Lost Wax Casting Method
Creating the model.
An artist or mold-maker crafts a unique model using materials such as wax, clay, or others. Wax and oil-based clay are frequently favored due to their ability to maintain softness.
Mold-making.
Mold-making involves creating a mold from the original model or sculpture. The rigid outer molds house a softer inner mold, which serves as the precise negative of the original model. Inner molds are typically crafted from latex, polyurethane rubber, or silicone, all supported by the outer mold. While the outer mold can be constructed from plaster, it may also be made from fiberglass or other materials. Most molds are composed of at least two parts, and a shim fitted with keys is inserted between the sections during construction to ensure accuracy of the mold. If thin protrusions from the model exist, they are removed from the original and molded separately. Sometimes multiple molds are required to replicate the original model, in particular for larger pieces.
Wax.
Once the mold is ready, molten wax is poured in with a swirling motion to ensure the inner surface is uniformly coated, usually to a thickness of around 3 mm. This process is done many times until reaching the desired thickness. Another method involves filling the entire mold with molten wax and letting it cool until the preferred thickness forms on the surface of the mold. Subsequently, the excess wax is drained, the mold is turned upside down allowing the wax layer to cool while solidifying. This technique makes it difficult to achieve a uniform thickness throughout the wax layer.
Removing the wax.
The hollow wax replica of the original model is taken out of the mold. The model-maker can reuse the mold to create many original copies.
Chasing.
Each hollow wax copy is then chased with a heated metal tool that is used to rub out the marks that show the flashing where the pieces of the mold came together. The wax is then dressed to hide any imperfections. The wax now looks like the finished piece. Wax pieces that were molded separately can now be heated and attached; registration marks are used to show exactly where they go.
Spruing.
The wax model is fitted with a treelike arrangement of wax that will ultimately create channels for the liquid casting material to flow through and for air to escape. This spruing usually starts at the top with a wax cup, that connects via wax cylinders to different areas of the wax model. The spruing does not need to be hollow, it will be eliminated later.
Slurry.
A wax model with a sprue is immersed in a mixture of silica, followed by a coating of sand-like stucco or dry crystalline silica of a specific grain size. This combination of slurry and grit is known as ceramic shell mold material, even though it isn’t technically made from ceramic. The shell is allowed to dry, and this process is repeated until there is a coating of at least half an inch around the entire piece. For larger pieces, the shell needs to be thicker. The interior of the cup remains uncoated, and the flat top of the cup acts as the base for the piece during this procedure. Additionally, the core is filled with a fire-resistant material.
Burnout.
The piece coated with ceramic shell is positioned upside down in a kiln, where the heat solidifies the silica coatings into a shell, allowing the wax to melt and drain out. The melted wax can be reused. At this point, the only remnants of the original artwork are the voids that were previously filled by the wax, found within the hardened ceramic shell. The feeder, vent tubes, and cup are now hollowed out.
Testing.
After the ceramic shell has cooled, it is examined to determine if water can flow easily through the feeder and vent tubes. Any cracks or leaks can be repaired using a thick paste. To assess the thickness, holes may be drilled into the shell, and sealed if necessary
Pouring.
The shell is reheated in the kiln to harden the patches and remove any traces of moisture. It is then placed cup-upward into a tub filled with sand. Metal is melted in a crucible in a furnace, and poured into the shell. The shell has to be hot because otherwise the temperature difference would shatter it. The filled shells are then allowed to cool.
Removal.
The shell is either hammered or sand-blasted off, freeing the rough casting. The metal sprues, which are accurately reproduced, are cut, and t
he material is recycled.
Chasing.
The cast is refined until any evidence of the casting process is eliminated, resulting in a product that resembles the original model. Imperfections caused by air bubbles in the casting and the remnants of the sprues are smoothed out and then polished.
Prior to the use of silica-based casting molds, several other heat-resistant substances were employed in the creation of molds, with plaster mixtures combined with grout and clay molds being the most common. Before rubber molds became available, gelatine was the main material used.