The brass has made a natural cut in the plaster mould, thus dividing it into sections. These are removed from the model, washed with water, soaped with green soap, melted in hot water and allowed to cool. Then with a dry brush all soap is carefully removed from the mould and a slight coating of olive oil is given it to insure the proper separation of the mould from the cast. The sections are now put together and held in place by burlap dipped in plaster, or tied with wire, and the mould is ready for filling. In order to remove this mould it must be chipped from the cast, because the undercutting of the cast does not allow the sections to be lifted off as in the piece-mould.
The cast of a bust or other simple work can be made in one piece. The sections of the mould are united, and the plaster at a proper consistency poured into it. By continually turning the mould the plaster may be evenly distributed. A thick ness of three-quarters of an inch is sufficient to make a strong cast, especially when it is reinforced inside with burlap dipped in plaster, or with iron pipes. Where the model is small it may be entirely filled, thus forming a solid cast.
In the case of large works, in which there is no way of moving the mould around, the cast is made in sections. When set, the sections are joined together and properly strengthened on the inside, great care being taken that all joints are clear of plaster so that they may fit perfectly.
Sufficient time should be allowed for the plaster to set properly, then, with a mallet and chisel the outside iron pipes are removed and next the natural coloured plaster, leaving the thin coloured coating. This should be removed with care to insure a plaster cast without a blemish. The plaster cast can be re-touched with plaster mixed to a thinner consistency than the casting plaster and care should be taken to wet the model before applying.
Casts.—Plaster is made of equal parts of plaster and water, the plaster being slowly poured into the water and given sufficient time for absorption, after which it should be stirred gently. There are various ways of tinting plaster casts so that they may resemble bronze, terra-cotta, etc. One of the most common is the oil paint method. One coat of shellac should be given as a prime. After this has dried thoroughly a coat of oil paint of brown with a little dryer is applied and allowed to dry. if a greenish blue tint is desired a very thin mixture of light green oil paint is applied to the brown coat. When this is almost dry it is wiped off with a rag, allowing the green colour to remain in the deep parts; for a metallic effect, a little gold or bronze powder mixed with dryer may be applied to the high parts. When thor oughly dry it is rubbed with ordinary prepared furniture wax. This will subdue any unpleasant shiny effect and bring the whole thing together. Similar procedure may be used with any other colour desired.
(See other sections of SCULPTURE TECHNIQUE preceding this article, such as CASTING AND FINISHING; MODELLING.) (L. LEN.) PATINA Patina, strictly defined, is an artistic product of the corrosion of copper or copper-rich alloys, notably the bronzes. It is an
"incrustation which forms on bronze after a certain amount of exposure to the weather, or of ter burial beneath the ground. When perfectly developed it is of a dark green colour and has nearly the composition of the mineral malachite, a hydrated car bonate of copper." In the broader interpretation of the term, patina includes mineral or mineral-like coatings or deposits on metal objects of art of a large variety of shades—greens, blues, reds, greys, blacks, etc.—the deposits having been produced by either natural or artificial means.
In this article we are primarily concerned with the patina formed on bronzes, the verde anticho or verdigris, the usual con stituent being the basic carbonates of copper, malachite and azurite. At times the patina will be distinctly red (cuprous oxide) ; at other times we will find it to be greyish green and frequently, as is the case with many Chinese and Japanese bronzes, it is almost black, resembling polished ebony.
A good patina of the proper composition and texture serves a double purpose. It enhances the beauty of the bronze object and secondly it protects and preserves the bronze from destruction by the very agents, such as carbonic acid, so necessary to form the patina. The second purpose of the patina, therefore, is like that of a permanent coat of protective paint.
Most patinas are green or greenish blue. The grey patinas are usually due to a predominance of tin oxide or lead carbonate in the crust. The black patinas are usually found on bronzes containing appreciable quantities of lead. How ever, there are many different intermediate shades of patina; likewise there are many different textures. The main factors that influence the shade and texture of the patina are (I) the chemical composition of the bronze; (2) the metallographic struc ture of the bronze, whether fine or coarse grained, presence or absence of intercrystalline impurities, etc.; (3) the location in which the bronze has remained before its discovery; and (4) the time or duration the bronze has remained in the particular loca tion. Thus, for example, some of the finest greens are found on bronzes of pure tin-copper alloys and on objects that were fash ioned of native copper. The addition of zinc and lead to the bronze darkens the shades very appreciably. A fine grained crystalline structure of the bronze favours a uniform fine tex tured patina. As regards location, absence of moisture and salts usually results in the characteristic bronze brown. Presence of sulphur compounds in the air to which the bronze is exposed, as for example in the path of the winds passing over active volcanoes, gives rise to very dark almost black shades of patina. Slow oxidation in alkaline soils but in the absence of carbonic acid will produce the comparatively rare red (cuprous oxide) patina. A short time in a moist salty location will produce a heavier or thicker patina than a comparatively long exposure in an almost dry location.