Molten iron

Molten iron is the melted, liquid form of iron ore that has been smelted by a foundry. On Vulcanus, molten iron can also be generated by processing lava. Both methods of generating molten iron require calcite. Molten iron cannot be placed into barrels, so it cannot be sent to other planets.

Once generated, molten iron can be cast into iron plates, but it can also be cast directly into steel plates as well as a number of other iron derivative. Many, but not all, of these casting recipes are more resource efficient than casting to plate and using an assembling machine to make the derivative.

A fluid wagon of molten iron represents more iron plates than a single cargo wagon can store. Even without productivity modules, 50,000 molten iron represents 7500 iron plates, nearly twice the storage of a cargo wagon (4000 iron plates). Additionally, even with legendary quality productivity module 3s, the 2000 iron ore that fills up a single cargo wagon would only generate 50,000 molten iron, the exact size of a fluid wagon. So fluid wagons are always at least as efficient per wagon as ore or plate.