< MELTING & POURING METALS
FURNACES
There are two types of furnace design commonly found in art foundries. The first, and most often encountered, is the LIFT OUT type, the use of which was outlined in the preceeding description of a typical casting procedure. As the name suggests, the lift out furnace is designed to melt metal alloys in a free standing crucible that can later be lifted clear of the furnace body, allowing the charge to be decanted into a refractory mould.
The largest lift out furnaces can handle crucibles with a capacity of some 400lbs+ (180kg+) of COPPER ALLOY. The capacity of crucibles is usually quoted as the maximum mass of BRASS or ALUMINIUM containable, with 1 inch (25mm) or so of freeboard to the crucible lip. Any crucible containing more than 250lbs (110kg) of metal CHARGE is usually lifted and handled in the foundry with some form of mechanical assistance, typically a running block and tackle system. A foundry with a 300lb (136kg) BRASS WEIGHT pouring facility should be capable of producing a cast sculpture up to and including a life scale figure, without having to resort to an excessive cutting up of the artwork to accommodate furnace capacity.
Foundries regularly casting refractory moulds containing in excess of 300lbs brass weight may additionally have access to a TILT furnace. This type of furnace commonly has a capacity of at least 500-1000lbs (225-450 kg), and is often associated with foundries that have sand casting facilities. Unlike the lift out type furnace, the crucible in the tilt furnace is fixed within the furnace body and is removed only for replacement purposes. The metal charge is introduced into the crucible via an access point at the top of the furnace, with the exhaust vented via an outlet, often positioned to one side or at the rear of the furnace casing.
Upon reaching a pourable temperature, the molten charge in the tilt furnace can be decanted either into a smaller pre-heated crucible held in shanks (for the manual pouring of multiple small moulds), or else into a geared LADLE for pouring large capacity moulds. The charge exits from a tilt furnace via a spout running from the lip of the inner crucible, passing through the furnace casing. This is enabled by pivoting the entire furnace body (hydraulically assisted), through 90°+ off vertical, to a horizontal axis.
There are a number of alternative furnace designs available to founders, including the traditional CUPOLA type – though the use of this and other designs in fine art foundries is now so rare as to be of passing interest only. Some of these furnaces alternative are found in commercial foundries and are featured briefly both in the following section and the section of the website covering SAND CASTING and COMMERCIAL FOUNDING.
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