MOULDING AND PRESSING.
On this point, Mr. G. H. Brown speaks as follows : " Having got your clay well tempered,' the next thing is to pass it through a machine which will compress the utmost amount of clay into a given space. Your bar of clay must be as dense, solid and hard as you dare to work your machinery. A soft bar of clay means a great shrinkage in the drying and burning, and an exaggerated change in the size of the brick in passing from the green to the dry state is sure to entail more or less check ing and warping, and the result be a weak or distorted brick, to say nothing of the difficulty of handling, and a greater per centage of loss. Everybody knows there is a vast difference in the shrinkage of different clays ; but in my judgment, when you are making a stiff-mud brick, make it stiff as you can, both by heaviest possible pressure in the machine, and by avoiding too lavish use of water." If the clay is to be molded into brick by a stiff-clay brick machine, by which class of machines nearly all paving-brick are now produced, it passes by gravity from the clay-bin to the hopper of the brick machine, and is tempered and molded into brick, which are uniformly end-cut after the bar of clay reaches the cutting table.
Soft-clay brick machines are used in molding some clays, and side-cut stiff-clay brick machines are also sometimes em ployed.
If the clay is to be molded into brick by any of the ordinary forms of sewer-pipe machines, it passes from the clay-bin by gravity through a chute into a 7-foot wet, or tempering pan, where the different clays are thoroughly mixed and incorpo rated into a plastic mass by the addition of a sufficient quantity of water. After being thus made plastic, the tempered clay, on being discharged from the wet-pan, is again elevated into the second story of the building and discharged into an automatic feeder, which feeds the material into a sewer-pipe press. The brick come from the press usually in eight streams of suitable width and thickness for paving-brick, and are cut off into suit able lengths.
Paving blocks can be molded by either a stiff clay brick machine or a sewer pipe press. If the blocks are being molded on a pipe press, the clay passes through a die which allows only six streams to be emitted from the press, and these blocks are cut into 9 inch lengths, and it is common to make 20,000 of them in one day of ten hours.
It has not heretofore been usual to re-press paving brick made on stiff-clay brick machines. Some manufacturers of paving brick do, however, re-press them, and it pays to do so, as the brick by re-pressing acquire a greater density. The brick in
this way become non-absorbent, and are of uniform size and finish. Such brick present a uniform surface for the passage of vehicles when the brick are laid in the roadway. The liability to flake or spall is also lessened by re-pressing.
The time has come when manufacturers of brick and blocks intended to be used for the paving of public roadways can no longer afford to put such brick upon the market without re pressing them. The only reason why paving brick and blocks have not, in all cases, been re-pressed in the past, is the fact that manufacturers of this class of clay wares have been so crowded with orders that they have been enabled to put upon the market large quantities of brick and blocks which ought never to have left the yards where they were made. The rapid extension which is now going on in both the construction of new plants devoted to the manufacture of street paving brick and in the enlargement of the capacities of such plants already in operation, make, in a great measure, the shipment of inferior paving brick and blocks in the future almost impossible—the reason of this being that there is now a more general and thorough knowledge concerning the qualities which are neces sary to be possessed by vitrified brick and blocks, if they are to be used for the paving of roadways. A paving brick, if it is properly made, should be so dense as to make it impossible for such a brick to absorb even an ounce of moisture. Such a brick must, in addition to great density, be true in shape, hav ing good corners, heads, faces and arrises in order that the brick, when laid in the roadways, shall lie close to the neigh boring brick and present a smooth, uniform surface. There is no possible way by which these desirable qualities can be im parted to a paving brick, except by re-pressing.
Paving brick, with of course fusible toughening elements, are properly prepared for the greatest constructive element, fire, by pressure ; hence, as a sequence, their absorbent conditions are practically wiped out.
Common building brick generally, on account of their uses, are not subjected to great pressure ; hence absorption rules at from ten to seventeen per cent.
What gold and silver are to commerce and the arts, what chemistry is to life ; what order is to nature, so is pressure to all paving brick products.
The Chicago Roadway Paving Ordinance, which was recently passed, provides that all brick used for paving purposes in that city " shall be of the kind known as re-pressed brick, and shall be re-pressed to the extent that the maximum amount of mate rial is forced into them." •