The lost-wax process for casting art hasn't changed much since the Renaissance.  There have been technological advances made in materials engineering, which allow for more efficient production and more consistent quality, but for producing sculpture in bronze, we use very similar techniques  to the ones our ancestors used.

Step 1

 The Pattern  

The first step in creating a sculpture in bronze is the production of the pattern.  The pattern is a positive incarnation of the sculpture. During the lost wax casting process there are three positives produced; the original pattern, a wax pattern, and finally the bronze sculpture.

The production of the original pattern is the job of the artist and is the creative part of the process. The pattern can be made of virtually any material but most artists work in an oil-based clay called 'plasticene' or 'plastilene'. This clay material is easily manipulated and doesn't dry out like water-based clay. This allows the artist to work on a sculpture over a long period of time without having to keep the clay wet or covered. Usually the clay pattern is built over a sturdy armature which acts as a skeleton, keeping the clay from sagging or falling apart under its own weight. Once the artist is satisfied with the clay, which the bronze will be an exact copy of, it is ready for molding.   

Doug Byars puts the final details on his clay sculpture "Running Late"

  Step 2

 The Rubber Mold

Now that the original pattern is made, a mold (negative) must be made.  This will be the master mold, which is made of rubber, and from which all future editions of the casting will be made.  During the lost-wax casting process there will be two molds (negatives) made; the master mold, from which a wax pattern will be cast, and a ceramic mold, from which the bronze will be cast.    

The clay pattern is first prepared by applying a release agent, usually a silicone based, liquid spray.  This release keeps the mold material from sticking to the pattern allowing for easy removal after the mold is constructed.  Sometimes it is necessary to cut parts off the pattern  and mold them separately because of the shape and the necessity to remove the pattern from the mold after it is constructed. 

Next, a polyurethane or silicon mold material is applied to the pattern one coat at a time.  Each coat of rubber is allowed to cure before the next is applied.  The first two coats are applied in a very thin and liquid consistency ensuring that every detail of the pattern is recorded and that no bubbles or air pockets form on the surface.  Plastic shims with spherical indentions are inserted along pre-determined seam lines after the first couple of coats of rubber are applied.  These seam lines are the joints where the mold will be taken apart to remove the pattern.  The spherical indentions act as keys which ensure that the mold fits back together correctly after it has been taken apart.  More rubber is applied in a thicker consistency until the desired thickness is achieved (usually about 1/8" to 3/16").

Shims have been attached and rubber is being applied to a mold

 Now that the pattern is completely covered in rubber, a hard shell is constructed that encases the whole thing.  This shell is called the 'mother mold' and is made from moulding plaster reinforced with hemp fiber.  The mother mold will keep the rubber, which is malleable, in the correct shape once the pattern is removed.  The rubber mold captures the detail and the mother mold holds the overall shape of the pattern.

 Moulding plaster is applied to the rubber to create a mother mold

 Finally, the mold can be opened along the seam lines and the pattern can be removed.  Any residue from the pattern that may still be imbedded in the rubber is cleaned out and the mold is now ready to cast another pattern made of wax. 

A rubber mold is removed from the original pattern

Step 3

  Wax Production

Next, a wax pattern is cast from the rubber mold.  Microcrystalline wax, a petroleum based wax used by the foundry industry, is melted and poured into the mold.  It is sloshed around, ensuring that the entire inside surface of the rubber is coated. The excess is poured back out.  This is repeated until the desired thickness is achieved (usually 1/8" to 1/4").  This creates a hollow casting which means that the final bronze will be hollow as well.  It needs to be hollow for weight issues and because thick bronzes will shrink more when they cool causing flaws in the surface.

\Molten Microcrystaline wax is poured into the mold, sloshed around to coat the rubber, and the excess is poured back out

After the wax cools sufficiently, it is gently removed from the mold.  The wax casting is then 'chased', meaning the seam lines from the mold are smoothed out using soldering irons and small wood or metal tools.  Next, any flaws in the wax, such as air bubbles or other surface imperfections are repaired and any parts that were cast separately are reassembled by welding them with a soldering iron.  These seam lines are then chased. 

A soldering iron is used to chase a wax casting

Step 4

  Spruing

'Sprues', or wax rods, are attached to provide channels for wax to be romoved and for molten metal to flow through to reach the pattern.  Smaller sprues are sometimes attached to high points on the wax pattern to allow gases to escape as the molten metal fills the mold.  The sprues that feed the pattern with metal are called 'gates' and the ones that allow the gas to escape are called 'vents'.  The sprues are welded to the wax pattern using a soldering iron.  A block of wax is attached to the main gate that will create a pour cup when the wax is melted out. 

