Lost-wax casting is an ancient technique for replicating an object in metal. In casting a hollow piece, the interior of a mold is coated with layers of wax and the core of the wax shell is filled with heat resistant material. When heated, the wax drains off to leave a cavity that will later be filled with molten metal. A solid piece can be cast by encasing a solid wax model to form a casting mold. The process is also known in art as cire perdue and in industry as investment casting.
Examples of lost-wax casting have been found in multiple cultures dating to 3,000 BC. The technique itself has essentially remained the same over the years. The components used have evolved from the original bronze, beeswax and pottery clay to modern alloys and synthetic materials.
Typically, the procedure begins with the creation of a model either by hand or by injection into molds taken of an original design. Wax or a similarly workable material is usually used. A mold of the result suitable for metal casting is then made. The interior of the mold is the exact negative image of the model. This allows any detail that can be worked into the model to be reproduced in metal.
For a hollow lost-wax casting, the interior of the model mold is layered with melted wax to a desired thickness. The wax shell is removed and touched up to erase imperfections. It is then fitted with a funnel-shaped pouring channel, and wax rods called sprues. These will form vents to allow gasses to escape when the metal is poured. The interior of the shell is filled with heat resistant material, which may be held in place by metal pins.
To cast a solid piece, the interior of the model mold is filled with wax. Upon removal, it is touched up and fitted with a pouring funnel and sprues, as well. In either method, the entire wax construct is then encased in a material that produces a mold that can be handled, heated in a kiln and receive molten metal. This mold might be made from layered plaster or gradually built up from layers of very fine to increasing coarse aggregate material.
When the mixture has hardened, the mold is placed in a kiln and heated to about 1,250 degrees F (677 C). The wax will be melted out or vaporized leaving a cavity in its place. Molten metal is poured into the mold filling this cavity. Upon cooling, the encasing material will be removed and evidence of the venting tubes polished away, or chased.
Special encasing materials are necessary when using metals with a very high melting point. When lost-wax casting is used to create small jewelry, techniques involving the use of vacuum and centrifugal force are sometimes needed to properly fill the molds. Industrial use of the process may be costly, but it allows intricate parts to be cast from almost any metal with high accuracy and an excellent finish.