< FURNACES
FUELS
Most of the furnaces found in art foundries are fuelled by either town gas (including liquid petroleum gas [LPG] for portable furnaces), or else oil (diesel). Gas fired furnaces are the most common. The tendency towards gas pick-up in the metal CHARGE requires particular care to be taken with the gas furnace’s atmosphere control (should be slightly OXIDISING), and casting procedure (the use of a COVER FLUX with DEGASSING of the charge prior to the pour). Oil fired furnaces have the advantage of being able to attain higher melt temperatures than gas fired versions. This characteristic can be of particular benefit when working with cast iron; at about 2,700°F (1500°C) or so, most cast iron alloys have a melting point some 5-700°F (3-400°C) higher than the more common copper casting alloys.
The ‘cleanest’ melts are achieved in ELECTRIC furnaces of the RESISTANCE and INDUCTION types. Resistance furnaces are comparable to domestic electric coil heaters, insofar as they radiate heat via an electric element. The resistance furnace is primarily designed to melt, and then hold low melting point alloys (zinc and aluminium are typical), within BASIN CRUCIBLES. This type of furnace is closely associated with repetitive industrial die casting processes – the charge held within the basin is decanted into individual metal DIES by means of a hand held ladle, or sometimes a mechanical pressure pump. The power output of resistance furnaces is rather limited so higher melting point metal alloys like copper are often unsuitable for melting in this type of furnace.
The electric induction furnace (also rarely found in art foundries), is more suitable for melting higher melting point alloys than the electric resistance furnace. Electric induction furnaces are worthy of note because of their minimal transference of contamination to a contained charge, and their ability to melt special alloys such as stainless steels – a material normally beyond the scope of most other furnace types. The induction furnace consists of an insulated/water cooled induction coil which carries an alternating current to a frequency of up to 2000 Hz or more (compared to most domestic mains supply grids of about 50 Hz). The high frequency induces a current in the metal charge, causing it to melt relatively free of gas contamination, yet capable of attaining a high temperature. Few fine art foundries can justify the financial outlay required to purchase such sophisticated and expensive equipment, though some may be able to prepare suitable investment moulds and arrange for pouring at a specialist facility.
Whilst by no means the only (or an infallible) guide to the suitability of a foundry, obtaining some basic information on their furnaces and pouring capacity can help indicate the ability of a founder to carry out work of a particular type or scale.
VACUUM CASTING >
|
