Lost Wax Bronze Process

Creating the art work

Generally for art-bronze casting a full scale wax of the artwork must be created. For smaller pieces the artist can work directly in sculpture wax.  For larger sculptures the sculptor often works in non-hardening oil base clay, applying the clay to a metal armature or skeleton.

If the sculpture was created in clay or some other non-wax medium, a rubber mold must be taken from the original.  For larger sculptures metal shims are used to divide the clay into sections, for easier handling.  Wax is then painted onto the rubber mold until a 3/16” buildup is achieved.  After the painted-up waxes are pulled from the rubber molds and parts are reassembled, the surface imperfections are “chased” away with wax-working tools.

The finished wax is then prepared for casting by attaching a series of wax tubes called sprues and vents. The sprues will later become the entry tubes for the molten bronze and the vents the exit for air and hot gases. Often large sculptures remain in pieces and the parts are sprued and vented together for placement in the same mold.

The sprued and vented waxes are then placed in a fireproof mold called an investment or shell made from liquid plaster or ceramic. The ends of the vents and a wax cup at the top of the main sprue are left exposed at the top.

These molds are then placed in a kiln and burned out, at a temperature of around 1200 degrees. During this burnout the wax drips and burns away leaving an open cavity exactly like the wax original with channel ways where the sprues and vents were.

The burned-out molds are very fragile, so they are buried in a sand pit for support when the bronze is poured in.  

Bronze is then melted to a temperature of between 1950 and 2300 degrees Fahrenheit depending on the complexity of the mold.  Before pouring, impurities are skimmed from the surface of the bronze.

During the pour, care is taken to agitate the bronze as little as possible.  The bronze is poured in the burned-out cup at the head of the main sprue.  The bronze flows through the sprueing system, pushing air and bronze gases ahead of it, finally flowing out the vents at the top of the mold and completely filling the cup. The cup is covered with a refractory material to slow its cooling, for shrink marks appear where the bronze last cools and shrink is preferred in the cup and not the artwork.

After the bronze has cooled to a manageable temperature the mold material is carefully broken away,  revealing the bronze, which should look just like the original wax with the sprues and vents attached.

Since the spures and vents are now bronze they must be cut away to free the pieces of the sculpture.  Large sculptures are hollow with 3/16” bronze walls, which means they have an investment core that must also be broken away.

Once the mold material is removed, the pieces are rough-chased with a variety of metal-working tools.  This process removes air bubbles and other imperfections that occur during the casting process.

Now the pieces are fitted and welded together.

After welding a more detailed chasing is done, which blends the weld seams and adds any more desired detail.

Chasing completed, the bronze is sandblasted to blend all working marks. 

The sandblasting also creates a slightly rough surface that will give the chemical patina a chance to bite. The final color of the sculpture may be made up of many layers of patina. Patina is not paint but a build up of chemicals that react with the bronze. 

 After the patina the surface is sometimes further chased and then lacquered or waxed to stop the chemical reaction. Other patinas are left unchecked and the patina process allowed to continue.