Sculpting Methods and Materials

Bronze Casting – The Lost Wax Process

Bronze sculpture is among one of the most popular mediums within the art world. They are long lasting, difficult to damage, and don't lose value. A broad spectrum of artists use Bronze as their chosen medium to work with. There are variety of techniques and methods used to cast Bronze works, and among them the most common is the 'Lost Wax' method.

The lost wax method of casting is as popular as it is ancient. Dating back as far as c. 3700 BCE, this simple and effective process has become extremely widespread. Proof of the longevity of the works that this process can produce, Bronze pieces that are over 5000 years old still remain to this day. The process has a 12 step working method which is simply to follow, but can be used to make intricate and elaborate designs suitable to almost any type of piece an artist would wish to produce.

Bronze Casting - Lost Wax Process

The 12-Step Process

An artist begins by making the original model of their design - traditionally in wax or clay – but can use another material so long as it is oil based which keeps it soft.

A mould is made of the original model using two shell like halves, one soft- normally made of latex or silicone- and one hard which is commonly plaster. It is common for larger works to be made using more than one mould. If the original model consists of thin expanses from the body of the model (e.g. arms, legs etc.) these would be moulded separately. While producing the moulds a shim with keys is placed between the two parts to allow the two pieces to be put back together accurately.

Molten wax is poured into the two halves and moved around to allow an even layer to set on the mould – normally the aim is for 3mm of wax to reside on the mould. Every artist is different however and will have found their own preferred quantities of wax to make their ideal cast. Excess wax is then poured away, leaving a hollow wax copy of the piece inside.

The wax copy is then removed from the mould. The mould may be used again to make duplicates.

Each copy in then “chased” – this is where a heated metal tool will be used to hide any imperfections and remove the line which shows where the two pieces have been moulded together. Any additional pieces that were moulded separately would also be added at this point.

The wax copy is next 'sprued' with a treelike structure of wax that will eventually provide paths for the molten casting material to flow and for air to escape. The carefully planned spruing usually begins at the top with a wax "cup," which is attached by wax cylinders to various points on the wax copy. The spruing does not have to be hollow, as it will be melted out later in the process.

The sprued wax copy is then dipped into a “slurry” of silica and then into a sand-like stucco – the combination of these two materials is known as the ceramic shell mould. This process is repeated until ½ an inch of the ceramic shell mould coats the wax copy.

Once there is a suitably thick ceramic shell of the wax copy, it is fired in a kiln. This hardens the ceramic shell to enable it to be filled with molten bronze later. At this point, it is also where our process gets it's name- the wax model that was inside of the ceramic shell melts and drains away through the sprued vents/tunnels and is lost. And so, we get our name!

Before the Bronze is added, the mould will first be tested using water to find any weak points or holes where leaks may occur. Any damage can be repaired, holes patched or to aid draining in some cases, additional holes may be drilled.

Now we know it is secure and strong, the shell is then heated once again in the kiln to remove any remaining moisture, and ensure the cast is at a similar temperature to the molten bronze. Now, our final mould is ready for casting our Bronze piece. First the bronze is melted in a furnace until molten; the molten bronze is roughly 1200 °C. It is then transferred into a crucible, to be transported and poured into our mould. The reason our ceramic mould must be heated is because if it is not similar to the bronze, the temperature difference would shatter the shell. Once the mould is filled with the sufficient amount of bronze, it is then left to cool.

Once cool, the bronze casting is now released by hammering away the shell and removing the sprues. The bronze in the sprues will be melted back down and used again, but the ceramic shell will be lost.

The metal chasing process is now used, the same as with the wax cast previously, this removes any signs that the piece has been through the casting process. Imperfections or pits from air bubbles can be removed by filing the surface.

Finally you have a finished bronze cast of your original sculpture!

From here, artists may want to look at finishing or putting a patina on their work.

More information about this can be found at: https://www.thesculpturepark.com/patination/