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Two processes are used to switch from plaster to bronze models: lost wax casting and sand casting, which was used in Rodin's time but is very rare today. In both cases, the molten bronze is poured into a mold around a core which is then removed. In the end, the bronze work is hollow, which lightens its weight and cost. The lost wax casting process begins by making a soft mold - formerly made of gelatin, taken by impression on the plaster or clay model. After removal from the mold, the model is carefully stored. The flexible mold, held by a plaster cap, is filled with a very resistant material that will serve as a core. Its shape, slightly reduced by a thickness that will be that of bronze, is placed back into the mold and held at a distance from the walls by metal rods. Between the core and the wall of the mold, liquid wax is poured, which hardens. The mold is opened. The gelatin mould is cleaned for possible reuse. Porte de l'Enfer, designed for a project that had no sequel, becomes a repertoire of shapes with infinite potential. We thus find, in his research for the monument to Victor Hugo, Rodin's propensity to make the figures migrate, to combine them, to modify their dimensions or their position in space. In the project for Balzac, going beyond the criteria of resemblance, he opts for a synthetic and symbolic vision of the great man who upsets the notion of a public monument. The marbles evoke subjects drawn from mythology and illustrate abstractions in a symbolist spirit, whose eclecticism is served by a combinatory aesthetic. Rodin, prized as a portraitist, also produced variations on the face. In affinity with the Impressionists, he is interested in perceptive phenomena and the mobility of forms under light. In 1900, Rodin was at the height of his fame. For his retrospective at the Pavillon de l'Alma, he reread his own work, and oriented the way he wanted the public to see it.
The wax print, still containing the core, is retouched and signed by the artist. The number of the print and the stamp of the foundry are inscribed on it. A network of wax rods is created around it. The whole is wrapped in a refractory earth capable of withstanding the temperature and pressure of the molten bronze. This casting mold is introduced into a furnace to bake it and melt the wax. Inside, the emptied wax rods have become: the drains, through which the wax has been evacuated; the jets, which will receive the molten bronze; the vents, which will allow the air to escape. The mold, still reinforced, is lowered into the casting pit. It receives the molten bronze, which must spread rapidly inside, in the void left by the wax that has flowed out. When the bronze has cooled, the mold is broken. Then begins the long work of finishing: cutting the supply network, crumbling the core, chasing and polishing the surface. Finally, metal oxides are applied to the bronze. Through controlled corrosion, they form the patina, which protects the surface of the work and colors it with a brown, green, blue or black tone.
