Making a bronze sculpture is a challenging but rewarding task. It allows you to showcase your creativity in one of the most beautiful and oldest metal alloys on the planet. The “lost wax” casting method I’ll be discussing dates back as far as early Egyptian dynasties.
Step 1 — An Original Design
We don’t actually create the original design in bronze. To start the process of making a bronze sculpture you first need a sculpture in some other medium. Sculptures can be created using a variety of materials, such as wax, clay, wood or stone. Most artists find clay as the go-to material for their initial creations. There are a multitude of different clay types you can use, such as oil-based, water-based, and self-hardening.
Oil-based clay is typically used since it always remains flexible and doesn’t harden like earthen clay, giving you the option of going back and adjusting the piece.
If you’re working on a smaller sculpture, wax allows you to skip the molding stage of the process: The original piece can be taken to the foundry for casting. When this is done the piece will always be cast solid.
Step 2 — The Armature
Often delicate or complex sculptures require an armature. The armature is the interior structure used to keep the sculpting material from slumping or possibly breaking off during creation. Picture a person extending their arm out.
Think of the armature as a skeleton for the sculpture to be built on. The armature material will vary depending on the size of the sculpture. For larger pieces, one may need to use large pipes to support the clay. In areas where there is going to be substantial volume you can attach foam to the pipe before adding on the clay. With smaller sculptures, you can use heavy but flexible wire. Aluminum is common and comes in various gauges. The lower the gauge number the thicker and stiffer the wire.
Step 3 — The Sculpting
Once you’ve got your armature complete it’s time to get into the sculpting! There’s a lot to take into consideration when designing a piece. Do you want it to be smooth or textured? Maybe a mix? I like the contrast between the two, as it can add dynamism to the piece when well executed. Each has its own challenges.
It’s almost impossible to get the clay perfectly smooth without indents or little pinholes. The options with texture are endless. Whatever you choose, know that the mold will duplicate everything exactly as it is sculpted. One thing to think about when creating your texture is that it be repeated in the metal chasing process. More on that process later on.
If you have sculptured your piece out of water-based clay and you have an interior armature, make sure it doesn’t dry out before getting it to the mold maker. In the drying process, the clay will shrink and crack as the water evaporates. Most sculptors who use water-based clay make the mold when the clay is leather hard
Step 4 — Mold Making
Now it is time to make the mold. I used to make my own molds when I was sculpting bas reliefs. But when my designs became 3D and more complex it made more sense to my pieces to a mold maker. The last thing you want to do is make a poor mold of a perfect sculpture.
Here’s a little overview on molding should you choose to give it a go: Typically, a polyurethane or silicon rubber mold will do the trick and capture the level of detail you are looking for. You may also run into a mold material called Smooth On. This is perfectly fine to use as it is latex-based and will do a great job!
Molds for really large sculptures are almost always made in pieces. Before painting on the mold material, the mold maker will determine where parting lines are required and attach parting shims to the sculpture. On complex pieces there will be multiple parting lines. The parting line is just the division line for the mold.
The molding material is then applied directly to the original clay sculpture using a chip brush. Usually, three to five layers are applied to the original sculpture with a minimum of 24 hours between application. The ambient room temperature will have an effect on the curing time.
After the layers are completed, a final outer “jacket,” made from plaster, hydrocal, resin, or epoxy, is applied. This is often referred to as the “mother mold.” Once all of this is dry, the original clay sculpture is removed from the mold. More often than not it’s destroyed in the process. Oh, well!
You now have a “negative” of the original.
Step 5 — Wax Pouring and Chasing
Once the mold is complete and the original sculpture removed it’s time to pour a wax. Some of the molding materials require the mold be preheated before the first layer of wax is poured into the mold. Some manufacturers recommend a product called mold release be applied to the interior of the mold previous to pouring the wax, this stops the wax from sticking to the mold.
Generally, the wax casting requires four individual coats of wax. First, wax is heated up to about 220° Fahrenheit and poured into the mold. The mold is then rotated to coat the entire inner surface. Any remaining wax is poured back into the wax vat.
The second coat is then poured in at a lower temperature — around 200° Fahrenheit. The final coats go on between the temperature of 180° to 185°. When finished, you should expect to see a 3/8″- to 1/4″-thick wax.
