Here is some more info on making steel from iron.
Both wrought iron and cast iron have their uses, but since neither form of iron has ideal properties, the smith will probably want to make steel, at least in small amounts. To do this, he needs another type of furnace. If he is faced with high-carbon cast iron, he must use an oxygen-rich furnace to try to "decarburize" or reduce the carbon content. If he is faced with low-carbon wrought iron, he must somehow produce a carbon-rich environment that would encourage limited amounts of carbon to combine with the iron.
Both tasks are hard to control in practice, but there is evidence that both methods were practiced in different parts of the world. Western smiths usually followed a process of heating the low-carbon wrought iron in some type of sealed container containing carbon, the idea being to promote the migration of carbon atoms into the metal. It was tricky, and often produced only small amounts of steel, but steel was simply so useful for tools and weapons that even small amounts were important. Steel edges were usually welded to a wrought iron core or blade to make a steeled tool in the most economical manner.
Broadly speaking, Europeans were devoted to the bloomery process until late in the Middle Ages, while the Chinese followed the opposite path, producing high-quality iron castings from the Chou Dynasty onward. The Chinese made steel or wrought iron by decarburizing their high-carbon cast iron, while Europeans made steel from their low-carbon wrought iron and seem not to have used cast iron at all. (There is some evidence the Romans made small amounts of cast iron by accident and discarded it as a "waste product.")
The job of the smelting furnace is to reduce the metal from its chemically combined state to a metallic state. Iron is a reasonably common substance in the earth, but as any owner of an old car will attest, most of the time it takes the form of rust, iron oxide. In the smelter, iron oxides and other chemically combined forms of iron have their chemical bonds broken. This allows the iron atoms to combine into a mass of metal. Rust goes in; iron comes out.
The smelting furnace has two tools to bring about this transformation: heat and carbon. Smelters, like all furnaces, burn carbon fuels to produce heat; that much is obvious. But burning is never complete, and the hot gases within a smelter are rich in carbon that is chemically active. Hot carbon has a strong affinity for oxygen, and the oxygen atoms are literally stripped away from the iron by the gaseous carbon. Left without any chemical partners, the iron atoms form a mass of nearly pure metal.
The temperature inside the furnace is a critical variable. Most early smelters in Europe could no reach average temperatures of about 700 degrees. Now pure iron has a very high melting point, about 1530 degrees. So when the newly-formed mass of iron coalesces at 700 degrees, it remains a red-hot, slightly plastic solid called a bloom. The smith can hammer on this hot mass to shape it (and to make it extrude lumps of impurities that it might otherwise congeal around).
The bloomery type of smelter must produce wrought iron. No carbon can dissolve in the iron bloom at 700 degrees. But what happens if we start raising the temperature inside the carbon-rich conditions of the smelter? Simplistically, we would expect to have to get to about 1500 degrees before the bloom would start to melt. But this isn't the case at all.
At much lower temperatures, around 1150-1200 degrees, the iron starts to flow as a liquid. What has happened is one of the great "tricks" of physics -- a so-called eutectic point. When the temperature in a smelter rises, more and more carbon is absorbed by the iron. At about 3.5% carbon content, the iron-carbon alloy has a melting point much lower than either element would have by itself. It liquifies and begins to try to flow out of the furnace.
Iron has an extremely strong tendency to behave this way. The energy in its chemical bonds is such that the iron will absorb free carbon up to the 3.5% mark very quickly once the right temperature is reached, and the iron liquifies itself in the process. There is simply no way to stop the process once it starts, and that is why no master smith can realistically expect to make steel in a smelter.
What you get out of a smelting furnace depends on the amount of heat it generates. At lower temperatures, the iron is reduced without ever becoming liquid; it is drawn out as a spongy solid, red-hot and malleable, with virtually no carbon in its crystalline structure. At higher temperatures, the iron flows from the furnace into moulds, but it is virtually saturated with carbon and it cannot be shaped any further after it has cooled and been removed from its mould.
Throughout the European Middle Ages there is a great deal of iron in use. There are many centers of production, and a great deal of experiment in changing technique. One constant theme is the application of water-power to the "muscle jobs" of hammering the bloom and blowing air onto the smelter fire. The larger bellows that a water-wheel can operate mean a hotter fire in the smelter, and along with changes in the size and shape of the furnaces, they make it possible to reach the critical temperatures.
Current evidence from archaeology indicates that cast iron was first produced in Europe at two sites in Sweden, Lapphyttan and Vinarhyttan, sometime between 1150 and 1350. This suggests a possible connection with the much earlier Chinese practice of iron casting perhaps via the Mongols and the "Viking" settlements in the Volga region. This suggestion is supported by the general shapes of the furnaces as well. On the other hand, it may simply have been that bigger furnaces and bigger bellows led inevitably to cast iron flowing from the smelters.
One mystery remains: even if Europeans were making cast iron is Sweden by the thirteenth century, they weren't using it as iron castings. We have no pots or pans or bells or firebacks from such an early a date. Most likely the Swedish smiths were decarburizing the product of their smelters to make common wrought iron. Indeed, it is even possible that their effort failed and their knowledge lost for a time.
The market for cast iron objects in Europe appears late in the fourteenth century when cannonballs came to be in demand. Iron casting could make cheap, uniform cannon shot in vast quantities, and with this as a base, iron masters learned to produce and sell other simple objects for household use. Smiths also became skilled at making different forms of steel from cast iron, objects of high value when made into weapons.
In time, they would learn to make cannons as well as cannonballs out of cast iron, but the bulk of the smelter's output was destined to be converted into wrought iron, the familiar and easily-worked form of iron on the European market. It wasn't until the mid-nineteenth century that Henry Bessemer learned how to make steel in vast quantities and at prices that could compete with wrought iron.
