more than 6 per cent of iron will burn into a white brick, provided it is strong enough to stand & sufficient quantity of chalk mixed with it. In the case of very refractory clays the mixture with a large proportion of chalk will render the resulting brick friable.
The processes usually gone through in the manufacture of white brick? do not differ very materially from those applied to other bricks.
"The best mode of manufacture is to grind the clay dry, mix it thoroughly with sand while dry, and then through a press."1
White bricks are frequently burnt in close kilns, carefully protected from smoky flames and soot, thoroughly burned in a dead heat, and allowed to cool down; gradually, or the faces will be full of cracks.
The clays from which white bricks are made are generally heavy, and they are in such case lightened by being made hollow or perforated.
These stains can generally be rubbed off when the brick is dry; if they reappear they can be permanently removed "by mixing up a wash of clay and sand of which the brick was made with sulphate of copper, painting over the brick with it, and leaving it till it is perfectly dry, and then rubbing it off with a brush." l
White bricks may be procured from several parts of England. Some of the best come from Suffolk, Essex, Arsley, Ewill in the district of the Med-way; from Dorsetshire (Beaulieu bricks and others); from the London brickfields; from Exbury and Cowes. Others are made in Cambridgeshire, Devonshire, Lincolnshire, and the Midland counties.
A few of the best known varieties will be further noticed.
Gault Bricks are made from a band of bluish tenacious clay which lies between the Upper and Lower Greensand formations.
This clay in its natural state contains sufficient chalk to flux the mass, and to give the brick a white colour.
The bricks made from this clay are of very good quality; extremely hard throughout, very durable, but difficult to cut.
They are generally white, but the lower qualities have a pink tinge caused by irregularities in burning.
Bricks made from Gault clay are generally very heavy. To remedy this a large frog is sometimes formed in the brick, or it is perforated throughout its thickness.
Bricks of this description are manufactured by the Burham Company at Burham, near Bochester, and at Aylesford, near Maidstone 3 also at Folkestone, near Hitchin, and at other places.
Suffolk white Bricks are also made from the Gault clay.
They contain a very large proportion of sand which makes them useful for rubbers.
Beaulieu Brides, of a light straw colour, are made from clay dug upon the Beaulieu river, near Southampton.
Ballingdon Bricks, made by Beart's process near Sudbury, in Suffolk, are much used for facework.
Beart's Patent Bricks are made at Arsley, near Hitchin, from the Gault clay.
There are different classes. "White rubbers, hand-made, moulded, solid brick, equal to the best Suffolks. No. 1, best selected white facing brick (pierced) and ordinary. These two are of uniform colour, hard and well burnt, and used extensively for facings. No. 2, mingled red and pink, vary from the above only in colour, and are equal in every respect to the heat made stock bricks." l
1 Building News, Sept. and Oct. 1874.
The clay contains lime, and requires to be burnt with great care, or the lime will remain in a quick state, and slake after the brick is in use.
Staffordshire Blue Bricks are made from the clays and marls of that county, which contain from 7 to 10 per cent of oxide of iron.
They are burnt in circular ovens with domed tops, being raised to a very high temperature, which causes the peroxide or red oxide to be converted into the protoxide or black oxide of iron.
These bricks are generally of a dark-blue or nearly black colour, with smooth glassy surfaces. They are very durable, impervious to water, and will resist enormous pressure.
For building ordinary strong work the second-class Staffordshire bricks are more suitable than the first quality, as the former have rougher surfaces to which the mortar adheres more firmly.
An inferior class of these bricks is made by the use of a surface wash of iron. These look well for a time, but the colour does not wear well.
Dust Bricks are blue bricks, for which coal-dust is used in moulding instead of sand. They have glossy surfaces, are very hard, and are used for paving.l
Red and Drab coloured bricks are also made in Staffordshire. The former are used for building, and the latter chiefly as a fire-brick, where intense heat is not required,2
Tipton Blue Bricks are Staffordshire blue bricks from the neighbourhood of the town after which they are named.
Black Bricks are obtained from Cowbridge in South Wales, from Maidenhead in Berkshire, and from other places.
Some inferior black bricks are made with a mixture of soot, and are weak and almost useless.
Fareham Red Bricks are made from a moderately plastic clay, which is found in very deep beds around the town of Fareham, and in other places in the neighbourhood.
