Typically, they didn't handle molten iron.

There are three main processes used to shape metal:

• Casting. The metal is melted, and poured into a mould, and it cools into solid metal with the same shape as the interior of the mold.

• Forging. The metal is shaped by hammering or pressure. This can be done hot (where the metal is heated before forging), or cold (at room temperature).

• Stock removal. Metal is removed from the workpiece by processes such as grinding, drilling, and cutting.

All three methods were commonly used for copper and bronze. Sometimes, all three were used for a single item: for example, a bronze sword might first be cast, and then the edges forged to give a thinner edge (made harder by work-hardening), and then the edge ground to give a sharp edge.

Before any of this can be done, metal must be obtained. Usually, one starts with ore, which is a chemical compound of the metal and other elements (often oxygen, but possibly sulphur or other elements). Sometimes, the crystal structure of the ore included water. The process of converting ore into metal is smelting. The typical smelting process, once one has collected/mined the ore, and broken it into suitably small pieces, is:

1. Roasting the ore. This converts sulphide and carbonate ores into oxides, and also gets rid of water in the crystal structure. If the ore is an anhydrous oxide, roasting isn't necessary.

2. Reduction. The oxide ore is converted into metal by a chemical reaction where another substance (usually carbon monoxide) that binds more strongly to the oxygen than the metal does pulls the oxygen from the ore, leaving the metal. This chemical reaction needs a high temperature. Generally, the ore is heated with burning carbon (usually charcoal in the pre-modern world, but the Chinese also used coal). The burning charcoal (or coal) can provide both the heat and the carbon monoxide.

With copper, the temperature needed for smelting is higher than the melting point of copper, so you get molten copper as the product. So this process is best done in a crucible so that you can move the molten copper to where you want it. Since it's expensive in fuel to heat room temperature copper to its melting point and melt it in order to cast something from it, it's convenient and cheap to cast things when the copper is still molten from smelting. For bronze, add tin (and/or other alloying elements), and you have molten bronze. As with copper, cast while it's molten from the smelter.

Iron is different! The first trick with iron is that smelting (the reduction step above) can occur below the melting point of iron. In pre-modern iron production, the chemistry takes place with both the ore and the iron produced staying solid. This produces a spongy mass of iron (called a "bloom") that's full of slag (the silicates and other non metal-oxide parts of the ore). The bloom is hammered to consolidate it, and the iron is usually repeated folded to get rid of excess slag (and to make it uniform).

One can then go on to make something out of the iron using forging and stock removal. The iron is never molten is this process.

The second trick is that the carbon in the smelting furnace can dissolve in the (solid) iron, producing steel. To do this efficiently needs a higher temperature than the minimum needed for smelting. Iron + carbon = steel, and if the carbon content is high enough (about 0.3% at minimum), the steel can be hardened by quenching.

The third trick is that as the carbon dissolves in the iron, the melting point is reduced - steel melts at a lower temperature than iron. If the smelting furnace is hot enough, and enough carbon dissolves in the iron, the melting point can drop below the temperature in the smelter. Carbon dissolves a lot more readily in molten iron than solid iron, so even more carbon will dissolve further dropping the melting point. The molten iron ends up with as much carbon dissolved in it as possible, typically 3-4%. This iron with 3-4% carbon is called "cast iron". The problem with cast iron is that is it usually very brittle (because flakes of graphite (i.e., carbon) form in it as it cools, and it breaks very easily along these flakes). If you try to forge cast iron, it will tend to shatter rather than deforming.

On a bad day, the whole bloom can melt. Since the temperature in the smelter isn't uniform, often only some of the iron melts, and mix of iron, steel, and cast iron results. Often, the cast iron was discarded (because it was unforgeable). If the goal is to produce as much steel as possible, you want those high temperatures, and accept the loss of some iron as cast iron.

It is possible to use cast iron, but this only appears about 2,000 years ago, in China. First, in a bloomery furnace, any cast iron produced ends up in a puddle on the furnace floor. If cast iron is considered unusable, then there's no motivation to make a furnace from which you can extract cast iron. Enter the blast furnace: air is forced into the furnace so that the fuel burns faster, giving a higher temperature (air can be forced into a bloomery furnace to give a higher temperature and more efficient steel production; this is a "blast-bloomery" and not a true blast furnace). If you reliably get to the temperature needed to produce cast iron, you get a puddle of molten cast iron and can tap it from the furnace. It can be run along a channel straight into moulds to produce various items (e.g., farm tools, woks, and many more). These cast iron objects are relatively brittle, so if you want something that needs the toughness of iron or the toughness and hardness of steel, you either need to use a bloomery furnace instead of a blast furnace, or convert the cast iron into steel or iron. The Chinese did both of these. If the cast iron is broken into small pieces and heated with oxygen, the oxygen pulls some of the carbon out of the cast iron. When the carbon content has been reduced enough, you have steel instead of cast iron. The extra fuel and labour that this requires make steel more expensive than cast iron. The lower fuel efficiency of a bloomery furnace makes bloomery steel more expensive than cast iron too. So the relatively brittle cast iron tools and woks are the cheap version, and the iron and steel objects are more expensive (they also need more labour and fuel to forge them).

The modern Bessemer process gets rid of the excess carbon while the cast iron is still molten, but the idea and the chemistry is basically the same.