KILN & CASTING – A SUMMARY
The BURNOUT (de-wax) procedure removes the wax pattern and wax casting attachment (ASSEMBLY), contained insise the refractory mould. Evacuation leaves a negative impression of the wax image within in the refractory mould's cavity, which is then filled with metal to form a 'positive image' casting. FIRING also alters the refractory mould’s chemical composition, converting it from a GREEN condition, to a fully functioning mould with optimum refractory properties.
Burnout and firing can either be done as a single continuous operation, or in two distinct steps (by first removing the wax via a steam autoclave or by chemical means, then later firing the evacuated mould in a standard kiln).
The kiln's temperature and firing cycle (which may be extended over a period of some days), is controlled to prevent an under or over firing of the refractory mould. Maximum kiln temperatures for firing the refractories used in fine art founding do not usually exceed 2000°F (1100°C).
CERAMIC SHELL moulds can be FLASH de-waxed and fired simultaneously. The highly efficient thermal properties used for this moulding system, allows the founder to remove a red-hot mould from the kiln and immediately pour in a metal charge.
If minor faults are detected in the fabric of a (ceramic) refractory mould after firing, these can usually be repaired simply by applying a refractory cement patch and briefly re-firing the mould. Patches may be held securely in place on the mould by wrapping with wire for extra security.
Most art foundries melt metals in either LIFT OUT or TILT type furnaces. These may be GAS, OIL, or occasionally ELECTRICALLY fuelled. The capacity of the largest crucibles can be as much as 1000lb+ (450kg)+ of copper alloy.
A metal CHARGE is melted as rapidly as possible, avoiding the uptake of atmospheric gases and other impurities. The charge is then DEGASSED with an alloy specific tablet or cartridge before pouring commences. Care is taken throughout the melt not to burn off light alloying elements with low boiling points.
Most of the COPPER ALLOYS used in art founding are poured at an optimum temperature of between 2000-2200°F (1100-1200°C). Pouring is carried out as a rapid, but controlled sequence of actions avoiding excessive spillage, turbulence and the entry of foreign material (INCLUSIONS) into the refractory mould.
VACUUM and CENTRIFUGAL casting is carried out with the aid of specialist equipment. Either process allows intricate and detailed works to be accurately reproduced in metal by artificially ‘forcing’ the molten charge into the refractory mould cavity.
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