The word “wrought” is an archaic past participle form of the verb “to work,” and so “wrought iron” literally means “worked iron”. Wrought iron is a general term for the commodity, but is also used more specifically for finished iron goods, as manufactured by a blacksmith or other smith. It was used in this narrower sense in British Customs records, such manufactured iron being subject to a higher rate of duty than what might be called “unwrought” iron. Cast iron, unlike wrought iron, is brittle and cannot be worked either hot or cold. Cast iron can break if struck with a hammer.
In the 17th, 18th and 19th centuries, wrought iron went by a wide variety of terms according to its form, origin, or quality.
While the bloomery process produced wrought iron directly from ore, cast iron or pig iron were the starting materials used in the finery forge and puddling furnace. Pig iron and cast iron have high carbon content which make them very brittle, but they have a lower melting point than iron or steel. Cast and especially pig iron have excess slag which must be at least partially removed to produce quality wrought iron. At foundries it was common to blend scrap wrought iron with cast iron to improve the physical properties of castings.
For several years after the introduction of Bessemer and open hearth steel there were different opinions as to what differentiated iron from steel, some thinking it was the chemical composition and others believing it was whether the iron heated sufficiently to melt and “fuse”. Fusion eventually became generally accepted as relatively more important than composition below a given low carbon concentration.9 Another difference is that steel can be hardened by heat treating.read more