One of our key specialities at Hickmet Fine Arts is sourcing bronzes of exceptional quality. Be they Animalier, Art Nouveau or Art Deco, we have a real appreciation for the finest antique bronze sculptures.
‘Lost Wax’ Bronze Casting - In this article we are delving deep into the process of wax casting to tell you how bronze sculptures are actually made today. The “lost-wax” method of casting bronze sculptures has been the predominant technique across history. In fact, the method pre-dates the discovery of bronze itself, as it was used to cast sculptures with other metals during the early dynasties of Egypt nearly 7,000 years ago, and the process has changed remarkably little over the centuries. The other key method of casting bronze sculptures was sand casting, which overtook wax casting in popularity during the 19th century, only to be superceded by wax casting in the 20th century when materials became available that made the latter more practical once again. We can tell so much about past cultures, religions and society through the many ancient “lost-wax” bronze castings that have withstood the centuries. From these we know that Chinese bronzes depicted ceremonial images, Indian and Egyptian castings symbolised deities, Africans cast images of nature and the Greeks recreated the human form.
Although elements of the “lost-wax” process have been refined over the thousands of years that have passed, today bronze casting is essentially the same as it was in 2,000 B.C. The key steps are detailed below.
1. Making the Original Clay Sculpture
For most sculptors, the process of making a bronze sculpture starts with making an original out of water-based, oil-based, or self-hardening/low-fire clay.
Particularly small sculptures are often modelled directly in wax which, though more difficult to model, allows for maximum detail.
2. Making An Armature
Sculptors will often need to use an armature to help support the weight of the clay, particularly when molding a figure in motion or in a standing posture. The armature is commonly made out of wire, pipe or aluminium.
Once the armature is built, the clay sculpture can then be formed around and on the armature.
3. Finishing the Clay in Preparation for Molding
The final procedure before molding is touching up delicate details and smoothing the surface of the clay to perfection. Any error or imperfection in the clay would be copied in the molding process and appear in every subsequent stage, so ensuring the clay is smooth now means less work is required on the wax replica and the finished bronze casting itself.
Once this process is finished, the piece needs to be allowed to dry thoroughly before continuing to the molding process.
4. Creating a Mold of the Clay Original
Next, a sculptor needs to make a mold of the original clay, which nowadays is done with a polyurethane mold compound or a high-quality silicon rubber. The mixture is “painted” directly to the surface of the clay using a brush in 3-5 coats that are applied over the course of several days. Pieces made over a century ago would have used gelatin rather than silicone.
Both methods then require a “mother mold” be applied; a firm outer “jacket” made out of plaster, Hydrocal, resin, or epoxy. This helps retain the shape of the flexible rubber/gelatin mold when pouring the wax replica.
When the entire mold is dry it is removed and cut away from the clay, taking the sculpture from a positive form to a negative form.
If the sculpture is large or complex, it will usually have to be divided into smaller pieces, with each piece needing its own individual mold, to be rejoined later, after the sculpture is cast in bronze.
5. Pouring a Wax Replica From the Mold
A wax replica can now be made of the original clay sculpture by pouring wax (in four stages or coats until approx 4 inches thick) into a hole in the “mother mold”. The mold is rotated while the wax is quite hot in an attempt to coat the entire internal surface of the mold with the melted liquid wax. When the wax cools and the mold is removed, a wax positive of the sculpture emerges.
6. Wax Chasing
There is now another stage of repairs, known as “wax chasing”, where all of the imperfections created during the pouring process are removed, such as air bubbles, seams, and mold lines.
A network of wax rods, called sprues and gates, are next attached to the positive wax model to serve as a type of channel system, which will feed the molten metal to all of the areas of the sculpture, as well as allow gases and air to escape.7. Spruing a Wax
8. Ceramic Shell (Investment Casting)
The wax is first dipped into a solvent to clean any loose particles or debris from the surface. Next, it is dipped into a solution called prewet, followed by two coats of a very fine-grained slurry. This “primary coating” is where all of the fine detailing in the piece is picked up. The shell is then dipped into a series of 7-9 slurry mixtures of different grades that gradually become coarser. The ceramic shell that results is a hard, durable shell around the entire sculpture that will be the vessel to receive, hold, and shape the molten metal to produce the bronze figure.
9. Melting/Burning Out the Wax
When the ceramic shell is complete, it is placed in a high pressure sealed oven, known as an autoclave. High temperatures (1500 to 1800 degrees Fahrenheit) and pressure force the wax from the shell and the wax melts out, thus becoming “lost”, leaving behind the detailed impression of the clay sculpture within the shell. This step also hardens the ceramic shell in order to prepare the shell for the extreme temperature of the molten bronze which it is about to receive.
10. Casting/Pouring the Bronze
When the ceramic shell is empty of wax, it is then re-fired and made ready to have the molten bronze poured into it. Hot ceramic shells are taken to the pouring floor, where they are either placed in sand to stand by themselves, or they are wired to a support frame to be held in place.
11. Break Out
The solid blocks of bronze are heated to a temperature of approximately 2250 degrees Fahrenheit to create liquid bronze that is then (very carefully!) poured into the ceramic shell. When full, the ceramic shells are left to cool for several hours.
The ceramic shell is broken off with hammers, tools, power tools and sandblasters (yet again very carefully!) to separate it from the bronze metal. Once the entire ceramic shell has been removed, the sprues (which have now become metal) must also be cut away or sawed off.
At this stage, the uncovered bronze is considered a “raw metal”.
12. Metal Chasing
“Metal chasing” is the process of finishing the metal back to the appearance of the original and usually involves a fair amount of welding with a high-frequency welder.
13. Metal Welding/Assembly
If the sculpture being created was rather large or complicated and was cut into pieces during the mold stage, these individual bronze castings will now be welded back together to create the whole piece.
14. Sand-Blasting the Bronze
When the final piece is all welded together and chased to perfection, the bronze is next sand-blasted (or bead-blasted) to make it very smooth and shiny.
Intriguingly, the ancient Asians buried their bronzes, sometimes for years, to naturally oxidize them in order to create patinas.The “patina” is the colour of the bronze and “patinisation” is when one applies acidic chemicals and high temperatures to oxidise the surface and change the colour or texture of the bronze as the chemicals interact with the metal.
After the patina is applied, it is protected with a sealant. Traditionally this was several thin coats of clear paste wax, and nowadays a metal protectant is recommended for any sculptures placed outdoors.
16. The Finished Sculpture
The sculpture is now complete and will be truly unique due to the handmade nature of the process. If the sculptor wants to make another one, they have to return to the “mother mold” and go through all of the steps again.
If you own any bronzes, do take a look at our post ‘How to clean and take care of bronzes‘.