< INTRODUCTION TO INVESTMENT
INVESTMENT MATERIALS
Investment moulds are primarily constructed REFRACTORY (heat resistant) materials. Some form of clay derived product is the principal constituent of almost all the investment systems used in art foundries; historically metalsmiths without access to today’s highly refined refractory materials 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 in comparison to the local clays used for the production of ceramic designs and utensils.
Freshly extracted clay is of limited use as an effective investment refractory and some amount of further processing would be usual – even in the most elementary of metal casting cultures. Processing would vary according to the sophistication and technology possessed by the founder/metalsmiths, but modification would typically include weathering and blending of different clay types, as well as adding organic material in the form of animal and/or vegetable matter. Other refractory materials than clay might also be added during the construction of the mould, these materials might include charcoal and ground (fired) ceramic particulate from crushed crockery and the like.
The refractory properties of a clay are inherited from the parent rock. Granite, which produces a pure clay of uniform crystalline structure, is a particularly important 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 investments also contain significant levels of high refractory minerals, 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 investments – 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 used with GREEN MOULDING SANDS.
Modern, processed investment 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 refractory materials plaster & grog), are wetted before application to the wax assembly, usually by mixing with tap water to a specific proportion. Modern dry investments (fused silicas), adhere to a wax pattern by immediately dusting grits onto a wet ceramic slurry coating.
The various components of the investment systems used in art foundries are developed and processed by specialist chemical and mineral companies. Modern investments are therefore subject to extensive 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 founders to modify the contents and constituent proportions of a commercially supplied investment to suit their own individual preferences and techniques.
Although the precise composition of the investment used can differ from foundry to foundry, all investment systems universally feature the following three basic elements:
1. A BASE REFRACTORY
2. A BINDER
3. ADDITIONAL ADDITIVES
COMMON FEATURES OF INVESTMENT MOULDS >
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