"When the newly introduced plaster is set the mould is carefully knocked away with chisels, and a true cast appears beneath, giving an entire fac-simile of the original model. Some skill is required in making moulds, in order to provide for projecting parts and under-cuttings; practice alone can teach the artist how to deal with those difficulties when they occur. The above general instructions sufficiently explain the ordinary processes of moulding and casting in plaster.
"In metal-casting or founding great attention must be paid to the strengthening of the parts to bear the weight of the metal; but the principle described in plaster-moulding applies also to the preparation for metal-casting. The mixture of metals to form bronze, the proper heating of the furnace, burning and uniting parts, chasing and other processes of founding cannot be fully described in this place. They belong to a distinct practice, and to be well understood must be studied in the foundry.
"If the model--now reproduced in plaster--is to be copied in marble or stone, the first step is to procure a block of the required size. Two stones, called _scale-stones_, are then prepared, upon one of which the model or plaster cast is placed, and upon the other the rough block of marble. The fronts of these stones have figured marks or 'scales,' to use the technical term, exactly corresponding. An instrument capable of being easily moved, and which is fitted up with socket-joints and movable arms, is then applied to the scale-stone of the model, and a projecting point or 'needle' is made to touch a particular part of the model itself. This is carefully removed to the scale-stone of the rough block, and the marble is cut away till the 'needle' reaches so far into the block as to correspond with the 'point' taken on the model. A pencil-mark is then made to show that the _point_ is found and registered. This process is repeated all over the model and block, alternately, till a rough copy or shape of the model is entirely made. These 'pointing' machines are not always precisely alike in their forms, but the principle upon which they act is exactly similar in all. The statue being thus rudely shaped out, the block is placed in the hands of a superior workman, called a 'carver,' who, having the plastic model near at hand to refer to, copies the more minute portions of the work by means of chisels, rasps, and files, the pencil-marks made by the 'pointer' showing him the precise situation of the parts and the limit beyond which he is not to penetrate into the marble. When the carver has carried the work as far as the sculptor desires, he proceeds himself to give it the finishing touches, improving the details of form and expression, managing the different effects produced by two different materials--one, the plastic model, being opaque; the other, the marble, being considerably diaphanous; giving the proper varieties of texture in the flesh, hair, and drapery, and, more especially, harmonizing the whole.