From Clay to Bronze; The Foundry & The Lost Wax Process

Getting to here…


From here…


As I was working on this piece, the size of the head got bigger than I originally intended, and because the armature, which holds the clay up, is fixed, I knew I would run out of room to make the neck the proportion I wanted. But in the lost wax casting process, that is not a problem. I knew I could adjust it easily in the “wax” phase. After I completed  the clay part of the piece, I brought it to the foundry. They then coat the clay in latex, and put a supportive coating of plaster around the latex. That creates a mold which is an exact negative of the clay.

The mold on the left is mine, closed up. Bottom left, is a view of the underside. On the top left, is an example of an open mold that shows the latex covered by the hard, plaster shell.The original clay apple you see here, is no longer needed. It is this latex, or rubber mold, which allows you to make as many castings as you wish, because it is a perfect, exact negative of the original clay. The next step is to pour hot wax into the now hollow mold and let it cool. The mold is then opened up, and you have a wax version of the original clay.


Here is me with the “wax.” Special workers at the foundry then, clean, or “chase” the wax. A heated metal tool is used to scrape the wax. One can  also patch imperfections with a tin of hot wax that can be applied to small places. You can also carve into the form with several kinds of heated tools. I always end up doing a bit of that at this point. We also built an extension piece out of wax to make the neck a bit longer, and smoothed it out. If there are pieces sticking out of the main body of the sculpture, this is the point at which they’re re-attached. Normally legs, arms, etc, are cut off when you build the first latex shell, and molded seperately. Then one heats up the wax and using wax struts, attaches the extruding elements.

A tree like series of attachments, “spruing,” is then attached to the wax. These will provide paths for the bronze to flow and air to escape in the final stage of casting. The wax is then dipped in series of fine to coarse flats of a silica slurry.

Above are the silica beds. You can see the top cap, which is always added in the wax spruing stage, to pour the bronze through, later. The silica slurry and grid must coat the wax to varying degrees. The bigger the piece, the thicker the coating. The thinnest coating is about 1/2 inch, all around. The picture on the bottom left, is a piece with the strut – like supports, drying. On the right, the apple we saw the mold for earlier, can be seen with its spruing visible.

Now, the silica hardened shell, with wax inside, is placed in a kiln, bottom left. This causes the coating to harden even more, and the wax melts and runs out. You can see the melted wax in the bucket, on the left.

What we have now, is a negative space that corresponds exactly with the positive wax form. Once the shell has cooled, water is poured through the vent tubes to make sure there is no obstruction. The thickness of the casing is also checked, by drilling holes, then patching them. The piece is then placed in the oven on the right, to harden any patches, to make sure all traces of moisture are gone, and to super heat the mold.

The metal, in this case, bronze (an alloy of ca. 80% copper with ca. 20% tin ) is heated to over 1200 degrees in a furnace, and then poured through the opening of the super-hardened shell. If the shell itself was also not heated in the prior step, the hot metal would shatter the mold. The bronze filled shell is now allowed to cool. Finally, when cool, the shell is hammered open and the raw metal piece emerges. ( The “sprues” are also cast, and are cut away for future use.) This is the raw metal casting, before patina, of my piece.


As with the wax, the metal must be checked for imperfections and air bubbles, and filed smooth, or “chased,”  just like the wax was.

Now comes my favorite part, the patina, There are many ways to patina a bronze sculpture. I use liquid chemicals, mostly with metallic bases, such as ferrous oxide. They are applied to the sculpture and heated with a blow torch. The duration of exposure to the heat, as well as the make up of the chemical compound, determine the color it turns on the metal.

Above, George, the patina specialist at the foundry, pours chemicals on the bronze, and then heats it with a blow torch. We go back and forth with different chemical combinations, depending on the look I’m going for. In order to see the real color, you have to throw water over the  piece. I enjoy trying to get the right mix, and look, to enhance the particular piece. Some artists use the same couple of patinas, all the time. I play around, and experiment with patinas. Sometimes, however, I go in the wrong direction, and have to sand – blast the experiment off, and start again. This is time consuming, and adds an extra expense.

Here is the final patina before it is waxed. The wax protects it, and also makes it a bit darker. The piece as I’m about to load it in the truck to take back to the studio.

Compare the image, above, with the photo of the piece that was taken, in the studio, at the top of the post. It appears darker in inside light. This sculpture was one where I did a fair amount of work in the wax; extending the neck, carving into the back, and fine tuning the frontal outline. In the end, I felt that the patina I achieved, really enhanced the piece.

I find the whole foundry process incredibly satisfying. It  makes the final result a grand synthesis between individual and group effort. Even more, it gives me a feeling of connection to both America’s great manufacturing past, and an ancient artisic process that has been used for centuries.

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20 Comments on "From Clay to Bronze; The Foundry & The Lost Wax Process"

  1. kathy says:

    I love seeing the process of going from clay to bronze. I’ve never really understood the whole thing before. Great post.

    • David Leeds says:

      The transformation is really powerful. You go from the clay, which always seems very immediate, and temporal to me, to a state, in bronze, which feels permanent, ancient, and monumental. You feel part of the ancient, timeless, line of all past sculptures.

  2. edward jamieson says:

    yes, great birthing process

  3. cynda valle says:

    thanks, great, informative…at what point in the casting is the crack in the clay repaired???

  4. Marisa says:

    Amazing to think of the ancient artisans developing this process. A great reminder of how close art, science and invention really are.

  5. cynda valle says:

    but how do you avoid having the crack replicated in the wax copy?? thanks, c

    • David Leeds says:

      Cynda, I think I was not clear the first time. The crack wasn’t in the clay when the piece was wrapped first in latex, then plaster. The damage occurs when the mold is cracked open. The latex around the clay, where the impression is made, is perfect. It’s after that, in the opening that there can be the damage. I’ve had some clay remain in perfect shape, and some that was really damaged. The force of the process can tend to pull the clay off the armature. Hope that clarifies things.

  6. Ida says:

    Thank you for an interesting post it is so easy just to look at the finished piece without any thought of the work involved in its creation.

    Do you name your sculptures? Ida

  7. Noel says:

    Wow! What a fascinating process to be involved with …. a little voyage of sorts with your work to a beautiful conclusion: a fine work of art that will endure for ages.

  8. Noel says:

    You are far too humble.

  9. cynda valle says:

    thanks for clarifying!!! also very helpful to know you can perfect/change the sculpture in it’s wax incarnation, didn’t realize that….

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Sculptor, painter, poet. Currently living in Los Angeles and Martha's Vineyard