Once the wax is cooled it can be removed from the mold. Remember, the wax pouring is often done to just a section of the original design at a time. The wax version of the sculpture will now need to be “chased.” Chasing removes any imperfections in the wax casting, such as air bubbles.
This is also when the piece is reassembled, now in wax, to exactly match the original design. That means all the pieces are put back together and all of the seam lines from the mold are removed. Any imperfections at this point will show in the bronze casting.
Think hollow chocolate Easter bunny at this point.
Step 6 — Spruing
Once the wax is back to the perfection of the original sculpture, it’s time to take it to the foundry. Larger pieces are cast in smaller sections. That means the sculpture is cut up again!
To each piece the foundry attaches a series of solid wax rods called sprues. Think of the sprues as channels. As you’ll see during the shell step, they allow the molten bronze to appropriately flow into all areas of the sculpture. They also serve as vents to release any air or gases. A larger wax funnel is also added to the sculpture. This is where the wax will melt out of and where the molten bronze will be poured in.
Step 7 —Shell
The sprued wax then moves from the wax room to the shell room where a second one-time mold is created. The wax is dipped multiple times into a very fine ceramic fluid called slurry. The wax is typically dipped about eight times, with each layer adding thickness and strength.
In the image below you can see some of my Desk Buddies (Cat-itude and Bottoms Up) going through the slurry process. If you look closely you can see the sprues underneath the bottom of the cat body and one for each tail. This ensures the molten bronze will flow properly and fill the void where the wax has been melted out. That will be explained in the next section.
Step 8 — Finally, Lost Wax!
Once this ceramic shell is complete and totally dry, it is placed in an large kiln at 1,500° to 1,800° Fahrenheit. At this stage, the wax melts out the pour hole, the funnel added earlier. It is “lost” from the ceramic shell. You are now left with a highly detailed, heavy-duty ceramic mold that is ready to receive the melted bronze.
Step 9 — Pouring the Bronze
The ceramic shell is then brought to a pouring area and placed on sand with the ceramic funnel facing up.
The foundry melts the bronze in a crucible to a temperature of 2,250° Fahrenheit. Protected by a great deal of safety gear, the pouring team carefully pours the molten bronze from the crucible into the ceramic shell.
Step 10 — Removing the Shell
Once the ceramic shell and bronze have cooled, the ceramic mold must be broken apart and removed. This is usually done with sledgehammers, as the ceramic shell is super tough.
After the raw bronze is released from the shell, the sprues are cut off of the sculpture, typically with a torch. The piece is then sand blasted to remove any remaining shell, which can get caught in hollows and tight spaces.
Step 11 — Metal Chasing
The next step is called metal chasing, and it’s particularly important for smooth work like mine. As mentioned, the sculpture will likely come out of the foundry in sections, unless it is a small piece that has been cast solid. Now it’s time to put it all back together and make it perfect again.
The metal chasing process is intense, requiring many tools and talent to bring the pieces back to the perfection of the original design. The sections of raw bronze will need to be TIG (tungsten inert gas) welded back together with silica bronze rod. If the piece is textured, the chaser will use the rod and pneumatic tools to recreate the texture.
The surface should be consistent across the entire sculpture. A quality sculpture looks like it was cast in one piece with no imperfections. There should be no indication of seam lines, short pours, or porosity (design areas missed by the bronze during the pour).
Step 12 — Patina
And at last, it’s time to apply the patina. The patina is a very complex process, a fine art like oil painting but with chemicals — typically oxides and nitrates — and heat. It starts with heating the bronze to a golden color using a propane torch.
Many factors determine the final patina, including how hot the metal is when applying the chemicals. It is also determined by the saturation of the oxides and nitrates along with how they are applied. They can be applied with a patina brush, spray bottle, or airbrush for example.
The final stage is to spray a few layers of lacquer over the piece to protect it for years to come. There are certainly more ways to accomplish a beautiful patina than the one explained here. Some artists use cold patinas and wax for sealing, others use acrylic paints, some use epoxy and auto body paint. To see the process I just described you can watch this bronze patina process here!
Voila! A beautiful piece of art has been created. 🙂
One of our most loved bronze horse sculptures after it has gone through this entire process.
You can also view the process in detail through our lost wax casting process videos here.