< INTRODUCTION TO INVESTMENT
INVESTMENT MATERIALS
Investment moulds are usually constructed using a combination of refractory and non-refractory materials. In almost all cases though, some form of clay derived product is the principal constituent of the investments used in art foundries. Historically, metalsmiths without access to today’s refined investment materials relied heavily upon local REFRACTORY material. In most instances this material would be extracted from a location yielding clays of an optimum quality for metal casting. Clays suitable for use as lost wax investment refractories can be quite distinct in texture and working properties from those used in the production of ceramic artifacts 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 usually included the weathering and blending of different clay types, and the addition of organic material in the form of animal and/or vegetable matter. Other refractory material might also be added to an investment clay, typically this would be in the form of charcoal, or a ground ceramic particulate from crushed crockery and the like.
The refractory component of a clay is inherited from the it’s 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), and this causes the rock to partially decomposes into the mineral kaolinite. The clays most suited for processing into lost wax investments also contain significant levels of the 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 prepared for use in the foundry by kiln firing 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 case the material is found principally in GREEN MOULDING SANDS.
Modern, processed investment refractory products are supplied to the founder either in the form of a dry grit or powder (grog, plaster or fused silica), or else as a wet preparation (colloidal silica). Traditional types of dry refractory material are wetted before application to the wax assembly, usually by mixing with tap water to a very specific proportion. Modern dry investments adhere by immediately dusting dry grits refractory over a wet ceramic slurry deposit on the wax pattern.
The various components of the investment systems used in art foundries are developed and processed by specialist chemical and mineral corporations. 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 usual 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
The principle moulding materials, systems of investment, and the methods of application to a wax assembly, are outlined in more detail later in later pages. First, a brief explanation of common physical features that make up typical art foundry investment mould.
COMMON FEATURES OF INVESTMENT MOULDS >
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