The English idiom to describe profuse sweating is sweating like a pig. However, pigs lack the ability to sweat properly due to ineffective sweat glands, which is why they roll in mud to cool their bodies. The idiom is not wrong though, as it is not referring to the animal, but pig iron.
When refining iron, the iron ore is smelted in a furnace until it is liquid. As the liquid metal is extremely hot, it is poured into a mould where it can cool. The iron was set in a branching mould that apparently resemble many piglets suckling from a mother sow – hence the name: pig iron.
Back in the old days when technology was not as advanced, it was difficult to tell when the pig iron was cool enough to transport safely. This is where science came in handy. As the metal cools, the surrounding air reaches a dew point, causing droplets of water to form on the pig iron as condensation. The condensation made it look like the pig iron was “sweating”, hence the idiom. When the pig iron sweats, it is a sign that is cool enough to be moved safely without spilling and burning everything around it.