< JEWELLERY INVESTMENTS
INVESTMENT – A SUMMARY
The word INVESTMENT can be used to refer both to the materials and the application techniques used when making refractory moulds for lost wax casting.
A WAX PATTERN that has had various casting attachments added, including cup, runners and risers, is known as a WAX ASSEMBLY.
Investment moulds are designed to resist thermal shock and the pressure of expanding wax and later, entering metal CHARGE. Investment moulds are also constructed in a way that allows gases from the metal pour to partially vent through the mould’s walls. The moulds are sometimes reinforced in their outer layers with metal clips or mesh, chopped fibres, sand other materials.
All art foundry investments consist of three basic components: a BINDER, a REFRACTORY, and modifying ADDATIVES. The binder ‘glues’ together any non-adhesive refractory body. Modifying additives are added to improve the properties of the binder and/or the refractory material. The use of additives varies according to the investment system used.
The four most common lost wax investment systems used in art founding are:
1 PLASTER & GROG (traditional investment). Uses a gypsum based binder in combination with a fireclay grog body, this is formed into a BLOCK MOULD around the wax assembly. Progressively coarser grades of grog are applied as the mould wall thickness is built up. The investment materials are less expensive and easier to prepare than those used in shell systems; however, moulds of this type can be prone to damage and breakdown during kiln firing and casting.
2 CERAMIC SHELL investment is built up through a repeated immersion of the wax assembly in a ceramic SLURRY. Fused silica STUCCOS of progressively larger grit sizes are applied to the wetted assembly in successive layers until the mould wall is fully built up. A solid refractory core is often omitted, instead the founder builds up a continuous shell jacket covering both the internal and external surfaces of the wax. Ceramic investments produce moulds with excellent strength to weight ratios, capable of faithfully reproducing fine pattern detail; however, the system does require the use of some specialist equipment, together with frequent slurry monitoring and maintenance schedules. COLLOIDAL SOL and stucco materials are relatively expensive to purchase when compared to plaster and grog investments. Ceramic shell moulds are also suitable for use when casting alternative materials such as glass.
3 The HYBRID investment mould is constructed using both ceramic shell (facing layers), and plaster/grog (backing layers), investments. Combined with sand venting techniques, hybrid moulds are especially effective for the production of large scale lost wax casts. Using both investment systems allows the founder to combine the high quality of surface reproduction and stability offered by ceramic shell, with the economy provided by less costly plaster and grog refractories. Use of hybrid investment does require the founder to stock a wide range of materials, and to have access to specialist equipment – the plaster and grog component of the investment system also lengthens kiln firing times.
4 JEWELLERY investments for use in either VACUUM or CENTRIFUGAL casting processes, are usually composed of a gypsum binder and a crystobalite (silica) refractory body. The investment, binder and any added ingredients, are normally supplied to the founder in the form of a single pack combined powder preparation. Unlike other investment systems, jewellery and dental investments are not graded, with coarser grits towards the outer wall of the mould, instead they are made up of a single fine grade. The investment powder is mixed with water or silica sol, degassed, then applied in a single ‘shot’ to a single item or cluster of wax patterns mounted on a SPRUE TREE. The tree is contained within a FLASK during investment pouring. The resulting block mould is normally cast with vacuum or centrifugal assistance. Investment powders for this process are relatively expensive; likewise, the sophisticated machinery required for degassing and casting can involve some considerable financial outlay by the founder. Suitable for the production of small scale works only, relatively few art foundries have direct access to these investment systems.
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