Lost-wax casting
Lost-wax casting – also called investment casting, precision casting, or cire perdue (French: [siʁ pɛʁdy]; borrowed from French)[1] – is the process by which a duplicate sculpture (often a metal, such as silver, gold, brass, or bronze) is cast from an original sculpture. Intricate works can be achieved by this method.
The oldest known examples of this technique are approximately 6,500 years old (4550–4450 BC) and attributed to gold artefacts found at Bulgaria's Varna Necropolis.[2] A copper amulet from Mehrgarh, Indus Valley civilization, in Pakistan, is dated to circa 4,000 BC.[3] Cast copper objects, found in the Nahal Mishmar hoard in southern Israel, which belong to the Chalcolithic period (4500–3500 BC), are estimated, from carbon-14 dating, to date to circa 3500 BC.[4][5] Other examples from somewhat later periods are from Mesopotamia in the third millennium BC.[6] Lost-wax casting was widespread in Europe until the 18th century, when a piece-moulding process came to predominate.
The steps used in casting small bronze sculptures are fairly standardized, though the process today varies from foundry to foundry (in modern industrial use, the process is called investment casting). Variations of the process include: "lost mould", which recognizes that materials other than wax can be used (such as tallow, resin, tar, and textile);[7] and "waste wax process" (or "waste mould casting"), because the mould is destroyed to remove the cast item.[8][9]
Step 1: A model of an apple in wax
Step 2: From the model a rubber mould is made. (The mould is shown here with a solid cast in plaster)
Step 3: From this rubber mould a hollow wax or paraffin cast is made
Step 4: The hollow paraffin apple is covered with a final, fire-proof mould, in this case clay-based, an open view. The core is also filled with fire-proof material. Note the stainless steel core supports. In the next step (not shown), the mould is heated in an oven upside-down and the wax is "lost"
Step 5: Liquid bronze at 1200°C is poured into the dried and empty casting mould
Step 6: the bronze cast, still with spruing attached. The sprue will be cut away and the final shape polished
Casts can be made of the wax model itself, the direct method, or of a wax copy of a model that need not be of wax, the indirect method. These are the steps for the indirect process (the direct method starts at step 7):
Prior to silica-based casting moulds, these moulds were made of a variety of other fire-proof materials, the most common being plaster based, with added grout, and clay based. Prior to rubber moulds gelatine was used.
Jewellery and small parts[edit]
The methods used for small parts and jewellery vary somewhat from those used for sculpture. A wax model is obtained either from injection into a rubber mould or by being custom-made by carving. The wax or waxes are sprued and fused onto a rubber base, called a "sprue base". Then a metal flask, which resembles a short length of steel pipe that ranges roughly from 3.5 to 15 centimeters tall and wide, is put over the sprue base and the waxes. Most sprue bases have a circular rim which grips the standard-sized flask, holding it in place. Investment (refractory plaster) is mixed and poured into the flask, filling it. It hardens, then is burned out as outlined above. Casting is usually done straight from the kiln either by centrifugal casting or vacuum casting.
The lost-wax process can be used with any material that can burn, melt, or evaporate to leave a mould cavity. Some automobile manufacturers use a lost-foam technique to make engine blocks. The model is made of polystyrene foam, which is placed into a casting flask, consisting of a cope and drag, which is then filled with casting sand. The foam supports the sand, allowing shapes that would be impossible if the process had to rely on the sand alone. The metal is poured in, vaporizing the foam with its heat.
In dentistry, gold crowns, inlays and onlays are made by the lost-wax technique. Application of Lost Wax technique for the fabrication of cast inlay was first reported by Taggart. A typical gold alloy is about 60% gold and 28% silver with copper and other metals making up the rest. Careful attention to tooth preparation, impression taking and laboratory technique are required to make this type of restoration a success. Dental laboratories make other items this way as well.
Textiles[edit]
In this process, the wax and the textile are both replaced by the metal during the casting process, whereby the fabric reinforcement allows for a thinner model, and thus reduces the amount of metal expended in the mould.[10] Evidence of this process is seen by the textile relief on the reverse side of objects and is sometimes referred to as "lost-wax, lost textile". This textile relief is visible on gold ornaments from burial mounds in southern Siberia of the ancient horse riding tribes, such as the distinctive group of openwork gold plaques housed in the Hermitage Museum, Saint Petersburg.[10] The technique may have its origins in the Far East, as indicated by the few Han examples, and the bronze buckle and gold plaques found at the cemetery at Xigou.[11] Such a technique may also have been used to manufacture some Viking Age oval brooches, indicated by numerous examples with fabric imprints such as those of Castletown (Scotland).[12]
A wax model is sprued with vents for casting metal and for the release of air, and covered in heat-resistant material.
A cast in bronze, still with spruing
A bronze cast, with part of the spruing cut away
A nearly finished bronze casting. Only the core supports have yet to be removed and closed
