The operations involved in brickmaking are very numerous, though not intricate; they differ in several particulars in different localities, according to local custom, generally influenced by the nature of the clay.
To describe these operations in sufficient detail to be of any practical value would require a separate treatise of considerable bulk and profusely illustrated.
Such descriptions would be beyond the province of these Notes, and would be unnecessary, for the practices in the brickfields of different localities are fully described in Dobson On Brick and Tile Making, one of Weale's series of very valuable technical works.1
It may be useful, however, to give a general sketch of the operations involved in brickmaking - not such details as would be of practical use to the brickmaker, but just so much as will enable any one using bricks to understand and appreciate more clearly the qualities and peculiarities of different varieties, many of the characteristics of which are caused by differences in the processes of manufacture.
With this object the various operations will now be rapidly and shortly sketched in succession.
The surface of the site from which the clay is to be obtained is first stripped of its turf and mould, which is removed to a spoil bank and kept to be respread over the site after the clay has been dug out.
The mould is sometimes called encallow, and the process of removing it encallowing.
In the autumn the clay is dug, and the various descriptions which it is intended to mix, together with the ashes which are to be incorporated in the mixture, are wheeled to heaps, in some places called kerfs, in which they remain during the winter, sometimes during two or three winters, so that they may be thoroughly disintegrated by the action of frost.
This mellowing of the clay renders the bricks made from it less liable to warp.
If the clay contain pebbles or pieces of ironstone they must be carefully picked out by hand; or if they are found in large numbers, the clay must be washed in small quantities and strained through a grating so as to separate all the stones from the mass.
When the clay is of a hard marly character and full of lumps, or contains fragments of limestone, known by brick-makers as race, it requires to be ground between cast-iron rollers, which must be set sufficiently close to reduce the hard particles to powder.
This is done after the winter's frosts, generally in March or April, before the brickmaking begins.
1 Now published by Messrs. Crosby Lockwood and Company, Stationers' Hall Court, E.C.
It consists in digging and turning over the kerfs or heaps of clay; sometimes the clay is also well trodden under foot, in some places it is passed through a pug-mill, and occasionally, for the very best bricks it is kept damp in cellars for a year or two to ripen.
The clay is dug in the autumn and at once tipped, together with a proportion of ground chalk in pulp, into a wash mill. This consists of a brick-lined circular tank in which are revolving harrows, knives, or implements of some kind to disintegrate and mix up the clay and chalk.
The exact proportion of the chalk differs according to the composition of the clay, but in some cases the chalk is about 1/16 of the bulk of the clay.
The mixture having been reduced to a creamy consistence is strained off through fine gratings into large shallow tanks called backs, and there left till it is nearly solid.
After that it is soiled in layers from 1 to 3 feet deep, i.e. covered about 1/6 its bulk with screened cinders, and allowed to remain during the winter.
In the spring the backs are dug out, the layers of clay and ashes being thoroughly incorporated in a pug-mill.
In some places the preparation of malm is known as washing.
Malm brides are made with the mixture of clay and chalk described above.