These are not cut up, they are passed between cylinders to crush the lumps, then soaked, alone or with anti-plastics; the mixture is then pugged, and afterwards moulded by cylinder or screw machines. Let us now examine in a more detailed manner each of these operations.
Crushing and stone-removing require no special remarks; it will be sufficient to refer to what we have previously said on the subject. As to rolling with cylinders, it must not be forgotten that the action of the cylinders is more rapid when their diameter is greater. If the clays are not held by the smooth cylinders, recourse must be had to those with points or flutings.
Moistening and soaking should receive special attention from the manufacturer. In brick - making we have not to bear in mind, as we must in tile-manufacture, certain considerations in favour of working on soft clay in preference to firm or hard clay. We must choose a convenient degree of dampness, which gives products neither too soft nor too firm. Pastes which are too soft undoubtedly require less power, but the products are not easily handled without loss of shape; hard pastes have the advantage of giving bricks firm enough to bear a pile of five or six without loss of shape; but considerable force is expended, machines are quickly worn out, and there is a risk of producing exfoliated pieces; moreover, the output is small. The best thing is to choose the happy mean between these two methods, that is to say, to mould with a semi-firm paste, the products of which can be placed in stacks, three or four bricks high, without loss of shape. In this way the production will reach its maximum with a moderate use of motive force.
The more plastic a clay is, the better can it be expressed in a firm state, subject to the remarks above.
Pugging is always effected by blade or screw machines: this is the best way to get a homogeneous paste, and in this respect cylinder-pugging is not to be compared with it Horizontal pug-mills are preferable to vertical ones for effecting difficult mixtures with substances of different density.
Moulding is done by machines with cylindrical or screw propellers; the latter are not recommended for the treatment of thin clays, which cannot bear the same manipulation as rich clays. For the latter both kinds are suitable, but some manufacturers are inclined to prefer cylinder to screw machines. In any case, when using the latter, it is advisable to take machines in which the screws have a high pitch but not too great a length.
When the same machine is to make, besides bricks, chimney-tops and pipes, we may use a cylinder machine of the type of the Joly machine (Fig. 101) on account of its arrangement, which allows of its producing these articles (Fig. 575).
Another important question to be decided is, whether it is better to put up one single machine capable of producing the whole daily number of bricks required (machines can be found which produce up to 10,000 bricks an hour), or to divide the production between several machines of moderate power. In our opinion the latter course is better in most cases, far it must not be forgotten that a powerful machine is not suitable for the manufacture of hollow bricks, and it is easier to regulate production with several machines than with a single one.
It may be useful to point out the precautions which should be taken to ensure the regular working of machines, to whatever system they may belong.
When empty, a machine should be easily turned by hand; if there is a difficulty, the cause should be ascertained (tightness of bearings, want of oil, etc.). The distributing or propelling cylinders should be two to three mm. (about 1/10th in.) apart, and even less if the product to be expressed is of small section.
The scrapers are placed near the cylinders, but not touching them. Whenever the thickness of the clay sticking to the cylinders becomes too great, the scrapers should be brought nearer. This is very important, because the output diminishes if the cylinders are not properly cleaned. The dies should be kept very clean, and care should be taken not to drop oil on the leather of those with water-face. Before fixing them to the machine, they should be dipped in water for a few minutes. It is better not to leave them on the machine during a long cessation of work; they should then be cleaned.
The cutting-table should be placed quite horizontal in front of the die; the skin-covered rollers should be moistened, and should be on the level of the lower edge of the die. The wires are placed at the proper distances starting from the pallet and tightly stretched. In order not to strain them uselessly, they may be loosened during stoppage of work. The rollers should always be kept clean, and those made of plaster should be renewed as soon as they are perceptibly worn.
When starting, it is recommended, especially in the case of thin clays, that the interior of the compression chamber should be filled by means of the hopper with soft clay; the first products which issue arc rejected; this precaution facilitates the passing of the clay under treatment. It will also be advisable, if it is observed that the production diminishes, to take advantage of a stop to take out the closely compressed clay fitting the chamber, and to substitute some soft clay for it
In screw machines fitted with cylinders, the production of these latter should be equal to that of the screws, otherwise there will be an obstruction which will force the clay under the scrapers; this defect can be remedied by bringing the cylinders closer together.
Generally speaking, whenever the output of a machine diminishes, we must seek the cause, which is often found to be an error in damping, too firm clay, cylinders too far apart, scrapers too far away, die out of order, screws worn, etc., all of which causes are easily remedied.
The maintenance of the machines belongs to the domain of the ordinary routine of any mechanical factory; we will not therefore lay stress upon it.