When should machine manufacture be preferred to hand manufacture? The question is not always easy to answer. It is evident that the first condition is a considerable annual output, which we should estimate at two million bricks. In a factory of such importance every machine working by hand or animal power should be rejected - pug-mills, expression machines, etc. The work of these is as good as those driven by steam-power, but their small output makes their employment a costly one. They can only be recommended in small works for making hollow articles; they have also their advantages in new countries where animal-power is the only kind available.
But a large output is not the only condition in favour of mechanical manufacture. Factories doubtless exist which produce millions of bricks by hand, and such a quantity of productions can only find a market in a great centre of consumption; but in such a centre many hollow bricks and chimney-pipes are used, that is to say, many hollow articles which require machines. As soon as the manufacturer needs power for one part of his work, it is certainly advantageous to use it for the remainder. It is true that his capital sunk is increased, but, on the other hand, he is no longer at the mercy of the inclemencies of the weather, which cause serious losses, and he no longer needs that vast space which hand-work requires.
In case the brick-maker is also tile-maker, there can be no doubt machinery is necessary.
Once this question answered, another presents itself: what machine is to be chosen? That is more difficult to answer. In a general way it can be said that all machines are good; but to get good results we must choose the one which suits the clay to be worked. Many disappointments have occurred through a mistaken choice of machines and the use of one unsuitcd to the clay under treatment. We must insist, then, on this important point: It is not every machine which is suited for treating a given clay. Anyone who knows the diversity of clays will understand that only a practical examination of a clay will teach us the most suitable way to treat it. Nevertheless, taking the principal varieties of clay, we shall be able to give a few general hints as to the tools to be used in the manufacture of bricks by expression machines.
Weathering is always to be recommended but is not indispensable. Clays which have not weathered need to be passed between cylinders to crush the lumps and make sure of a better damping. The latter will be done in the mass, or more quickly with a moistening machine. Then the clay will be pugged and moulded by a machine with propelling cylinders; we do not advise screw machines. We pass over clays containing stones; it is better not to use them. Nevertheless, if the stones do not inconvenience the firing, such a clay can be used after being passed between crushing cylinders and reduced to powder. In the contrary case, and if no other clay can be used, stone-removing machines must be employed. To summarise, the following processes will be carried out, according to the circumstances of the case: -
Clays with stones. Crushing or stone-removing by cylinders.
Clays without stones.
Weathering or Rolling. Rolling or Damping.
The same remarks as to weathering apply. Treated in the fresh state, these clays are cut into pieces in a mixing mill, then dipped into ditches with or without the addition of anaplastics. The mixture is pugged, and then moulded by means of cylinder or screw machines. When more or less dry clays are used, the mixing is replaced by crushing with antiplastics or other clays. Sometimes the sand used as an antiplastic contains hard lumps which have to be crushed; then it is passed with the clay between cylinders which effect a first blending. Then the treatment will consist of the following processes, according to the state of the substances employed: -
Pugging and shortening.
Rolling with antiplastics.