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Lost wax casting for sculpture


Phil White

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Hi all,

 

In response to the recent discussion regarding the lost wax technique of casting metals, I decided to post a presentation that I wrote a couple of years ago, documenting the creation of a portrait bust of HRH Queen Elizabeth II that I did for the House of Commons.

 

The process is the standard traditional technique for casting sculpture, regardless of size or form.

 

The presentation was intended to be about 45 minuits long, so there are quite a few photos, but the sequence is pretty much complete, and in detail.

 

The photo-documentary begins with the finished sculpted clay model and ends with the completed bronze sculpture, as returned from the foundry, prior to mounting.

 

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Photo 1:

The clay model or maquette is sculpted from china clay on a wooden armature. The scale of the sculpture is “heroic”, or slightly larger than life-sized.

 

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Photos 2 to 4:

A rubber mold is made by brushing on in layers to build up thickness. Pigment is added to the rubber on every other layer to make the coatings obvious. The coating is built up to a thickness of about ½ inch thick, making sure to fill and under-cut areas. The rubber mold is the best means of capturing the surface detail in the clay, but is floppy once removed and requires support.

 

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Photos 5 to 11:

A plaster mother mold is made to support the rubber. The Plaster is built up in three sections by laying on pieces of loose-weave burlap that has been soaked in plaster. This is done in three sections, starting at the back. The joints for the sections are first planned out and drawn onto the rubber, considering that each section has to pull straight off without and obstruction or under-cutting locking them in place. Each section is built up, allowed to set, then trimmed and cut with a depression that will act as a key when reassembling. The trimmed joints brushed with soapy water solution to prevent sticking, and the next section is built up. When the mother mold is finished, it is allowed to cure overnight. The pieces are separated by carefully driving a chisel into the joint line, and gently prying until the joint splits. The three sections are set aside, and the rubber mold is removed by cutting up the back of the head and peeling forward. Excess clay is washed out, and the rubber is allowed to dry.

 

Plaster cast begins. Traditionally, this was done to produce a copy of the clay for reference, but could also be done for approval or promotional purposes.

 

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Photos 12 to 15:

The rubber mold is given a coat of commercially prepared mold release and the three pieces are reassembled, starting at the back. The cut in the back is then carefully re-aligned from the inside to minimize and seam lines in the casting. The mother mold is held together with an elastic cord, and the keys cast into the joints prevent it from moving around.

 

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Photos 16 to 20:

A small amount of plaster is cast into the mold to coat the inner surface, and rotated until it hardens. This is repeated with second layer, followed by layer of plaster-soaked burlap to re-enforce the piece. Support rod is cast into the back for mounting and holding in vise, and the plaster is built up to the desired thickness. The plaster is then removed from the mold and repaired while still wet, filling any voids or cracks and removing excess beads left where bubbles in the rubber mold formed.

 

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Photos 21 to 25:

After repairs are complete the piece is given a coat of a clay/water wash and allowed to dry. The surface is coated with shellac to seal it, allowed to dry, waxed with paste wax applied with a brush, then buffed with a cloth.

 

The finished plaster can then be used for reference when casting and finishing the bronze, or as a reference to create a copy in stone or wood.

 

As this sculpture was intended to be cast in bronze the following photos deal with the lost wax casting process. This is a technique of casting in which a wax copy of the sculpture is created and covered with a ceramic shell. The ceramic is fired, and the wax runs out, (lost wax) leaving a hollow void, which is then filled with liquid bronze.

 

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Photos 26 to 30:

A hollow wax casting is made, using the same mold as used for the plaster cast. The mold is reassembled, and hot liquid wax is poured in, rolled around, and poured out, leaving a layer of wax about ¼ to 3/8 inch thick lining the mold. The piece is de-molded, and the wax is examined for imperfections. Any beads are cut out, and voids are filled with wax using a hot spatula. Edges are trimmed, and the piece is made to look as it is intended to look in bronze.

 

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Photos 31 to 33:

The wax is prepared for casting by adding wax rods to act as “gates” for the liquid bronze to flow in, and as “risers” to allow gases to escape, and the bronze to flow through. The whole assembly is then covered with a ceramic shell, inside and out. This is built up in several layers by applying the clay slurry, followed by a coating of silica sand, and allowing to dry. The clay slurry goes on green and dries to yellow, which is particularly useful when trying to judge when inaccessible areas, such as inside, are dry. The slurry and sand are reapplied to build the shell up to about ½ inch thick.

