Lost-wax casting is an old method of making a metal replica of an object. 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 casting a hollow piece. When heated, the wax drains away, leaving a cavity to be filled with molten metal later. A solid piece can be cast by encasing it in a casting mold made of solid wax. The process is also known as cire perdue in art and investment casting in industry.
Lost-wax casting has been discovered in multiple cultures dating back to 3,000 BC. Over the years, the technique has essentially remained the same. From the original bronze, beeswax, and pottery clay to modern alloys and synthetic materials, the components have evolved.
The process usually starts with the creation of a model, which can be done by hand or by injection into molds of an original design. Typically, wax or a similar workable material is used. The result is then turned into a mold that can be used for metal casting. The interior of the mold is a negative representation of the model. This allows for the replication of any detail that can be worked into the model in metal.
The interior of the model mold is layered with melted wax to a desired thickness for a hollow lost-wax casting. The wax shell is removed and retouched to remove any flaws. It’s then outfitted with a funnel-shaped pouring channel and sprues, which are wax rods. When the metal is poured, these will form vents to allow gasses to escape. The shell’s interior is lined with heat-resistant material that is held in place by metal pins.
The interior of the model mold is filled with wax to cast a solid piece. It is touched up and equipped with a pouring funnel and sprues after removal. The entire wax construct is then encased in a material that creates a mold that can be handled, heated in a kiln, and receive molten metal in either method. This mold could be made out of layered plaster or layers of very fine to coarse aggregate material built up over time.
The mold is placed in a kiln and heated to about 1,250 degrees F once the mixture has hardened (677 C). The wax will be melted or vaporized, leaving behind a cavity. This cavity is filled with molten metal poured into the mold. The encasing material will be removed after cooling, and any evidence of the venting tubes will be polished away, or chased.
When using metals with a very high melting point, special encasing materials are required. When lost-wax casting is used to make small jewelry, vacuum and centrifugal force techniques are sometimes required to properly fill the molds. The process is expensive for industrial use, but it allows intricate parts to be cast from almost any metal with high accuracy and a great finish.