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.
After having examined the bust so as to be thoroughly acquainted with its difficulties, the workman proceeds to cut off with a twisted wire the projecting portions of the heard, and the hair, which, from the cavities of the locks and curls, would present difficulties for casting. The parts thus removed are afterwards easily replaced. The bust is now reduced to a very simple instead of the complicated form it at first presented. The plaster mould is then made in the ordinary way: the bust being laid on a table, face upwards, is fixed in that position by lumps of modelling clay so that one-half of the thickness of the bust is completely covered, the remaining half presenting the appearance of a figure floating on its back in water. The workman then begins to make the pieces of the mould: taking the liquid plaster, which is of the consistency of thick cream, he forms a cube 2 in. high, and the same length and width, which he squares as soon as the plaster begins to harden; with this cube of plaster he covers a first portion of the surface of the bust; close to this first cube a second is formed, and so on until the whole bust is covered with an irregular mosaic of plaster cubes, care being taken to prevent them from adhering to each other or to the bust by the application of a strong solution of soap.
The surface of these cubes, after being well wetted with this solution, is covered over with a very thick coating of plaster, which is called the cope, the place of each cube having been previously marked; the first half of the piece-mould is now complete. The moulder then turns the bust with the face down on to the table, fixing it as before, and proceeds to cover the back in the same way with cubes of plaster, so that when this second half is also covered with a thick plaster cope, a complete mould is formed in 2 halves. The great art of the moulder is to make the piece-moulds at the same time simple and solid, and fitting so closely together as to leave the least possible trace of the joints on the plaster cast produced from it; care must also be taken that in handling the mould none of the small pieces should detach themselves from it. The mould being completed, it is opened, that is to say, the 2 plaster copes are separated, the bust which is intact is taken out, leaving a complete mould in which other busts can be cast just as bullets are cast in a bullet-mould. The next operation is the reproduction of a bust in wax, precisely like the original in plaster.