 

The assembly is fired in a kiln, and wax runs out through the gated and risers, and is collected for re-use. At the same time the bronze is heated to the proper temperature. The clay shell is removed from the kiln while hot and placed in a drum with sand to support it, upside down. Liquid silicon bronze is poured into the gates and pouring stops when it comes out through the risers. (sorry, no photos of the actual pour) The piece is allowed to cool, and then removed from the sand. The shell is chipped off, and discarded. Photo 33 shows the discarded pieces of shell, with details of beads and hair preserved.

 

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Photos 34 to 41:

The rough bronze casting is cut away from the risers and given a quick sandblasting. Any imperfections or voids in the casting are filled by welding in bronze rod with a TIG welder. In this case a hole was left in the top of the head, which had to be filled. The welds and beads in recesses are cut out with a combination of files, chisels, and tungsten carbide rotary bits, as applicable. Any imperfections or porosity in the surface are also cut out. A bronze mounting plate is also welded to the bottom along the inside edge. The piece is sandblasted to prepare for patination

 

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Photos 42 to 53:

The patina chosen was a green/brown, and is applied in 3 layers. The first coating applied is a liver of sulphur solution applied cold. This prepares the surface, and leaves an uneven dark brown. The second coat is a pale green cupric nitrate, applied with a brush, while the bronze is heated slightly with a propane torch. The patina is built up to a thick opaque green. The third coat is a ferric nitrate solution, also applied to a hot surface. This darkens the green with a brown color, and provides depth. The surface is washed, and slightly heated to dry. Commercial paste wax is applied to the warm surface and allowed to soak in, then buffed with a cloth once cooled. The finished sculpture is then ready to mount.

 

Best regards,

 

Phil

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Hi Jim,

 

Thanks, and by all means, take your time. I realize it is a long post.

 

I had hoped to take photos of the pour, but I was out of town at the time. The casting was done at a local sculpture foundry called "Lost and Foundry" with relatively simple equipment. The foundry uses a home-made propane-fired furnace that consists of a modified 40 gallon drum lined with fire brick with an air boost provided by the exhaust from an old vacuum cleaner. Sounds crude, but it is actually very efficient.

 

 

Phil

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Thanks for the informative post, it was great to see the molding process. I knew that large bronzes were made hollow but I didn't fully understand the process by which it was done.

 

I've been wanting to do some more bronze casting, and I've also had plans to make a mold of my head, this should be very helpful with both projects.

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Hi Phil,

That was an excellent tutorial on bronze casting. And if I might add and excellent piece of sculpture. You get a chance to do everything. Great Job!

When I ran my foundry I had a commercially bought furnace for my small pieces (which was what I used most of the time) but I built a garbage can, vacuum cleaner furnace to melt metal for big pieces. It worked great.

Thanks again for the tutorial.

Dick

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Hi Phil

 

A most interesting tut and informative too. Just like others from you :blush:

 

As a retired toothy lost wax casting is very familiar to me especially as I did much of it myself using precious metals not bronze. I would be interested in more detail about the materials used eg the rubber for the mould and the wax for starters. I was given a rough formula for wax using pitch and candle wax (I think!) but the lady who gave it to me and was going to help me, emigrated to the States about 15 -20 yrs ago before this could take place. She did some really super sculptures of the cat family but I don't remember her name (began with an 'S'?).

 

The sculpture is really first class as well :blink:

 

Thanx

Toothy

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Thanks to all for the comments, and compliments, I'm glad you will find this useful.

 

Dick, Yes, I have absolutely no doubt that I have the best job in the world, at least for me.

 

Toothy, So that's where you get your handle. The molding rubber that I use varies, according to the job. In larger pieces like this I use the "brush-on" Urethane rubber from Smooth-On Smooth-On website I find that their variety of products, and customer support is excellent. For example, I once e-mailed them a question about which rubber to use for a project, and promptly received a phone call from a mold-maker on staff that lasted for 1/2 hour. For smaller work, I prefer to use a silicone RTV, as it it much better at flowing over very fine detail.

 

The wax is a standard bronze casting wax, available from most sculpture supply stores. It is quite soft and easily sculpted at room temperature, unlike jewelers carving waxes. Some sculptors who work in the small scale will sculpt their wax directly, without the clay-mold-plaster stages, but I always considered this a bit risky, as it is a one shot deal.

 

If anyone has any further questions, please don't hesitate to ask.

 

Phil

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Excellent tutorial, Phil! I'm curious about the way you make moulds. I was taught to make them in two pieces, on the grounds that the fewer seams, the less finishing work on the cast piece, but I notice you've used more for this head. Is there a reason for that?

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Hi Freda,

 

The three piece plaster/burlap mold that you are referring to is actually the mother mold that supports the underlying rubber, which is relatively thin and flexible. The mother mold has no contact with the finished product. The rubber mold has only one seam, down the back, which keeps the clean-up work to a minimum.

 

Hope this clears things up.

 

Phil

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