Bronze Portraits ›› Lost Wax Process
The lost wax method of casting is by far the most common technique in use today and the basics of it has changed little in thousands of years. Contemporary foundries have the advantage of modern tools and equipment, but it is still a complex, time consuming, highly skilled craft.
The information below is just a brief and condensed explanation of the lost wax process, and what happens when your commission makes that wonderful transition from clay to bronze.
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Sculpting the portrait
The initial head is modelled life-size in clay, over a period of days or weeks. When complete, the artist’s job is almost over, and it is transported to the foundry to begin the casting process.
Creating a mould The clay head is used to make a master silicone mould, which is backed up with a plaster or fiber glass supporting jacket. The mould may have to be made in several sections depending on the complexity of the clay original.
Create a wax cast
Hot wax is poured into this mould and poured out again, building up an even thickness. Once cooled, the mould is opened and the flexible silicon is peeled away from the wax, to reveal, a hollow wax version of the original sculpture.
Apply sprue system
After the seams on the wax have been repaired and the surface texture restored where necessary, a complex sprue system of wax rods (runners and risers) are welded to the wax with a hot knife, these allow the molten metal to run into the sculpture, and the air to escape during the pouring.
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Investment
The inside and outside of the wax is coated with a fireproof material called the investment. When dry, the investment is heated in a kiln to melt out the wax, leaving a gap inside, shaped exactly like the hollow wax.
Pour metal
Molten bronze is poured into the investment via the runners and risers to fill this gap.
Break off investment
When cool, the investment is broken away, to reveal the rough bronze cast of the original sculpture, complete with its runners and risers.
Cleaning up the casting
The long and patient task of cleaning up the metal, removing the runners and risers and restoring the surface texture of the sculpture where necessary (chasing and fettling).
Patinating
One of the final processes is to chemically induce a colour to the surface of the bronze using oxidizing agents and heat.
Wax and Polish
Once the desired patination is achieved, a coating of wax is applied to the bronze to protect the finish.
Mounting
The finished piece is mounted on a plinth and ready for display.
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