This is effected by compressing the powdered mixtures in metallic moulds with powerful hydraulic presses worked in different ways. Some have a revolving table (Fig. 791), and the water, compressed by a steam pump (Fig. 790), acts directly, or through accumulators, which is preferable for the uniformity of the products.
Fig. 790. Pump for Hydraulic Press.
The table bearing the moulds, three or four in number, is turned by hand or with a small pump round one of the uprights of the press, in such a way that the compression of one empties the one just pressed, and a third is meanwhile being prepared. The compression is transmitted from the pumps or accumulators to the press by a distributer provided with a lever. Two accumulators are often used, one of which gives the low pressure (50 to 80 atmospheres) and the other the high pressure (200 to 250 atmospheres). One accumulator serves for four or five presses, but two will be required if both pressures are used.
Fig. 791. Hydraulic Press (Boulet).
The full mould is placed upon the piston, and the workman, by moving the lever of the distributers, gives the lower pressure in order to expel the air from between the grains of the powder. If this precaution were not taken, the bubbles of air disturbed by the pressure would accumulate in a horizontal plane; in the firing, they would become heated and would cause accidents, especially in insufficiently baked quarries. At the proper moment, therefore, this air must be allowed to escape, and then the high pressure is applied by means of the distributers.
When the quarry is finished, a workman withdraws it while another full mould is pushed over the piston. The distributer is turned to low pressure; the piston, by means of a series of levers, sets in motion in its ascent the demoulder, which presses on the quarry previously pressed and expels it from the mould.
Fig. 792. Hydraulic Press with Distributer and Accumulator (Laeis et Cie.).
Fig. 793. Hydraulic Press with Pump Distributor (Laeis et Cie.).
Working with accumulators and a single block, three workmen produce 180 quarries per hour; with double blocks, the production reaches 250 quarries. The direct action of the pump only allows of 140 quarries per hour.
Moulding of Incrusted Quarries- - This was the first object of manufacture with powdered clay. The pattern of the quarry is reproduced by means of a network of thin metallic bands soldered together, whose height is equal to the thickness of uncompressed incrustation (three to four inches). The cells thus formed receive the coloured powders. With the chambered network, thin metallic plates are prepared, in which are cut out for the punch the parts of the pattern which are to be of a different colour; each colour then requires a plate whose openings correspond exactly to the hollows in the chambered frame (Figs. 794, 799). Each mould must have its own network and its different lids.
All the moulding pieces being previously prepared, the work is then divided; each colour requires a workwoman. In front of her is a compartment from which she draws the coloured mixture; the latter she puts into a small sieve, and, a lid (Fig. 795) having been placed over the network, the workwoman shakes the sieve over a funnel with which the mould is provided (Fig. 801); the pulverised clay falls, and fills the disclosed empty space. When it is full, the lid is taken off, and the mould passes to another workwoman, who puts on it another lid having an orifice corresponding to another colour (Fig. 796). This process is continued until all the colours are placed.
Figs. 794 to 799. Networks for the Manufacture of Incrusted Quarries.
The moulds are made in two parts, which are placed one over the other; the lower part has the thickness of the uncompressed incrustation, and the upper part that of the paste. When the first has been filled as above described the caliber is removed, a slight pressure is applied with the hand or a mandrel, and the second part of the mould, filled with powdered ordinary clay, is put on. Chambered effects are produced by giving a greater thickness to the copper blades composing the caliber. When this latter is removed, a hollow is left which is filled with a black paste, and then, a slight shaking motion having been given to settle the powders, the filling is completed with ordinary paste, and the quarry is pressed.
Fig. 800. Press with Circular Table for Incrusled Quarries.
Fig. 801. Press for Incrusled Quarries.
For limited production the preceding presses may be used, or the one represented in Fig. 801; for large output, it is better to use presses with circular tables of large diameter (Fig. 800).
The moulds run on rails and pass in turn before the workwomen who are to fill them. When the pattern is completed, the second part of the mould is placed over and filled; then the whole is pushed under the press. Removal from the mould is effected by means of a small hydraulic press.
The kilns used are intermittent and reverberatory, in the style of that described on p. 205 and the following pages.
Figs. 802 and 803 are the section and plan of a kiln of the same kind which is working in an important factory in the North of France. The furnaces are ten in number, the draught passes through the centre of the kiln, and the flames follow the course indicated by arrows. The quarries are packed with sand in large saggers; the stacking must be judiciously done, the quarries whose colouring oxides best resist the heat being placed in the hottest parts of the kiln. Care should be taken not to put in the same sagger quarries whose colours have different properties, etc. The fire, which is pushed gently at first, reaches its maximum at the end of the operation; the temperature is then from 1200° to 1400° C, sometimes as much as 1500° C, that of dazzling white. The consumption of coal depends upon its quality; as an average, we must allow 25,000 kilog. for a kiln which contains 40,000 kilog. of quarries, which represents 1000 square metres of pavement.
Continuous kilns with separate chambers, even those using gas, do not seem to give satisfactory results. A semi-continuity may, however, be established by arranging intermittent reverberatory kilns in a battery of four or six.