This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The bust thus prepared is placed on the brick layer in the place in which it is to be fired; it is then surrounded by a wooden case, having the form of a 4-sided truncated pyramid. This case, which must bo sufficiently large to leave a space of 6-8 in. between it and the greatest projection of the bust, is made of frames placed one upon the other, 9 in. in height, the whole, when placed together, having the form of a pyramid; the first frame, namely that which rests on the brick layer, being naturally the largest. The case being ready, the cube measure of its capacity is calculated, and the upper frames are removed, leaving only the lower one resting on the brick layer. The mould is made of precisely the same material as that forming the core of the wax bust; the requisite quantity is prepared as well as the proper number of measures of water required for mixing the "potin." As the operation of filling the frames must proceed rapidly, and, once begun, cannot be stopped, care must be taken to have a sufficient supply of the material at hand.
For the formation of the cope of a large-sized bust, 3 men are required for mixing the "potin," 2 for pouring it into the frames, and 2 for throwing the mixture on to the bust, which is done with painters' brushes, and in such a way as to thoroughly fill up all the cavities of the sculpture.
The 3 mixers have each before them a vat or bucket containing one measure of water, into which they pour rapidly the dry "potin," which is in the form of fine sand or powder, and this not all at once, but gradually, by allowing it to fall through their fingers; when the "potin" is all in the water, the men work it into a paste with their hands. As soon as it is ready, the other men pour one after the other the contents of the 3 vats or buckets into the lower frame of the wooden case; in the meantime the mixers are preparing fresh vats of "potin." As soon as the first frame is nearly filled, the second frame is placed above it, the joints being closed with "potin" that has become almost hard, and it is filled in the same way; at the same time the other 2 men, armed with brushes, have been sprinkling the bust with the mixture so as to fill up completely all the cavities of the wax bust; if this is not done with great care and exactitude, any cavity that is not filled with "potin" will retain a certain quantity of air, and when cast the cavity will be entirely filled up with a solid mass of bronze which would require to be removed by the chaser at a considerable expense, or it may happen that the fault is one impossible to remedy.
When all the frames have been placed one upon the other and filled with "potin," the operation is completed, care having been taken to fill the upper frame only to the level of the top of the runner and the vent, so as not to cover them.
A third channel, required for draining off the melted wax, is formed in the same way as the other two, a stick of wax 1 1/4 in. in diameter being placed at the base of the bust on the slant, so as to facilitate the issue of the liquid wax, the stick of wax being fastened by one end to the wax of the bust, while the other end touches the wood which forms the case. The "potin" having been allowed to harden, which it does very rapidly, the wooden frames are removed, and the cope appears in the form of a block of stone, on the upper surface of which is seen, on the right the wax of the runner, and on the left that of the vent, and at the base that of the drain.