"In modelling in _rilievo_ of either kind, _alto_ or _basso_, a plane or ground is prepared upon which the design is, or should be, carefully drawn. This may be made of clay floated or laid upon a board, or the ground may be of slate, or even of wood, though the latter is objectionable, in large works especially, from its liability to shrink and to be warped by the action of damp or moisture. The clay is then laid in small quantities upon this ground, the outline being bounded by the drawing, which should be carefully preserved; and the bulk or projection of the figures is regulated by the degree of relief the sculptor desires to give to his design.
"If the final work is to be baked in clay (_terra-cotta_) there must be no iron or wooden nucleus, as it would interfere with the model drying regularly and uniformly, and probably cause it to crack in shrinking. The model is therefore prepared for drying without such support. When perfectly free from moisture the model is placed in an oven and baked slowly, by which it acquires great hardness and the peculiar brownish-red color seen in these works. This art has been brought to great perfection in England in modern days.
"If the final work is to be in marble, or bronze, or only in plaster, the next process after finishing the model is to mould it, in preparation for its being reproduced in a material that will bear moving about without risk of injury to the design. This is done by covering it with a mixture of plaster of Paris with water, which quickly sets or becomes consistent, forming a hard and thick coating over the whole. The clay is then carefully picked out, and an exact matrix, or form, remains. This is washed clean, and the interior is then brushed over with any greasy substance, usually a composition of soap and oil, to prevent the plaster with which it is next to be filled adhering too firmly to it. The fresh plaster is mixed to about the consistency of cream and then poured into the mould, which is gently moved about till the inner surface is entirely filled or covered, so that all parts may be reached. The thickness or substance of the coating depends upon the size of the work and the degree of strength required.