Lost wax casting for jewelry

 

 

 


Pembroke Welsh Corgi mother and puppy pin on stone and moss backdrop

Casting in molten metal

Lost wax or “cire-perdue” casting goes back at least as far as 3,000 B.C.E.  Archaeologists have uncovered evidence of early lost wax casting  on every continent except Australia.

The enduring popularity of the lost wax method is due in part to the fact that it captures very fine detail.  And it can be done without high-tech equipment.

Models are carved in a special hard green wax

Making the model

My first step is to carve a model in wax.  For jewelry, I start with a wax that’s 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.  It  takes fine details, and at the end of the project, it can be polished to a high shine.  

Wax colors are fairly standardized across brands, with green wax the hardest, followed by softer blue and purple or orange waxes.  Red and turquoise waxes are formulated for casting copies in injection molds, so they’re more fluid when melted.  And brown microcristalline wax is softer and stickier, excellent for large scale sculpture.

I start with reference photos and drawings.  I make the rough shape with a dremel, then carve details with wax tools.  Wax is a very forgiving medium.  Like clay, if you cut off too much, you can add material back on and try again.  When I get stuck, I’m glad to have my own living models close at hand. My dogs, cats, and horses are philosophical about being pressed into service so show me a pose, or exactly how a nose is constructed or how hair blows back on the run.  It usually takes several weeks of carving and tweaking (and walking away and coming back with fresh eyes) to perfect a design.  

The finished model is cleaned with a wax solvent that removes surface imperfections and loose bits of wax, (preferably without melting any detail at the same time.)

Wax model for Irish Water Spaniel bracelet link . The attached wax tube is the sprue, which will create a channel for the molten metal to travel from the outside of the plaster shell to the design.

A wax model with sprue attached

Preparing for casting

Next, I attach a sprue — a slender rod of wax —  to the model, using sticky wax or a hot wax pen.  Multiple sprues can be attached to a central wax cylinder like branches to the trunk of a tree. In fact, the process of attaching wax models to this central channel is called “treeing.”

Multiple waxes being sprued to a tree

Then the sprued model  or tree is centered in a rubber base and set upright into a stainless steel cylinder.  The cylinder is filled with a special investment plaster that tolerates high heat without crumbling.  Like plaster of Paris, investment plaster self-cures due to a chemical reaction.  But investment plaster is finer grained to capture more detail.  And it holds up to high heat.

After the investment sets up, the rubber base is peeled away, leaving a cup-shaped depression in the plaster at one end of the cylinder leading to the trunk of the tree.

When the plaster is fully dry, the cylinder is heated in a kiln to about 900 degrees.  At that temperature, the wax models, sprues, and tree melt and vaporize, leaving a complex set of channels in the plaster leading to the voids left by melting out the wax models, now “lost.” The cup-shaped depression at the top of the cylinder connects the tree to the outside air.

Melting metal with a torch

Casting the metal

While the plaster cylinder 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 metal is melted in a forge or kiln.

Gold melts at 1948 degrees fahrenheit, silver at 1763 degrees, and bronze at 1748 degrees. When the metal is hot enough to flow, the caster pours it through the cup-shaped opening into the still-hot cylinder.

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.  Small artisanal casters melt metal with a hand torch a few teaspoons at a time, and cast with a spring-operated desktop machine that uses centrifugal force to “throw” the metal out of a small crucible into the plaster cylinder.

Finish work

After the raw castings have cooled, the plaster is chipped away and the sprues (the channels that the metal followed through the plaster) are cut and ground away.

A rubber mold makes it possible to cast copies

If there is to be more than one copy of the design, a vulcanized rubber or silicone mold is made around the perfect first casting of the original design, so more waxes can be made with all the detail of the wax original.

Finally, pin backs, pendant bails or earring posts are soldered to the castings. Then each piece is polished and finished with a patina to enhance the detail and preserve the color of the metal through long years of wear.   Seeing a wax carving transformed into a finished piece in gold, silver, or bronze  is the closest thing I know to magic!

Finished Saint Bernard earrings in sterling silver