The first step is creating a scaled sketch in wax, which serves as a guide for the work. The next step is to model the clay statue (with internal reinforcement to prevent fractures) in its definitive dimensions, called the “core”, which is then baked to transform into terracotta. Because of the natural shrinkage caused by baking, the terracotta model is slightly smaller than the final result. A wax layer is applied to the model to reach the final dimensions of the piece. Modeling the wax must be carried out with care in all of its details, because the final appearance of the statue depends on the wax. Then a series of small tubes in various sizes are applied to the wax covered “core”, called vents, with support sprues. Then another layer of clay is extended along this “porcupine” surface (called “coating” or “mold”), from which the open vent holes protrude.
Then the model is once again baked in the kiln slowly to melt the wax and let all of it drip away through the vents. In the meantime the fire also transforms the clay into terracotta and the support sprues permit the creation of a hollow space between the “core” and the “casing”, where the molten bronze will then be poured.
Before proceeding with the final pouring, the entire structure is covered with mortar, creating the so-called “casting hood” which is reinforced through iron plates. The hood is melted away into a space underneath the kiln where the molten bronze will be poured. The bronze, entering into the hollow space, forms the statue, with a thickness equal to that of the eliminated wax.
Once poured, and after cooling (one or two days), the statue is erected and the casing and hood are broken away, and the statue appears as a form of bronze tubes (vents) and nails. To prevent any risk of dilation, the core in terracotta is extracted, usually from the base, or through openings which must then be repaired. Any incomplete parts must be poured again and welded. After the nails and vents are eliminated, the statue may appear very rough depending on the molten metal used, so an extensive period of “metal chasing” may be necessary, including sanding of the surfaces (milling and polishing), integration of any defects and elimination of improperly cast areas (with the insertion of the so-called blocks), refinishing of details (often through engraving and chiseling) and the elimination of all imperfections.
In some cases a final coating or gold-leaf operation is also part of the process, which is carried out by applying a thin layer of a mercury and gold mixture to the surface. When the piece is heated the mercury evaporates, leaving the gold coating.