Hollow bricks, first manufactured mechanically in France by M. Borie in 1850, are bricks pierced longitudinally, transversely, or perpendicularly with cylindrical or prismatic holes which pass from one face to the other parallel to an edge. In France, those most used are pierced longitudinally with prismatic holes.
The preparation of the clay, the soaking or moistening, is performed in the same way as for solid bricks; impurities such as roots and stones should be carefully avoided, as they would cause a loss in manufacture.
In some machines, all traces of impurity are removed by passing the clay through a sieve. The paste is generally firmer than in the case of solid bricks, since more consistency is required to prevent the brick from losing shape.
The moulding is done by expression machines, the compression of the clay being effected by means of propelling cylinders, screws, or pistons; all these machines are similar to those we have described in speaking of solid bricks, only the dies being different. Figs. 266 to 269 give different views of a die which gives passage to three hollow bricks having eight holes in each. The outer edges of the brick are cut by the die, and to produce the holes bronze pieces are added which carry well-finished parts of the dimensions of the holes, and these parts keep back the clay. The hollow separating them from the outer edge forms the thickness of the brick. Each row of holes requires a special piece, as may be seen by the view of the inner side (Fig. 267). These pieces are movable, for being of bronze they wear out rather fast, and they must be renewed as soon as the holes in the bricks become too narrow.
Sheet-iron combs are frequently placed against the die above and below the prism of clay; the sharp teeth of these bite into the fresh clay and dig grooves in it which will increase the hold of the plaster or mortar when the bricks are employed for partitions, the commonest use for these products.
Fig. 266. View of Outer Side.
Fig. 267. View of Inner Side.
Fig. 268. Section CD.
Fig. 269. Section A B.
The dimensions of the die as well as the distance between the wires of the cutting-table vary according to the direction in which the holes are to lie, as shown in the following table. We suppose a brick which is to be after firing: .22 x .11 x .055 -
Dimensions of the Hollow of the Die.
Distance of Wires.
Bricks pierced longitudinally .
,, „ transversely .....
,, ,, perpendicularly. .......
As in the case of solid bricks, the same die may pass several bricks at the same time, either the issuing orifices being separated, as in Fig. 266, or there being one single orifice, and the brick being divided, as it issues, by well-stretched wires (Fig. 130).
The dimensions are the same as for solid bricks. Hollow bricks naturally require less clay for their moulding than solid ones, therefore large machines of high production are not suitable for this work, machines of moderate production being better.
Figs. 270 to 287. Various Hollow Bricks (Perrusson and Desfontaines).
The drying and firing of hollow bricks do not differ from those of solid ones, but they require less time and less fuel, hence the sale price is lower.
In France, hollow bricks are generally parallelopipeds with holes in the direction of the length (Figs. 270 to 279, 295 to 297), but in Germany, where nearly all buildings are of brick, hollow bricks of the same shape as the solid bricks are used, and we find bricks with cut off corners (Figs. 301 to 303), laporte "hourdis" (Montchanin Manufacture) rounded bricks (Figs. 304, 305), and finally moulded bricks (Figs. 306, 307). These products being used in facings, the holes must be placed perpendicularly to their large surface; when they are headers, they may be perpendicular (Fig. 299) or transverse (Fig. 300). Finally, bricks with cut off or rounded corners are made having holes in the direction of their length (Figs. 290, 291, 292).
Fig. 288. For Arched Flooring.
Fig. 289. For Straight Floor.
Figs. 290 to 309. Hollow Bricks of Different Shapes (German Manufacture).
In giving to bricks special shapes like those in Figs. 310, 311, and 314, different pipings of great diameter are produced (Figs. 312, 313).
Hollow bricks are much used for the arches of I-shaped iron floorings (Fig. 316). We see from the figure that the beginning of the arch does not fit the shape of the iron on account of the wing"; therefore, to remedy this inconvenience, bricks of special shape are manufactured called "briques a sommier" (Fig. 275).