Another essential part of modelling is preaerviu; the moisture of the clay, which should be always uniform if possible ; it must never be allowed to dry, and it can be kept moist with very little troitble.
While the modeller is at work, and the figure is exposed, especially in warm weather, he should repeatedly sprinkle it with water. A plasterer's brush is the best instrument for this purpose, and much superior to a syringe or the mouth, which some sculptors formerly used to apply as a squirt, even in the presence of their sitter, when modelling a bust : Nollekens, according to his biographer Smith, adopted this elegant mode of keeping his clay moist while modelling the bust of George III., and in the king's presence. Bacon, on a similar occasion, used a silver syringe. At night time, or when the artist is not at work on the model, it should be covered over with a wet cloth or sheet. A figure may be kept moist for a long period without adding water, provided the air be kept away from it ; this may be done by an oil-silk or any air-proof bag, which can be made fast to the plinth of the banker by clay. Sculptors generally model flowing draperies from lay-figures. When the model is complete, the next process is to take the cast, to work the marble from, or to make tither casts from. The whole model, while wet, must be covered, in two or three masses, or more if necessary, with plaster of Paris ; when this is fixed and dry, the whole may be separated at the joints, with out any regard to the preservation of the model, for when the mould is taken the model is no longer of any value. When the clay is com pletely removed from the mould, the component parts of the mould must be again put together, and in the place of the original clay it must be filled with plaster of Paris, and when the cast is well set, the mould may be carefully broken off in fragments, and the cast is ex posed, and complete, the finished work. If casts of it are required, a new working mould, or safe mould, as it is termed, must be taken, in many parts. If the figure is to be executed in marble, it is copied by the carvers, with the assistance of the pointing-machine, which is so contrived that it can diminish or increase the scale of the model with perfect ease and nicety. It is always best to make the model of
the size of the intended figure if practicable, because any error in a small model becomes multiplied in a larger one in proportion to the difference of size. Flaxrnan was in the habit of making small models, and he had in consequence sometimes immense labour to go through to diminish errors in the full-sized marble work : it is hardly possible to completely rectify them.
The ancient sculptors used to bake their models, but this is not so good a plan as making plaster casts from them, though less trouble some and much cheaper : the clay in drying shrinks, and is apt to crack, and certainly never comes out of the oven the same shape that it was when put into it, or at least when originally modelled. These baked models are called terra-cotta (baked earth) figures, arc extremely numerous, and are generally of small dimensions, but there are a few of a Large size in various European museums. The ancients made also moulds of clay, which they likewise baked, and they formed their casts by pressure of clay into these : this practice of pressing clay, or any malleable substance, into a mould is still occasionally had recourse to iu works of fine art, and constantly in the potteries, and by frame makers.
The ancients used also wax for casting and in forming their models, especially those of the small bronzes, which are still so exceedingly numerous, and it is the common material used by goldsmiths and medalists. Modelling wax is prepared by melting virgin wax with a very small quantity of Venice turpentine and flake-white in fine powder : if coloured wax is required, a colour in fine powder must be substituted for the flake-white. The tools used in modelling in wax are made of wood and ivory, and have the same shapes as those of wood, already spoken of, for modelling in clay. Other processes in the art of sculpture will be found explained in the articles BRONZE; FOUNDING ; SCULPTURE ; and IVAx MODELLING.