 Sprued wax castings  stand up-side down awaiting the next step of the process 

Step 5

  Investment

Next, the wax pattern, with sprues attached, is 'invested' in a ceramic mold material which creates the mold into which the molten metal will be poured.  The wax is dipped into a liquid ceramic slip made from colloidal silica (a liquid silica material) and silica flour.  This slip, called 'slurry', has a consistency of runny yogurt.  Some slurry sticks to the wax while the remainder drains back into the mixing tank.  Silica sand, called 'stucco', is then poured over the surface sticking to the wet slurry.  Fine stucco is used for the first few face coats ensuring that the detail is captured.  Coarser stucco is applied to the outer coats to add structure and strength to the mold.  Each coat is allowed to dry for a few hours before the next one is applied.  This process is repeated until the desired thickness is achieved (usually about 3/16" to 1/4" for smaller molds and up to 1/2" for larger ones).  This usually requires 6-8 coats for smaller molds and as many as 20 coats for large molds.

A wax is dipped into the slurry and the excess is drained back into the tank

Stucco is poured over the wet slurry and the mold is then allowed to dry 

Step 6

  Burnout 

Now that the ceramic shell mold is constructed around the wax, the mold is put in a kiln up-side down.  The kiln is quickly fired to about 1200°-1400° F.  This melts the wax out of the mold leaving it hollow.  It also 'vitrifies' the ceramic material.  Vitrification occurs when the ceramic particles are hot enough to melt and fuse together.  This strengthens the mold so that it can withstand the hydraulic pressure of the molten metal when it is poured into the mold.  After the molds cool they are inspected carefully for any cracks that may have formed during the burnout process.  Any cracks found are patched with a refractory mortar so that the mold won't leak when molten metal is poured into it.

Step 7

  Metal Casting

Now that the mold is empty and vitrified, molten metal can be poured into it.  The metal is melted in a furnace that runs off propane or natural gas.  The metal is placed in a 'crucible', or pot, made of silica carbide.  A forced air blower mixes oxygen with the gas and pushes the pressurized mixture into the furnace.  The gas/air mixture enters the open chamber of the furnace at an angle, where it combusts and creates a flame that rotates around the outside of the crucible heating it evenly.

The bronze takes about 45 minutes to reach 2200°F when it is ready to pour into the mold. The crucible is lifted out of the furnace using steel lifting tongs and is placed into the pouring shanks. The pouring shanks consist of a steel ring with a handle attached to each side. The shanks are picked up by two people (one on each handle) with the crucible resting in the steel ring.

The metal is then poured into the mold which has been heated in a kiln to 1400°-1500°F while the metal was melting. 

The crucible is lifted out of the furnace with steel tongs 

 Molten bronze is poured into the ceramic shell molds

The freshly poured molds are allowed to cool

Step 8

  Fit, Weld, and Finish

After the metal has cooled (usually an hour or two) the ceramic mold material is gently chipped off of the bronze casting with a hammer and chisel.  The remaining mold material which is left in the crevices is sand-blasted off, revealing the raw cast bronze.

The ceramic shell has been mostly removed from Karen Kasper's trophies for the Dubai International Arabian Horse Championships which now await sand-blasting

Next the sprues, which are cast in bronze along with the pattern, are cut off using an angle grinder and metal cutting disc. The nubs left behind where the sprues were attached are sanded off using various grinding and sanding tools.

Parts of Karen Kasper's Dubai International Arabian Horse Championships trophies have been chased and await being welded together

The cast parts are then checked to make sure they fit together properly.  Necessary adjustments are made and then the parts are  welded together.  The welds are chased, also using various grinding and sanding tools.  Surface textures are replicated with various tools so that seam lines and sprue attachments are no longer visible.

 Karen Kasper's trophies for the Dubai International Arabian Horse Championships have been welded together and the weld seams are being chased

Step 9

  Patination

The sculpture is now complete other than coloration.  The surface is sand-blasted, leaving a clean, consistent surface. 

A patina is applied using various chemicals determined by the desired coloration.  Patination is a controlled oxidation of the metal.  Virtually any color can be obtained depending on the chemicals used, the application technique, and the temperature of the bronze when the chemical is applied.  Often, patinas are applied in layers of different chemicals creating background colors and visual depth.

Once the desired patina is achieved, the surface is coated with carnauba wax.  This gives the surface visual depth and shine as well as inhibiting further oxidation.  The sculpture is now finished and ready to be installed or mounted to a pedestal.    

 Karen Kasper's trophies for the Dubai International Arabian Horse Championships are finished and ready to ship