Baking The Mould

When the mould has stood for a few minutes, with the aid of a knife cut a small groove round the back, towards the iron frame. Turn the mould on its back, and lightly tap the frame, when the plaster will drop out in its entirety. Superfluous plaster is trimmed off with the knife, and notches are cut on the top sides of the plaster rim, that the molten metal may gain admission to the face when put into the dipping pan.

The plaster cast is next baked in the oven, whose proper heat is about 400° F. (204°C). The mould is introduced between 2 of the partitions in the oven on its side, and allowed to remain for about 1 1/2 hour, by which time it will have become sufficiently baked, and will assume a brownish hue. Meantime the dipping pot and floating plate are likewise put into the oven, on the bottom shelf, in order that they may attain the same heat as the mould.

Testing The Metal

Before pouring, it is necessary to test the metal, as unless it is hot enough, it will not flow freely under the cast, and the plates will lack sharpness or become chilled; if too hot, the mould is liable to crack when immersed. The test mostly applied is that of inserting a piece of paper in the metal, when the paper should acquire a straw colour. If the metal is too hot, the draught of the fire must be reduced or a little cold metal added. The dipping pot or casting pan, when sufficiently heated, is slid along the iron shelf to the front of the metal-pot, and the floating plate, which is of the same size as the bottom of the pan, is put inside, the workman being provided with pads of thick flannel while handling them.

Casting The Plate

The first precaution is to ensure that the pan, plate, and mould are of nearly one uniform temperature : if the plate is colder than the cast it will cause a sudden contraction of the latter; if warmer, a sudden expansion, either of which will probably crack or warp it. Some workmen prefer to heat the floating plate by immersion in the molten metal. There must be no delay between placing the dipping pot in position, the floating plate inside, the mould on the top, and fastening the lid. After removing the cast from the oven, should anything unforeseen occur to prevent its being immediately placed in the dipping pot, it must be put back in the oven till heated again, together with the pot and plate.

Should the cast be much smaller than the floating plate, small plaster cubes, previously prepared, may be placed round the sides, to prevent it moving about in the pan. The cover is next put on and secured by means of the clamps and screw, the clamps attached to the chain on the crane being fastened into the sockets on the side of the pan. The ratchet is wound up and the whole is swung above the metal-pot, then gently lowered until the top is on a level with the surface of the metal. By tilting the clamps with one hand, the side of the dipping pan is gently dipped at one corner into the metal, allowing the latter to flow in only at one corner, so that the air may be driven out at the other openings, the pan being entirely immersed only after all the air has been expelled. When the pan is full, gently lower the whole into the metal, allowing it to rest on the bottom of the pot. Care must be taken that the metal in the melting pot is not allowed to run too low, so as to ensure that when the mould is placed ready for dipping, there is sufficient metal to cover the top of the pan.

When new metal is added to the pot, the temperature of the mass will be considerably lowered, and no cast should ever be made without first testing the temperature.

By its greater specific gravity, the molten metal presses up the floating plate and the mould to the lid of the dipping pan, and forces itself through the notches cut in the side of the plaster into every part of the mould. The pan should remain in the metal for about 10 minutes, during which time the floating plate for the next casting may be placed in the metal, allowing sufficient to remain above the surface to enable the operator to obtain a firm hold for its removal.

Cooling The Cast

When the pan has remained in the metal for the time stated, it should be gently raised, swung round to the cooling trough, and allowed to rest on the supports made for the purpose. Care must be taken that it be swung in a perfectly horizontal position, or the metal will be liable to flow to one side and thus render the thickness of plate uneven. As the metal cools, it contracts considerably, and more metal must be poured in at the corners of the dipping pan, to make up the deficiency, and to exert the necessary uniform pressure on the cast. This pouring must be repeated several times during the cooling. As the water in the trough sinks owing to the rapid evaporation, further supplies should occasionally be added to maintain the required level.' The cooling of the cast (which properly occupies about 20 minutes) must not be hurried, or the mould will split, and the metal will run into the crack. If the cooling operation is hastened in the slightest degree, the sudden contraction of the metal on the surface of the newly-formed plate will cause the letters to lose their clearness of outline.

The water in the trough should be high enough to saturate the pieces of blanket, but it must not be allowed to touch the pan bottom.

Knocking-Out The Plate

When the pan has completed its cooling, it is lifted on to the knocking-out block; then loosen the clamps, and remove the lid by inserting a strong chisel at the corners. Turn the pan upside down, and give a smart blow with the mallet on the bottom, when a block of apparently solid metal will drop out. Let it stand for a few minutes to allow it to become still colder, then turn again, the widest part uppermost. When the metal is sufficiently chilled, strike off the extreme corners with a mallet, being careful to hit away from the bulk. Next break away the sides, striking from the top, and, as before, away from the body of the metal, or the plate will be injured.