520 BRICK 



There are few places in this country which do not possess alumina in combination 

 with silica and other earthy matters, forming a clay from which bricks can be manu- 

 factured. That most generally worked is found on or near the surface in a plastic 

 state. Others are hard marls on the coal-measure, New Eed Sandstone, and Blue Lias 

 formations. It is from these marls that the blue bricks of Staffordshire and the fire 

 bricks of Stourbridge are made. Marl has a greater resemblance to stone and rock, 

 and varies much in colour ; blue, red, yellow, &c. From the greatly different and 

 varying character of the raw material, there is an equal difference in the principle of 

 preparation for making it into brick ; while one merely requires to be turned over by 

 hand, and to have sufficient water worked in to make it subservient to manual labour, 

 the fire-clays and marls must bo ground down to dust, and worked by powerful ma- 

 chinery, before they can bo brought into even a plastic state. Now these various clays 

 also shrink in drying and burning from 1 to 15 per cent., or more. This contraction 

 varies in proportion to the excess of alumina over silica, but by adding sand, loam, or 

 chalk, or (as is done by the London brick-makers) by using ashes or breeze as it is 

 technically called this can be corrected. All clays burning red contain oxides of 

 iron, and those having from 8 to 10 per cent, burn of a blue, or almost a black colour. 

 The bricks are exposed in the kilns to great heat, and when the body is a fire-clay, 

 the iron unites with a portion of the silica, forming a fusible silicate of protoxide of 

 iron, which melts into an external glaze. Bricks of this description are common in 

 Staffordshire, and, when made with good machinery (that is, the clay being very 

 finely ground), are superior to any in the kingdom, particularly for docks, canal or 

 river locks, railway-bridges, and viaducts. In Wolverhampton, Dudley, and many 

 other towns, these blue bricks are commonly employed for paving purposes. Other 

 clays contain lime and no iron ; these burn white, and take less heat than any other 

 to burn hard enough for the use of the builder, the lime acting as a flux on the silica. 

 Many clays contain iron and lime, with the lime in excess, when the bricks are of a 

 light dun colour, or white, in proportion to the quantity of that earth present ; if 

 magnesia, they have a brown colour. If iron is in excess, they burn from a pale red, 

 to the colour of cast iron, in proportion to the quantity of that metal. 

 There are three classes of brick earths : 



1st Plastic clay, composed of alumina and silica, in different proportions, and con- 

 taining a small per-centage of other salts, as of iron, lime, soda, and magnesia. 

 2nd. Loams, or sandy clays. 



3rd. Marls, of which there are also three kinds : clayey, sandy, and calcareous, 

 according to the proportions of the earth of which they are composed, viz., alumina, 

 silica, and lime. 



Alumina is the oxide of the metal aluminium, and it is this substance which gives 

 tenacity or plasticity to the clay-earth, having a strong affinity for water. It is owing 

 to excess of alumina that many clays contract too much in drying, and often crack on 

 exposure to wind or sun. By the addition of sand, this clay would make a better 

 article than wo often see produced fromfit. Clays contain magnesia and other earthy 

 matters, but these vary with the stratum or rock from which thoy are composed. It 

 would be impossible to give the composition of these earths correctly, for none are 

 exactly similar ; but the following will give an idea of the proportions of the ingre- 

 dients of a good brick earth : silica, three-fifths ; alumina, one-fifth ; iron, lime, mag- 

 nesia, manganese, soda, and potash forming the other one-fifth. 



The clay, when first raised from the mine or bed, is, in very rare instances, in a 

 state to allow of its being at once tempered and moulded. The material from which 

 fire-bricks are manufactured has the appearance of ironstone and blue lias limestone, 

 and some of it is remarkably hard, so that in this and many other instances in order 

 to manufacture a good article, it is necessary to grind this material down into particles 

 as fine as possible. 



Large quantities of bricks are made from the surface marls of the New Eed Sand- 

 stone and Blue Lias formations. These also require thorough grinding, but from their 

 softer nature it can be effected by less powerful machinery. Chamberlain. 



Recently, some very valuable fire-bricks have been made from the refuse of the 

 China Clay Works of Devonshire. The quartz and mica left after the Kaolin has 

 been washed out are united with a small portion of inferior clay, and made into bricks. 

 These are found to resist heat well, and are largely employed in the construction of 

 metallurgical works. See CLAY. 



The general process of brick-making consists in digging up the clay in autumn ; 

 exposing it, during the whole winter, to the frost and the action of the air, turning it 

 repeatedly, and working it with the spade ; breaking down the clay lumps in spring, 

 throwing them into shallow pits, to bo watered and soaked for several days. The 

 next step is to temper the clay, which is generally done by the treading of men or 

 oxen. In the neighbourhood of London, however, this process is performed in a 



