Plaster of Paris models prepared by templatea should be made to the required ecale when a large repetition of the same object is likely to be required, and more than one plaster mould wanted for making olay impressions. Plaster is the best ground or framework for clay tuodels of enrichments and bassi relievi, when more than one piece of the same ornamental design is required. The com bined clay and plaster model are readily moulded with plaster in the ordinary way. If the model is entirely in clay, and full of undercut foliated work, say in alto relieve, or a statue, many pieces would be required in such a mould, and an experienced plaster-moulder only should be employed, or the clay model may be spoilt. It is best to make as few pieces as possible, and arrange to get, whenever practicable, all the small pieces into two outside cases, the back of a statue in one case, and the front in another ease. Grease or oil should not be used in making plaster moulds for terra-cotta, ae is done in the common way of making plaster piece-moulds for casting. A little clay and water, mixed about as thick ae good milk, will answer all purposes for the prevention of one piece of plaster sticking to another in piece-moulding.
Original, life-size, terra-cotta statues are modelled and burned, without the process of moulding, by building up the clay in a cellular form, and working with the fingers and modelling-toola on the surface. Extended limbs should be made solid in the first instance, and when the clay is stiff, hollowed out, jointed with soft clay to the body of the figure, propped up with clay supports of the same stiffness and sge aa the mass of the statue, aud slowly dried for the firing. Some difficulties and dangers arise in moving large statues into a kiln, and frequently they are finished by the sculptor in the kiln, which is for the time illuminated by gas.
Very large fountain-basins and vases are made by forming on a bench, with a trap-door in the middle, the core, or inside, with clay worked by a template from the centre. When the core is turned in this way, it should be covered with thin sheets of paper, and upon this paper the outer surface of the basin is turned with another template from the centre, by laying sheets of clay over the prepared surface of fairly uniform thickness, pressing and uniting this outer May which forma the basin well together. The template will now turn the outside of the basin. When the clay has stiffened, ornament may he rnodelled on the surface, or pieces of enrichment, made from plaster moulds, may be uted on with clay slip. It will be observed that, during this process, the basin is upside clown, and is resting partly on the clay core. After the clay has become tough, the trap door in the centre of the bench may be removed from beneath, and the clay core pulled away in pieces, leaving the basin gradually to dry alone ; when dry, it should be turned over very carefully, examined 118 to finish and truth of surface, and removed to the kiln, resting only on a roll or disc of clay at the bottom, leaving the rim and upper portion quite free for equal contraction in burning.
Basins, vases, and other articles, 12-20 ft. in circumference, have been made sound and true in every respect in the way described.
Many persons have a just objection to the piracy of architectural details especiEblly modelled for their own use. Working such details out originally in terra-cotta is a check to this piracy. If an artistic work is executed in marble or stone, a model in common clay is made and moulded, and a plaster cast is taken to guide the carver, and this rough model is often used for all sorts of cement and compcbition castings. But the terra-cotta clay model is wrought out, undercut, and finished at once by the sculptor and burned, and when only one or two designs alike are required, it is in a money sense the cheapest and best method of working. The following are a few of the forms which can be safely and economically made as original terra-cottas :—Altars, arches, architraves, balconies, brackets, bassi relievi, bosses, busts, columns, capitals, chimneys, crosses, cornices, coats-of-arms, finials, fire-places, friezes, park seats, key-stones, medallions, tracery, per forated panels, rustic work, tablets, tombs, animals, canopies, caryatides, festoons, statues, trophies, scrolls, corbels, date tablets, terminals.
When much repetition is required, the use of plaster moulds is common; now and then, for flat surfaces, metallic and wooden moulds are used. The workmen employed in making impressions in moulds are called "moulders." The plaster mould is usually dusted with a little finely-ground flint, grit, or sand, and the clay, having been properly prepared, is beaten out on Et very thick block of plaster of Paris, to the required thickness, and then firmly pressed by hand on to the surface of the mould. If the mould is in two parts, or eases, both pieces are partially filled with impressed clay, the edges of such clay are scratched with a tool, moistened with water, and a little soft clay is placed over tbe edges, when one portion of the mould is lifted over the other, and squeezed down so that the luted edges join perfectly. The clay and the plaster mould remain undisturbed until the porosity of the plaster has absorbed a portion of the water from the clay, when the outside casings and the smaller pieces of the mould are removed, and .the article pressed appears showing the joints or seams of the mould on the surface. These are in due time removed by a modeller, or careful finisher. If the pieces to be made are large, struts or cells are formed to strengthen them, as nearly all terra-cotta works axe hollow. The hollow parts of architectural details are generally filled up with good stiff mortar, selenitic, Portland, or Roman cement, with fragments of brick or tile. Great care should be taken if Portland cement is used that it does not expand, which it is apt to do, sometimes cracking the terra-cotta.