Smelting hot liquid iron was poured over sand to cool. The molds used for this process looked something like a suckling pig in silhouette, which gave the material the nickname of pig iron. Once it cooled, it became useful for the production of things like frying pans and cookware and fences and fireplace stuff. The sweating comes in when the iron begins to cool. It meets the dew point of the surrounding air, and moisture collects on the surface