Lost Wax Casting

The process of lost wax casting used by artists today is an adaptation on the traditional technique of ancient cultures. Originally, forms were constructed in beeswax and then coated with layers of clay. Straw or linen would then applied to the exterior of the clay mold and then placed into an oven to harden and burn out the wax within. Molten metal would then be poured into the remaining cavity and the clay shell removed to reveal the finished casting. Due to the destruction of the wax model this technique is referred to as lost wax casting.

Contemporary artists rely on a variation of this method. A wax form is constructed in a number of ways including building up layers with melted wax or by carving into hardened wax. Wax injection, when heated wax is injected into a rubber mold, allows the artist to produce multiple forms of a similar shape. Another method used is wax extrusion, when wax is pushed through a heated form to create unique wax forms.

Once the wax form is completed thin wires of wax called sprues are attached to the model and then to a wax button or base to keep the model in position while creating the mold. During the casting process sprues allow the liquid wax to escape and molten metal to enter the remaining cavity. In some cases vents are applied to larger wax forms to help ventilate excess gasses while casting.

The wax form with attached sprues and button is placed into a steel flask, which is then filled with a plaster-based material called investment. Before and after filling the flask air bubbles are removed from the investment by either vacuum or vibration to ensure the consistency and quality of the cast.

The filled flask is allowed to dry and then placed into a kiln and slowly heated to both harden the mold and burn out the wax form within. The flask, now an empty mold, is injected with molten metal by steam casting, vacuum assist casting, sling casting, or centrifugal casting. Steam casting uses the steam pressure from the heat of the molten metal to force the metal into the mold. Vacuum assist casting uses a vacuum pump that creates enough pressure to draw the molten metal into the mold. Sling casting uses centrifugal force created by swinging the flask by hand around in a large circle, forcing the molten metal at the top of the flask to enter the mold. Centrifugal casting uses centrifugal force created by a spring-loaded arm that when released spins in a circle, forcing the molten metal from a heated crucible into the mold.

Materials other than wax can be used to cast models including plastic, wood, and organic substances such as insects or leaves.

Reference The Complete Metalsmith written by Tim McCreight