The metal produced in the furnace, usually termed a bloom, requires further refinement whether it was a classic bloomery process bloom or an ingot of cast iron.
A classic bloom would contain significant amounts of slag, that need removing. This was achieved by heating and initially squeezing or gently striking the bloom. The risk being that if the temperature was too high the slag would melt and the bloom would fall apart. If the temperature was too low, the slag would be brittle and fracture, also causing the bloom to fragment.
If the furnace produced cast iron (3-5%C), then decarburisation was required to reduce the carbon content, either to steel or malleable iron.