The object of this process is to give to the bricks when they come from the ordinary machines the sharp edges and regular faces lost by them under the manipulations to which they have been subjected when in a soft state, however much care may have been given to their treatment. In other cases stamping is intended to further compress together the molecules of the clay; this is the case when bricks made by hand or by the lever press (Fig. 89) are so treated.
Fig. 157. Installation of Whittaker Machines.
Stamping takes place a certain time after manufacture, - twelve, twenty-four, and forty-eight hours, - in order that the bricks may have become firm, and may undergo no further change of shape after the operation. We must, however, avoid any drying of the surface, for a too dry clay would not take the shape of the mould. The hand-stamping formerly practised exists no longer since the introduction of stamping machines or "rebatteuses." We know that the invention of these ingenious machines is due to Brethon, a Tours manufacturer. All the other machines are only copies of that press. It is composed (Fig. 158) of two cast-iron standards bolted to a solid wooden platform and joined at the top by a cast-iron table which carries the mould. These standards carry a cast-iron piece called "poitrine," to which the cap of the machine is fixed by two rods. The "poitrine" also has a central opening fitted with little steel friction rollers; those at the top are movable about an axis, those below are fixed. Between these slides passes a special piece quadrant-shaped and fixed to a curved shaft which turns on two bearings in the two uprights. At the end of this shaft is fastened a lever with a counterpoise.
Fig. 158. Brethon Stamping Press (Chavassieux).
The mould is formed of a cast-iron piece (Fig. 160) placed on the table and held by bolts. The inside is often lined with copper; the bottom (Fig. 160) is movable. It rests on the table, and the lower rod with which it is furnished passes through an opening in the table and rests against the "poitrine"; it is this rod which removes the brick from the mould.
When the lever is raised (Fig. 161) the plate at the bottom just touches the upper part of the mould; the distance between the cap, which is furnished with an adjusted plate just sliding in the mould, and the upper part of the mould is large enough for a brick to be. introduced. This distance is regulated according to the thickness of the brick, by raising or lowering the cap, which is fixed by four bolts to threaded rods.
When the brick is placed on the movable bottom, the lever is sharply brought down with some violence (Fig. 162). The shaft to which it is fixed turns through a certain angle, and the quadrant-shaped piece, resting on the lower roller of the "poitrine," brings the latter down and with it the upper cap.
Fig. 159. Stamping Press (Jager).
Fig. 160. Stamping Press (Lacis et Cie).
The bottom of the mould, the rod of which is no longer supported by the "poitrine," drops, but soon meets the cast-iron table and stops. The brick is then compressed between this bottom and the upper cap, and thus takes perfectly the shape of the mould, at the same time acting as a spring and throwing the lever forward again. This movement is assisted by the counteqioise, and it is sufficient to seize the lever to bring it back into its first position. But in rising, the shaft has brought back the quadrant-shaped piece which it bears, and by this motion has raised the "poitrine" again, consequently also the cap and the rod of the movable bottom of the mould: the brick reappears in its former position, but stamped or "represst'e" as it is called.
Fig. 161. Stamping Press (Boulet).
The ascending movement of the "poitrine" is facilitated by the revolution of the friction rollers on which the curved piece acts. This revolution, to be as smooth as possible, requires lubrication. Moreover, the vertical rods, which run in guides and sockets placed on the cast-iron table, also require lubrication. These rods bear all the strain of the sliding motion, and are soon worn; this gives play to the machine. To remedy this wearing away, the cap is sometimes furnished with two other rods which slide in sockets (Fig. 158), or sometimes the ends of the cap are guided by fixed slide-bars, as in Figs. 159 and 160.
Fig. 162. Stamping Press (Lacroix).
Fig. 163. Stamping Press (Whitehead).
In these figures the "poitrine" is hidden by a sheet-iron plate to protect the frictional parts from dust, and the curved piece is surrounded by a sheath to avoid accidents.
The action of the machine is very simple, as can be seen from Fig. 162, which shows the workman bringing the lever forward and the boy preparing to put in another brick.
This machine and the following ones (Figs. 158-165) are based on the same principle as the foregoing. They only differ as to details, and especially in the way in which the lever moves the cap, and so exercises the compression. In the Lacroix and
Boulet machines, the framework is in one single piece instead of consisting of two uprights; this makes the machine more rigid.
In the Joly machine (Fig. 165) the movement is effected by means of a cam working on friction rollers; the whole piece bearing the rollers is guided below by a shaft sliding in a ring fixed to the base of the machine.
This press produces considerable effect, but it is less easily handled than the preceding ones.