Cire Perdue (Lost Wax) Method

A method of constructing moulds for metal casting. In the lost-wax method, a model of the object to be cast is made in wax and then invested with clay; when the clay is baked the wax runs out, leaving a clay envelope in which an exact metal replica of the wax model can be cast. If the metal object is to be hollow, it is only necessary to give the wax model a clay core (which will become the core of the casting and may be seen in X-rays of ancient artefacts). The lost-wax method is well-suited to produce objects of irregular or undercut shape. Since the thickness of wax applied to the clay core is easily controlled, it also helps the craftsman to keep the walls of a hollow casting uniformly thin, saving metal and reducing the risk of flaws due to uneven cooling. The earliest lost-wax castings yet identified come from a 4th millennium bc hoard of copper and arsenical copper objects, some cast around clay cores, found in Nahal Mishmar near the Dead Sea. The main alternative to the lost-wax process, called the piece-mould or sectionmould technique, constructs the mould without the aid of an evanescent model. In this technique, clay is packed around a permanent model, not of wax, and then removed from the model in sections, the sections being reassembled to form the mould. The number of sections into which the mould must be divided in order to free it from the model depends on how intricate or undercut the shape of the model is. Bivalve moulds, the simplest of section moulds, are adequate to produce many weapons and tools. Castings with mould marks corresponding to the divisions of three- or four-part moulds were among the metal objects unearthed by Woolley in the Royal Cemetery at Ur. For shapes that are not excessively complicated, the section-mould method is practical and straightforward (see The section-mould method was used extensively in ancient China, where lost-wax casting did not appear until about the 6th century bc (see Xiasi). Outside China, however, perhaps its main use has been as an adjunct to lost-wax casting. The lost-wax method suffers from the drawback that the model is destroyed in the process of making the mould, so that only one casting can be obtained from each model; moreover any accident to model or mould can mean the loss of all the effort invested in preparing the wax model. The solution to this problem, exploited in Greek foundries in the 5 th century bc if not before, is to begin with a permanent model that is not of wax and to form a section mould on this; the section mould is then used to shape duplicate wax models, each of which can be used to make a casting mould. Modern artfoundry work generally relies on some such combination of the section-mould and lost-wax techniques, using first the section-mould method to make a wax model and then the lost-wax method to cast a metal replica of the wax model. The procedure necessarily sacrifices some of the freedom of shape offered by the lost-wax method, since the model must not be so complicated that a mould cannot be conveniently removed from it in sections. By way of compensation, it allows duplicate castings and keeps the original model intact. The expression ‘lost-wax process’ in many contexts refers to the entire procedure just described, including the first step in which a section mould is formed on the permanent model.