Lost wax casting

Lost wax or “cire-perdue” casting goes back at least as far as 3,000 B.C.E.  Archaeologists have uncovered evidence of this type of ancient metalwork on every continent except Australia.  The enduring popularity of this technique is due in part to the fact that it captures very fine detail, even without expensive equipment.

Lost wax casting can be very high tech. Modern vacuum equipment can run tens of thousands of dollars. So can a commercial-sized burnout kiln or melting crucible. But skilled artisanal casters can still achieve excellent results with homemade equipment.

The rough beginning of the corgi head door knocker

The rough beginning of the corgi head door knocker.

The original sculpture

The first step is to carve a model. For jewelry, I started with a wax very similar in hardness and working properties to wood. Like wood, green jeweler’s wax can be carved with knives and tiny hand tools, or worked with a dremel. For sculpture, I occasionally use brown microcrystalline sculpture wax, but more often these days I model in plasticine or polymer clay. Starting with reference photos and drawings, I model the rough shape with my hands, supporting it with an armature if needed, then carve details with hand tools.

Clay is a very forgiving medium. If you cut off too much, you can add more clay and try again. But still, it can be a long, slow, process.  There are times when it’s hard to imagine that the piece will EVER look like what I want.  It’s kind of like Mount Rushmore. When they first started blasting, I’m sure the designer looked at the jagged standing rocks and the heap of rubble and wondered if it would ever pull together. But bit by bit, it does.   It usually takes several weeks of carving and tweaking (and walking away and coming back with fresh eyes) to perfect a design.

Finished sculpture for the corgi head door knocker

Finally, the finished corgi head. sculpture which will become a door knocker

The studio mold lets me cast a wax to work on, as well as a cold cast model for display.

The studio mold lets me cast a wax to work on, as well as a cold cast model for display.

The first mold

When the sculpture is finished, I make a temporary mold in my studio and make a cold-cast model, both to see the influence of color on the piece, and to have something to show to potential buyers. If it still looks good, I use the same mold to cast a wax. Using the cold-cast piece for reference, I carve and refine any details that need touching up. It’s this wax that goes to the caster.

The second mold

I use a small artisanal caster right here in Vermont.  He makes a professional-quality mold from my wax, which will allow him to make perfect copies of the original.   The model is set aside in case something ever happens to this mold.

The permanent mold is used to cast wax replicas of the original.

The permanent mold is used to cast wax replicas of the original.

The wax tree

Each new wax is checked carefully for defects.   Any lines left by the mold are trimmed and redetailed.   Then pairs of waxes are joined back-to-back to make what’s called a “tree”.   We usually cast each design in batches of six (three pairs of waxes).

The ceramic shell

Each pair of waxes is dipped in layers of ceramic slip and sand to form a shell. It takes a week or more to build up the layers, allowing the shell to dry each time. Then the finished shell is fired in a kiln, causing the wax to burn away from the inside.

Caster inspecting a row of door knockers dipped in ceramic slip and sand to create the shell for casting.

Casting in metal

While the ceramic shell is being heated, the artisan melts the metal For one or two pieces, the metal can be heated with a torch in a graphite crucible right on the top of the work bench. For bigger batches the metal is melted in a forge or kiln.

Bronze melts at 1748 degrees. When the metal is hot enough to flow, the caster pours it through the cup-shaped opening into the still-hot shell.

Metal finishing

After the raw castings have cooled, the ceramic shell is chipped away and the sprues (the channels that the metal followed through the plaster) are cut and ground away. If the sculpture has been cast in more than one piece, the parts have to be welded together, and every seam ground and redetailed.

Then the piece is polished and finished with a patina to enhance the detail and preserve the color of the metal through the years.

applying the patina to a freshly cast bronze dog doorknocker

applying the patina to a freshly cast bronze dog doorknocker

The patina

three irish water spaniel bronze door knockers in different patinas

The Irish water spaniel bronze door knocker by Elizabeth Trail