Being the occasionally interesting ramblings of a major-league technophile.
There are many types of iron. You have the actual element, pure Fe, which comes in several isotopes (same electron shell structure, same charge on the nucleus, but different numbers of neutrons in the latter) which all have the same chemical properties. Wrought iron is nearly pure, and has been in use from before the time of the classic Iron Age up well into the age of steel. (The last wrought iron plant in the US closed in 1969.)
Wrought iron gets its name from the traditional method of production. Iron ore was heated and "wrought" (from wreaked, or bent) to work the impurities out. During this process, a lump of iron ore is heated, then pounded, repeatedly, to drive out the impurities and forge the metal into a single, solid piece. During this work the hot piece of ore is beaten flat, reheated, folded, and beaten flat again, until the smith determines it is ready for forging. Wrought iron was - and still is - a fine material for many uses. It is ductile - that is, easily worked without splitting or cracking - and nearly immune to rust (largely due to traces of silicon but also due to the tiny amount of carbon). Making large objects from wrought iron using this traditional method of production is difficult, since the "blooms" produced by wreaking are small, maybe enough for a modest belt knife. Anything larger required welding or forging the products of two or more blooms together.
On the other hand, a weapon or piece of armor made in this way has some interesting properties. For example, sword blades were made by forging separate strips of wrought iron, heating them and twisting them together, then forging the result into a single mass, often folding one or more times along the way to get the piece to the correct dimensions. The result was a pattern blade. Named because breathing on the metal brought out a writhing pattern created by the twisting, folding and flattening. Some such blades were even acid etched, to bring out the pattern without the fog of breath.
Items of this sort are often referred to as "Damascus" or "Damascene" but most likely the process was developed independently multiple times in widespread locations through a long period.
Such blades had properties quite similar to those of the famed folded blades of Japan, such as the katana sword. Because no two blooms were identical, the twisting and forging fused metals with slightly different properties, making a composite which blended the different properties. Selective treatment of the edge made sure it was hard enough to stay sharp. The inherent resilience of the wrought iron in the body of the blade, combined with the springiness imparted by the twisting and folding, kept the blade from breaking or bending easily. Once the knowledge of how to produce steel became widespread the same practice was used to make early steel items of large size, since batches of steel were small during this period.
The process of making a sword or breastplate in this way was very labor intensive, however. With little change in technique for centuries, well-made swords and even knives were often handed down as working items for generations. Often, if a good-quality long blade was broken, the already forged metal was simply cut and ground the minimum amount to make the next smaller blade of the same general shape. Thus a fine long sword broken in battle might become a belt knife and short sword. Good metal was simply too precious to risk reforging into something which might not be nearly as good.
Cast iron became common some centuries after wrought iron. It is made by using a large, hot furnace to melt iron ore in a large batch. As the ore becomes hotter, the carbon becomes more soluble in the iron, and the melting point of the combination is lower than that of wrought iron. At around three percent carbon the iron will melt enough to run into molds. In the old days these were often nothing more than a channel dug into the dry sand floor of the foundry, leading to blocky cavities likewise excavated. The result resembled pigs lined up at a slop trough; hence the term "pig iron."
Cast iron is stronger and harder than wrought iron. However, it is also brittle, and it rusts easily. Like bronze, it can be cast and filed, but not forged or easily welded. So why wasn't it a big step backwards?
Because it can be cast and filed. Making items for which brittleness wasn't a problem was far quicker and easier with this method. Also, the reason large castings of wrought iron are rare is that - as mentioned above - pure iron has a higher melting point than that of cast iron. Melting enough cast iron to pour into a mold is, simply, easier.
As demand for iron increased and improvements were sought, people learned - independently, several times, in several locations - that blowing air through molten cast iron would remove the carbon, producing wrought iron in large amounts, without all that "wreaking." Only, if you didn't remove enough of the carbon, you got something else. Wrought iron has little or no carbon. Cast iron has a lot of carbon. And in between... lies steel.
Steel was known long before someone figured out how to make it from cast iron. Remember the comment above about treating the edges of forged wrought iron swords? One of the treatments was to put the finished object in contact with carbon (usually charcoal) and heat. This allows a small amount of carbon to dissolve into the forged wrought iron, face or case hardening it. That is, converting the outer layer of wrought iron into steel. Again, creating a metal composite.
Even before that, people were making use of meteoric iron. It comes naturally as a steel which is in many ways finer than anything produced deliberately until the Nineteenth Century. More than one mythic blade may have been made with this metal.
Oh, the stories which could be told of the long struggle to understand and master the process of deliberately making steel with the desired properties. The superstitions, the rules of thumb, the strict rituals and fickle results. For over a thousand years, the most successful makers of iron-based swords and armor were not those who made the best case hardening or the most impressive blade shape and balance, but those who could consistently reproduce their results. Doing this without modern chemical theory, metallurgical knowledge or even thermometers requires a keen eye (as well as ears and nose) and a fanatical dedication to one's craft.
All of which ended - though in reluctant stages - as science explained just what was happening, and provided tools to make it happen more easily.
Even the production of steel changed, with the introduction of the Bessemer furnace and its descendants. But wrought iron remained the most frequently useful member of the family until then, even if only as a starting point before case hardening. It continued strong for decades after, due to such factors as its corrosion resistance. Eventually, however, wrought iron died except for a few specialty mills, which produce it for the art trade. Cast iron still sees significant use, though. New methods of production have made it far more economical than wrought iron, either for making cast iron items, or as a precursor to steel.
Today, in fact, various types of cast iron are doing jobs once the province of wrought iron or even steel. There are cast irons which are corrosion resistant. Cast irons which are ductile.
These improvements require moving beyond the simple combination of iron and carbon, with other stuff - such as silicon - added to moderate the effects of large amounts of the latter when mixed with the former. In truth, there has always been other stuff in any iron, native to the ore or coming from the particular process - and even individual crucible, mold, hammer or anvil - used to make it. Those were present as accidents, though, and any benefit they provided was a mystery to those working the metal. Today it can be safely said we have now tamed cast iron - the metal once decried as "ruined iron" - and can make it do the job of true iron, the wrought iron of old. Sometimes even better than the original could.
This document is Copyright 2007 Rodford Edmiston Smith. Anyone wishing to repost it must have permission from the author, who can be reached at: firstname.lastname@example.org