Bricks are burnt in "clamps" or in "kilns" according to the practice of the locality.
1 Dobson. 2 Except in over-sailing courses or when they are to receive a layer of asphalte for a damp course.
Clamps are stacks of dried raw bricks dexterously built up over a system of flues roughly formed with burnt bricks, and leading from live-holes or eyes at which the fire is introduced.
Clamps in a permanent brickfield are made in a very elaborate manner, which is fully described in Dobson On Brick and Tile Making.
The following brief notes refer merely to the rough kind of clamp used in a temporary brickfield.
Bricks intended to be burnt in clamps should always be made from clay mixed with ashes or breeze, so that they contain fuel which enables the fire to seize upon them and burn them throughout more readily.
Before building a clamp the ground is first made firm, thoroughly drained, and sometimes formed to a dish-shaped section.
On the ground is placed a foundation of burnt bricks on edge close together to keep out the moisture from the earth, so that the bricks may lean inwards towards the centre of the clamp, and not tend to fall outwards.
[This foundation course is sometimes omitted, especially if the ground has been the site of a recently burning clamp, so that it is pretty dry.]
The clamp itself is commenced by laying two courses of burnt bricks on edge, in parallel lines. In the lowermost course of these two the parallel lines run diagonally across the clamp, with spaces about 2 inches wide between the lines. The uppermost course is laid crossways upon the lower, the lines of bricks being parallel to the end of the clamp, and the bricks close together. The spaces between the bricks are filled in with breeze.
In laying these courses the live-holes1 about 9 inches square are left right across the clamp, filled up with faggots, and the whole covered with breeze from 5 to 7 inches thick (marked p in Fig. 6).
Upon the layer of breeze, p, is placed the first course of raw bricks as headers; along the clamp, above this course, another layer of breeze, o, from 4 inches to 7 inches thick; and theu alternate heading and stretching courses of raw bricks on edge, tip to a height of 10 or 12 feet.1 On the uppermost course of raw bricks a 3-inch layer of breeze is sometimes spread, and the top of the clamp is covered with a casing of burnt bricks two or three courses deep.
1 In many cases there is only one live-hole - near the end of the clamp, as in Fig. 4. If, however, the burning does not keep pace with the building of the clamp, rive-holes are formed at intervals, so as to start the burning at other points.
In some cases a thin layer of breeze (about 3/8 inch thick) is spread over the top of each course of raw bricks throughout the clamp. In other cases a little breeze is inserted only at the edges of the courses.
In rough clamps used for temporary works, the ends and sides are smeared with clay. In permanent brickfields they are cased with burnt bricks.
The bricks nearest the outside of the clamp are tilted a little upward by the insertion of bats, so that they may not have a tendency to fall outwards.
Figs. 4 to 7 are taken from measurements of a clamp in one of the Kentish brickfields.
Fig. 5 is a plan of part of the second course.
Fig. 6 is an enlarged section of one of the sides of the clamp, showing at q how the bricks are packed so as to give a batter to the outer wall of burnt brick, o is a layer of breeze 4 to 7 inches thick, p a layer 5 to 7 inches thick.1
Fig. 7 is a longitudinal section of the clamp - which may be of any length.
The operation of burning takes from two to six weeks. A good deal depends, however, upon the situation and direction of wind. The nearer the live-holes are together the quicker the burning.
The completion of the burning process is indicated by the settling down of the top of the clamp, caused by the shrinking of the bricks as they become burnt.