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Bronze casting technique In Siam, as in South-East Asia generally, craftsmen almost always used the lost wax technique. (With very insignificant modifications it is still used to this day.) First a rough model of the future sculpture is made out of clay. The clay core is preserved in almost all sculptures, except the smallest statuettes. If the sculpture is to be a large one (taller than 150 cm), the body, limbs, head and other large elements are cast separately and then joined together. After it dries and hardens, the core is covered with a thin even layer of beeswax and tree resin. Then on this wax surface the craftsman cuts all the fine details of the future sculpture: facial features, folds and details of clothing, decorative motifs. It is at this stage that the professional standard of the craftsman reveals itself, together with his adherence to a particular style. The depth of the wax coating determines the thickness of the walls of the future sculpture. Funnels or sprues are attached around the bottom of the pedestal: the metal will be poured in through these, replacing the melted wax. When a large sculpture is to be cast, sprues are also placed on the back and forearms to ensure the better dispersion of the metal. Unremoved sprues can often be seen on seated figures - they resemble the feet of the pedestal. This is especially common in the sculpture of the Far North. On top of the wax, the craftsmen apply several layers of clay coating that effectively forms the actual mould. The most important layer is that immediately adjacent to the wax: it needs not only to perfectly fill all the fine detail and contours of the wax surface, but also to be of equal thickness all over. In all three inner elements of the mould there must be no roughness, empty spaces or other defects - otherwise these will later be repeated in the metal. Several layers of coarser clay mixed with sand are applied over the fine clay. Then, for greater strength and stability, the whole thing is bound around with a sort of metal net before being coated with several more layers of clay. At last the fairly shapeless massive construction is left to dry out completely away from direct sunlight. Several medium-sized sculptures are placed in the furnace together. For very large sculptures individual furnaces are constructed. If the mould for a sculpture is relatively small, it is first placed upright, with the sprues at the bottom; when all the wax flows out, the mould is inverted and metal is poured in through the sprues, which are now at the top. Heavy and awkward moulds are at once placed upside-down on a grille, after holes have been made on the opposite side to allow the molten wax to flow out. The metal for casting is prepared in large clay crucibles. The components for the alloy were always exceptionally pure, without impurities. But besides the pure metal ingots, in the course of smelting other things were added: captured war trophies, old damaged statues and pieces of them, and also the donations of local inhabitants inspired by motives of piety - domestic ritual utensils, jewellery, coins. Thus the composition of the alloy varied from crucible to crucible. Consequently in the course of a single casting session different sculptures cast in one furnace, or even parts of a single large sculpture could differ in composition. Once cast the "wrapped" sculpture is allowed to cool, then the mould is broken with a hammer, freeing the finished piece. Even when casting under the most ideal conditions minor technical errors are bound to occur and any cast article requires prolonged finishing: the sprues and burrs are removed, the casting flaws are corrected and so on. After all the mechanical operations are complete, they begin polishing and then gilding it. Gold placed directly on the metal does not hold well and quickly rubs off. To avoid this, a base coat of several layers of black lacquer was first applied and only then was the gold applied over it. Black lacquer was obtained from the resin of the hak tree (Melannorrhae usitata). In the Far North it was used not only as a base for gilding, but also as a decorative technique in its own right. |
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