In the Foundry
Lost wax - In the Foundry
The lost-wax technique is long and complex: all its phases are manual and require expert and very skilful workers to avoid errors that can compromise the casting.
Briefly, it is necessary to create a wax mould of the work that is then enclosed within two moulds made of refractory material, an internal one, known as “core”, and an external one called “mantel”. At a successive heating, the wax liquefies and burns leaving within the mould an empty casing. In this empty space the liquid bronze is poured. When mantel and core are removed, cleaning, chiselling and patination begin. This too is a strictly manual process that can last months or even years depending on the size of the work.
NEGATIVE MOULD
A negative mould is drawn from the original, which can be in any material, even in bronze if a replica is to be made. The negative mould must be in a material that when it solidifies maintains elasticity and can thus be peeled off even from very complex figures.
REALISATION OF THE WAX AND TOUCHING UP
The negative mold is used to obtain a positive copy of the work in empty wax. The author of the work or another artist will retouch it to eliminate any defects or imperfections. A red dye is added to the wax to facilitate the retouching work. Its thickness will determine the thickness of the bronze: it is precisely this wax that will be ‘lost’ and replaced by the molten bronze.
APPLICATION OF “DRAINS”
The wax sculpture is encased with a series of reed sticks, the “drains”, whose task is to allow vapours to leak out during casting and the bronze to reach all parts of the sculpture unimpeded.
COVERING IN REFRACTORY MATERIAL
Once the “drains” have been placed, the wax is filled inside and covered externally with refractory material, the so-called loto, a blend of ground brick and chalk. Remaining visible are the extremities of the “drains” and the hole into which the liquid bronze is poured.
HEATING OF THE MOULDS
The loto mould that contains the wax sculpture is heated in a furnace. This leads to the melting of the wax and burning of the “drains” thereby creating an empty casing within the mould. Heating and cooling must be gradual to avoid cracks: some ten days are required to heat and cool the moulds.
INTERMENT OF THE MOULDS
After heating, the moulds are covered with pressed wet earth. This prevents the deformation or cracking of the loto moulds during the casting of the bronze, whose specific weight is approximately 9 times higher than water.
CASTING
The bronze alloy (90% copper and 10% tin) is melted (880-1,020°C) and poured into the moulds. The gases produced leak out, together with air, along the “drains” created around the sculpture.
DE-CASING AND CLEANING
Once cooled, the moulds are lifted out and broken to remove the cast. The “drains” – now filled with bronze – and all loto residues must be removed from the rough bronze at this stage. When large sculptures are involved, casting occurs in separate pieces, which must now be assembled and soldered together.
CHISELLING
The sculpture is now handed over to expert bronze workers who start the filing and chiselling process. With great care and patience, for a time that can take as much a year, these artisans bring the bronze surface to its maximum splendour.
PATINA APPLICATION
Bronze can take up differing colours depending on the reagents that are applied. Patination is a process that accelerates the oxidation that would normally occur to the bronze through weathering.
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