The object of this is to take from the bricks coming from the machine or mould the uncombined water contained in them. The quantity of this is very variable: while it may be neglected in products made from dry clay, it reaches a maximum in bricks made from soft paste.
Drying is an important process; if it is complete, the firing will be easily and economically effected; if incomplete, the firing is rendered difficult and more costly.
In factories which continue work during the winter, drying should be carried out in closed drying-rooms warmed by direct heat, or, as is more generally the case, by transmitted heat from the kilns. In brickworks which are only open during the summer, the drying is done in the open air without shelter, or under sheds, or in storeyed drying-rooms.
This is the most economical method; although excellent in hot countries, it often causes disappointment in northern countries, on account of the frequent rain; very often it has to be abandoned, and sheds are constructed.
The arrangements made are as follows:
Near the place where the moulder works, a rectangular piece of ground of about a yard or a yard and a quarter in breadth, and of a length depending upon the space available, is raised in level. On each side of this bed two trenches (R R, Fig. 171) are dug in the following manner. A line is stretched in position, arid along the line lumps of earth are taken up with a spade, and laid in a row by the cord. These lumps form the boundary of the raised plot; the trench is about a foot and a half wide. The space between the two parallel rows of clods of earth is filled with soil taken from the trenches, and if necessary from elsewhere; when well rammed down and dressed this forms the foot of the hack P.
Fig. 170. View from Above.
Fig. 171. End View.
Fig. 172. Side View.
On this foot the bricks are arranged in open-work hacks, the bricks being placed in an oblique direction which changes for each layer (Figs. 170, 172).
The first layer is sometimes composed of fired bricks, in order that the dampness of the earth may not soften the unbaked bricks supporting the whole hack; sometimes a layer of straw is put down. These precautions, which are useful at the beginning and end of the season, are less so in the middle. Two layers at a time are stacked along the whole length, and thus four layers may be placed in position and then left to recover for a day or two so that they may harden and be able to support further layers. The vertical rows thus placed one over the other are called "feuilles"; the two middle "feuilles" are first erected, and as the air passes quickly between the bricks, drying is fairly rapid. For the sake of rigidity, not more than eight or ten rows should be placed on each "feuille," but the bricks at the foot are then dry enough for a new "feuille" to be begun on each side of them, so as to make altogether four "feuilles." When their height is sufficient to consolidate the middle "feuilles," more bricks are placed on them. Generally a height of 15 to 18 layers is attained, that is to say, about 5 to 7 feet. The top of the hack is so arranged that the straw matting 1 placed on it has the slope of a roof. Instead of matting, which soon wears out, tiles are often employed.
It will be easily understood that the total length of the hacks depends upon production, since only four layers at a time can be placed in position. For works producing 6000 to 7000 bricks a day, it will be seen that a length of 1 500 to 1800 bricks will be required, that is to say that, counting 0.08 m. for the thickness of the brick and intervals, we must have one, or, better still, several hacks, with a total length of 1 20 to 140 metres. In the heat of summer this length is not necessary, but it is better to have it arranged in advance in order to avoid inconvenience in any case of bad weather. The great disadvantage of open hacks is that they are exposed to this. Therefore in a large number of factories they are sheltered by light wooden sheds like those represented in Figs. 173 and 174 and called "hallettes"