REFRACTORY (INVESTMENT) MOULDING MATERIALS
Moulds for lost wax casting are primarily constructed from REFRACTORY (heat resistant) materials. Some form of clay derived product is the principal constituent of almost all the refractory moulding systems used in art foundries; historically founders without access to today’s highly refined refractory products relied heavily upon locally sourced clays. In many cases these clays would be extracted from a location yielding material of an optimum quality for metal casting, sometimes with quite distinct working characteristics and quality of texture when compared to other local clays used for the production of ceramic artifacts and utensils.
Freshly extracted clay is of limited use as an effective refractory moulding material and some amount of further processing would be usual – even in the most elementary of ancient metal casting cultures. Processing would vary according to the sophistication and technology possessed by the founders/metal smiths, but modification would typically include weathering and some blending of different clay types, as well the likely addition organic material in the form of animal and/or vegetable matter. Alternative refractory materials (other than clay), might also be added during the construction of the mould, these typically include charcoal and ground (fired) ceramic particulate from crushed crockery and discarded utensils.
The refractory properties of a clay are inherited from the parent rock. Granite, which breaks down to a pure clay with a uniform crystalline structure, is a particularly important parent rock in this respect. Granite bed rock is attacked chemically (by boron and fluorine), which causes the rock to partially decompose into the mineral kaolinite. The clays most suited for processing into lost wax refractories also contain significant levels of heat resistant minerals such as ALUMINA and SILICA. In addition to these minerals, complimentary chemicals may also be present in the clay, including magnesia, potash and metal oxides (these variously act as fluxes, colour and texture modifiers).
Such is the refractory resistance of some clay grades to high temperatures, that they are known collectively as ‘fireclays’. GROG – which features as a major constituent of traditional refractory moulds – is a ground fireclay body kiln fired to a minimum 2400°F (1300°C). Some other refractory materials with very similar properties to clays are also used in art founding. BENTONITE (derived from decomposed volcanic ash), is one of the most important of these, though in this material is principally associated with GREEN MOULDING SANDS.
Modern, processed refractory products are supplied to the founder either in the form of a dry grits or ground powders (GROG, PLASTER or FUSED SILICA); or else as a wet preparation (COLLOIDAL SILICA). Traditional dry supplied refractory materials plaster & grog), are combined and wetted before application to the wax assembly, usually by mixing with tap water to a specific proportion. Modern dry refractories (fused silicas), adhere to a wax pattern by immediately dusting the grits onto a wet ceramic SLURRY coating.
The various constituents of the refractory moulding systems used in art foundries are developed and processed by specialist chemical and mineral companies. Modern refractories are therefore subject to intensive quality controls and this attention to quality provides the founder with a consistent and predictable group of materials to work with. Even so, it is quite common for individual art founders to modify the contents and constituent proportions of a commercially supplied refractory system to suit their own individual preferences and techniques.
Although the precise composition of the refractory used can differ from foundry to foundry, all moulding systems universally feature the following three basic elements:
1. A BASE REFRACTORY
2. A BINDER
3. ADDITIONAL ADDITIVES
A traditional grog &
plaster 'block' mould.
Ceramic 'open shell' mould.
(Photo's cc ANPP).
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