IRON

Cast iron is free flowing when molten and very strong, if potentially brittle, when solidified. Various grades of common cast iron are nominally identified by the colour of an exposed TEST FRACTURE (GREY, BLACKHEART, WHITEHEART for example). Alloying elements such as CARBON, SILICON and PHOSPHORUS impart additional strength to the iron, hence cast iron's traditional use as both a structural and decorative material.

The use of cast iron in structurally oriented decoration was pronounced at the height of the industrial revolution during the mid to latter part of the nineteenth century. Some of the great masterpieces of engineering design were constructed at this time, including Sir Joseph Paxton’s (1801-1865) Crystal Palace for the Great Exhibition of 1851. A much earlier example of decorative ironwork was the railing erected around St. Paul's Cathedral, London, this was installed between 1710 and 1714. A cast iron head originating from the Ming dynastic period (China 1368-1644), can be found in the Fitzwilliam Museum, this artifact is believed to be one of the oldest examples of a cast iron artwork to be held in a British collection [ref].

COMPOSITION

Whilst the relatively high carbon content found in grey cast irons improves the material’s outright STRENGTH, this high carbon content also EMBRITTLES the metal and reduces it's ability to resist impact, shock and other stresses (esp. lateral loading). The application of a shock force may therefore lead to the fracturing of a cast iron design. Cast iron can be damaged through mechanical impact or heat application (hammering or welding for instance), or in service (usually due to vandalism or mishandling). For this reason the more resistant MALLEABLE and DUCTILE versions of cast iron are generally preferred for producing today's art and design works.

Ductile or SPHEROIDAL GRAPHITE (SG) iron – so called because the CARBON content of the metal chills into graphite nodules – is one of the more important modern iron grades. Whereas most of the carbon found in traditional cast iron crystallises into ‘flake’ like structures, the addition of magnesium to the SG iron charge combined with a low sulphur content, instead encourages carbon to crystallise as a spherical structure, imbuing the material with it’s ductile characteristics. The ability to manipulate a cast design to some degree without undue risk of fracturing is an especially useful quality in art founding, where sculptures are often extensively worked during the chasing/finishing stages.

Standard rutile type carbon steel MMA welding ELECTRODES can be used for joining ductile SG iron sections, these welding electrodes are far more economical to use than the more usual nickel bearing electrodes which are necessary for welding traditional cast iron grades (see below). In the US ductile cast irons are designated under ASTM A536-84:1984, in Britain and Europe either BS 2789:1985 or ISO 1083:1976.

The joining of cast iron sections is traditionally done with the aid of mechanical fastenings such as rivets or bolts, both of these methods minimise the likelihood of the cast fracturing – always a potential danger with the localised heat concentrations required for fusion welding processes. Many artists and designers working with grey cast irons use this feature to their advantage and treat fastenings as an integral visual feature of the artwork itself. Welding of grey and other traditional irons can be carried out with MMA (STICK) welding techniques, using a flux covered electrode containing a nickel bearing alloy (this type of electrode can be very expensive). The LOW HYDROGEN type welding electrodes and filler wires used for joining low carbon steel alloys are an alternative option, though the cast must be pre-heated before welding commences to minimise the possibility of cracking in the joint zone (SG iron excepted). Cast iron can also be joined by oxy-gas, in particular flux assisted BRONZE WELDING, which is a essentially a non-fusion brazing technique.

< BACK / NEXT >