Wrought iron was traditionally smelted at a relatively low temperature in the solid state to produce a spongey mass of metal called a bloom from which the impurities were driven off as liquid slag by hammering, hence the term "wrought" ie "worked" iron. The bloomery method was later replaced with others, more sophisticated, which achieved the same results. Wrought iron is very pure, with a carbon content of less than 1%, which makes it resistant to corrosion, strong in tension and malleable. It was much used for gates and railings, and for structural members, but the early methods of production, (the iron cooled quickly and could only be worked in small quantities) limited the size of parts which could be easily made. Long thin sections were impossible until the development of the rolling mill, which is still used in steel production, during the eighteenth century. The rollers could be shaped to produce bars of different sections for the smith, nailmaker etc and eventually patterned sections , which were known as fancy work.
By comparison, cast iron, only produced commercially after the introduction of cupola blast furnaces in 1794, was smelted at much higher temperatures in the liquid state, and so became saturated with carbon from the furnace fuel, up to about 5%. It was then poured out (ie cast) into a mould to produce blocks traditionally known as pigs, because the line of individual blocks connected to a channel looked like a litter of suckling pigs, hence the name "pig iron". The high carbon content makes cast iron very rigid in compression, but weak and brittle in tension, even when red hot, so it cannot be forged or rolled. However it also lowers the melting point so it can be easily poured into moulds to produce complex shapes. Moulds were commonly made of a sand or clay mixture.
Casting techniques improved and by the end of the Georgian period, the material was being used for all types of structural and decorative purposes. Both cast and wrought were in wide use for items such as cooking pots to gravestones, door furniture to machinery even whole buildings, until they were supplanted by steel, an iron/carbon alloy.
It can be difficult to tell wrought from cast iron. Generally, wrought iron is more delicate and is employed for the more fantastic shapes. Cast iron is usually chunkier and as the process would suggest occurs where detail is repeated. A broken piece of wrought iron will reveal a fibrous structure, while broken cast iron is more crystalline. The catalogues of the great foundries, such as McFarlanes and Carron, were fantastic works of art, now much prized. Most foundries signed and dated their work.
Wrought iron has now been totally eclipsed by steel production. Cast iron has come back into fashion as a structural material in the last decade partly due to the development of SG (spheroidal graphite) malleable cast iron. Certain additives cause the excess carbon to precipitate out of the iron into tiny globules, allowing the iron matrix to regain its flexibility and tensile strength, and function more or less like wrought iron.
Corrugated iron was invented in London in 1828 by Henry Robinson Palmer. It involves a rolling mill process, if it is stamped, it will have weak spots. It quickly became popular particularly for roofing and farm buildings, but was also used in more elaborate structures. There is a corrugated iron ballroom at Balmoral.
The Cast Iron Soil Pipe Institute an American organisation dedicated to aiding and improving the plumbing industry.