They are dressed or batted (as described at p. 94) when partially dry, and thus brought to a very true surface. They are also carefully burnt in small oven kilns holding from 20,000 to 30,000 each.
These bricks are of a fine deep-red colour, and have been much used in London for superior buildings.
The facework of St. Thomas's Hospital is of Fareham bricks, and many are being used in the new Law Courts.
Sometimes these bricks are rubbed so as to obtain very fine surfaces and thin mortar joints, but this removal of the outer skin is bad, as it tends to make the brick decay quickly under atmospheric influences.
Nottingham Patent Bricks are made by the dry clay process, the clay being ground and subjected to pressure of about 200 tons on the brick in moulding.
They are very close in texture, and have good surfaces and arrises, but they appear to be deficient in toughness, and do not "ring" properly or weather well.
They are of a dull red colour. Many of them are burnt in Hoffmann's kilns, in which, case the ends are generally of a yellowish shade. This is owing to the ends being exposed to the fire, whereas the other parts of the brick are protected.
Sometimes bricks are purposely packed on end, so as to protect the ends from fire, make them red, so as to afford headers of an uniform colour.
These bricks were used for part of the St. Pancras Station.
Lancashire Red Pressed Facing Bricks are made by Platfs patent brickmaking machine.
Dutch Clinkers are small bricks, well burnt, very hard, vitrified throughout, and sometimes warped.
They are used almost entirely for paving.
Adamantine Clinkers are similar bricks, harder, denser, and heavier, of a fine pink-white colour and smooth surface.
They are sometimes chamfered on the edge so as to give a firmer foothold when used for paving.
Terro-metallic Clinkers are bricks of the same size and shape, made from a clay which is burnt very hard to a dark-brown or nearly black colour.
Enamelled Bricks have a white or light yellow glazed surface like that of china.
This is produced by a thin coating of white material over the brick, which in inferior descriptions is apt to peel off.
Bricks of this kind are much used for the sake of cleanliness in lavatories, urinals, butchers' shops, dairies, etc.; also in order to obtain reflected light, as in some of the underground railway stations.
Salted Bricks have a thin glaze over their surfaces, produced by throwing salt into the fire during the burning process.
Moulded Bricks are produced in every variety of pattern, from simple sections like those of cornice, plinth, and string-course bricks, already mentioned, up to the most elaborately decorated blocks of different forms, such as voussoirs for arches, diaper patterns for walls, panels, string-courses, etc.
The simpler patterns are made in moulds furnished with shifting sides and ends on which the pattern is raised or sunk. These can be screened up against the soft clay, and then released so as to liberate the moulded brick.
Sometimes the pattern is formed on the stock-board, or on a plaster cast which takes its place.
In the more elaborate patterns iron moulds are used, which are opened and closed by simple machinery.
Pether's Ornamental Bricks are made by the Burham Company from Gault clay forced into a hinged iron mould.
They can be made to almost any design, however elaborate, and afford a cheap and very durable means of decoration.
Pallette Bricks "rebated on edge so as to hold a l1/2-inch fillet securely in the wall, splayed from 7/8 inch at one edge to 1/2 inch at the other, have been occasionally used of late but are not recommended, as the advantage gained is not to be compared to the extra labour and expense involved." 1
Concrete Bricks should hardly be noticed in this chapter, as they are not made of clay, and they do not require burning.
Bodmer's Bricks consist of a species of fine concrete, the constituent parts of which vary, some being of about 1/6 to 1/8 of its weight of sand with selenitic lime or cement, others of black furnace slag mixed with about 1/16 of its weight of lime or cement according to quality.
The ingredients are filled into moulds, and subjected to considerable pressure which binds the particles together.
The resulting bricks are hard and dense, with good arrises and surfaces, and they weigh about 58 cwts. a thousand.
The cost of labour for making these bricks is said to be from 3s. to 3s. 6d per thousand.1
Wood's Patent Concrete Bricks are similar to those just described. They are made at Middlesborough from slag reduced by agitation in water to the state of sand. The slag sand is ground and mixed with 1/7 its bulk of lime. The mixture is forced into moulds under a pressure of about half a ton per square inch. The bricks are dried in the air, and are then ready for use.
These bricks may be made with ordinary sand or crushed stone instead of slag sand.
Slag Bricks are made by running molten slag into iron moulds. The blocks are removed while the interior is still molten, and then annealed in ovens.