Wildcat bronze in progress

In 2014, I was commissioned to create a memorial for a long-serving and beloved superintendent of schools, Gehrig Johnson, who retired after 30 years of service.  The school mascot being a wildcat, a bronze figure embodying the spirit and energy of the school and it’s leader was selected.

I documented much of the process of creating this work, and thought I’d share what goes into creating a bronze sculpture.  It’s not a simple process – although the modern version uses some better materials, the casting process has been the same for about 7,000 years.  Bronze first appeared between 4,000 and 3,000 B.C.E.

After creating the concept, I created a model in orange polystyrene foam that roughed out the shape, and placed it in a simulation of it’s planned installation.  This confirmed that the concept would work at scale, and illuminated some of the engineering issues that would need to be addressed.  Then I refined the shape, and painted the foam with white shellac.  The shellac prevents the foam from contaminating the modelling clay overlay that is applied next.  The modelling clay can hold fine detail that the foam can not, and is a more workable material.  (Why not work entirely in clay?  It would be too heavy to support its own weight, as well as too expensive. )

Detailing the sculpture is fussy work – each hair of the fur as well as the larger parts of the anatomy need to be defined, with attention both to the abilities of both the clay and bronze to hold the shapes.  A steel armature acts as scaffolding  to stabilize the model.

Once the clay model is complete, it is coated with a liquid rubber, which cures to form a mold.  The cured mold is coated with a think layer of plaster for support.  The mold is removed along a parting line; the inside is now a negative image of the work.  It’s impractical to cast this form in one piece; I work with the foundry to decide how many sections are required, and where the parting lines go.  The inside of the mold sections are a negative image of the sculpture.

Next, the inside is coated with wax to the desired thickness of the bronze; the wax is then removed from the rubber mold.  This results in a positive image in wax.  Imperfections are removed by working the wax with hand tools; this is called “chasing”, and is similar to the process of modelling the clay originals.

The foundry adds wax rods, called sprues and gates to the wax.  These will form channels that allow the molten bronze to reach all parts of the sculpture.  They also allow air to escape as the metal enters the molds.  Then the wax (positive) model is coated with a high-temperature ceramic, which forms yet another mold (called the investment).  It is cured in an oven, melting the wax, which drains out and/or burns.  The ceramic molds are embedded in sand, and molten bronze is poured in via the sprues.

Once cooled, the sand is removed and the ceramic broken off.  Just as with the wax, imperfections must be chased from the bronze.  Then the sections are welded together, and the seams chased so that they become invisible.

The assembled work is sand-blasted to prepare the surface for patination.  A patina is applied using various chemicals at medium temperatures, and and a sealant applied.  Finally, the piece is ready for installation.

As we’ve seen, the process of creating a bronze sculpture is complex, and involves many skills and a massive commitment of time and materials.  The result can be as lifelike as this wildcat, or very abstract.  But the process of bringing it to life, while equally enjoyable, is quite different from the stonework described elsewhere on this site.

Although one might think that the use of molds allows for duplicates, the manual chasing of the wax and bronze, and the inevitable artifacts of casting and assembly ensure that every bronze is unique.

Is a bronze in your future?  Let me know…

For more views of work in progress, click here.

© Andreas von Huene