The Lost-wax Casting Process (Cire Perdue)
Lost-wax casting, also known by the French name of cire perdue, is the process by which a metal object is cast from an initial artists work. The process has been around for centuries in fact since the Bronze Age and today will vary from foundry to foundry, but the steps which are usually used in casting metal sculptures and objects in a modern foundry are generally quite standardised. The process itself can invariable be used for many types of metals but the most common use of Lost-wax casting would be for Bronze Sculptures.
The "Lost-wax" Process
The Art Piece.
The first phase is the creation of the initial sculpture. An artist creates an original artwork this can be from any type of material, obviously some materials are better than others to then go on to create the final bronze. Classic sculptures could be made from wax, clay, resins or another materials.
A mold is made of the original sculpture. Molds are usually comprised of two pieces. A locator key is placed between the two halves during construction so that the mold can be easily and accurately put back together. The mold is made from Silicone rubber with a fibre glass jacket. To preserve the fine details on the original artwork's surface a liquid of silicone is applied. Once that has gone off another batch of silicone rubber is made up with a fixative additive. This is applied onto the initial silicone rubber coating to make the rubber approximately 1cm thick. Then onto this a fiberglass jacket it applied. Unfortunately in most cases the original artwork is destroyed during the making of the mold. This is because the originals are solid, and do not easily bend when the mold is removed. In many cases the original art piece is molded using multiple molds especially when large or complex items are to be cast.
The clip (above) is a video, of the casting process, of the bronze panels that butterfly bronze did. It shows, the process, from being sprued to the final pour.
Once the mold is complete molten wax is either painted or poured in depending on the size and shape of the mold. The mold therefore creates a carbon copy of the original art-piece in wax. This hollow wax copy of the artwork is removed from the mold. The artist can reuse the mold to make more wax copies of the original art piece, but wear and tear on the mold limit their number. Once the wax has been removed from the mold the foundryman will "Chase" the wax to remove imperfections that may of occured in the wax molding process. Common imperfections may occur around the rim where the two halves of the mold met. Using heated metal implements and wax the foundryman melts, smooths and molds the wax copy into shape. The wax copy is "sprued" with a treelike structure of wax that will provide paths for molten metal to flow and the air to escape. Invariable there are two main sprus but for more complex models more sprus may be used. The two key sprus are the "Runner" and the "Riser". The runner has a funnel shape attached where the molten metal will be poured and the riser is where the air will escape as the metal moves through the mold. It will also show the foundryman that the mold is full as the metal rises up it.
Ceramic Mold Making.
A "sprued" wax copy is dipped into a slurry of liquid silica, then into a sand-like "stucco", or dry calcified clay of a controlled grain size. The slurry and grit combination is called "ceramic shell" mold material. This shell is allowed to dry, and the process is repeated until at least a 10mm coating covers the entire piece. The bigger the art piece, the thicker the shell must be. Only the inside of the funnel is not coated and the funnel's flat top serves as the base upon which the piece stands during this process.
The Wax Burnout.
Once the ceramic shell is in place the whole piece is moved to a kiln. Placed on it's head with the funnel facing down the mold and wax object is heated to extremely high temperature. This allows all of the wax to melt and burn out of the mold, leaving only it's intricate imprint.
Testing the shell.
The ceramic shell is allowed to cool, then is tested to see if water will flow through the feeder and vent tubes as necessary. At this stage any cracks or leaks can be patched with thick refractory paste.
Pouring the metal.
The shell is reheated in the kiln to harden the patches, then placed cup-upwards into a sand filled bin. Bronze ingots are melted in a crucible using a furnace. When the metal has reached approximately 1100-1150 degrees centigrade the metal is carefully poured into the mold using the crucible. By preheating the shell the molten metal flows more freely through the mold. The metal-filled mold is then allowed to cool.
Knocking out the sculpture.
Once cooled the shell is hammered and sand-blasted away, sometimes very high powered water jets are also used. Once released the metal sprus are removed using an angle grinder, and thrown back into the pot to be reused in the next casting.
Just as the wax copies were chased for imperfections so are their metal counter parts. Using air powered grinders and sanding tools the areas where the sprus were are reworked as well as any other areas where imperfections may be. In some cases welding may need to be used to cover up larger problem areas.
Patinating the sculpture.
Once cleaned and chased the artwork can be patinated. Patination colours the bronze to give a final effect and also protects the bronze from the elements and decoloration. The bronze is colored to the artist's preference, using chemicals applied to heated or cooled metal. Using heat is probably the most predictable method, and allows the artist to have the most control over the process. This coloring is called patina, and is often green, black, white or brownish to simulate the surfaces of ancient bronze. Different chemicals and techniques will give different results. Ancient bronzes gained their patinas from oxidisation and other effects of being around for many years. Many effects can be reached. After the patina a sealer is generally applied — traditionally a coating of wax, but sometimes lacquer over more unstable patinas. This helps protect the piece from ultraviolet rays, and can slow the discoloration of patinas by oxidation. Patinas invariable don't last forever so touch ups need to be done down the years to keep the piece looking fresh.