Traditionally bun ingots were seen as a primary product of smelting, forming at the base of a furnace beneath a layer of less dense slag. However, experimental reconstruction of copper smelting showed that regular plano-convex ingots are difficult to form within the smelting furnace, producing only small ingots or copper prills that need to be remelted. High purity copper bun ingots found in Late Bronze Age Britain and the Mediterranean seem to have undergone a secondary refining procedure.
The metallographic structure and high iron compositions of some plano-convex ingots suggest that they are the product of primary smelting. Tylecote suggested that Roman plano-convex copper ingots may have been formed by tapping both slag and copper in one step into a mould or pit outside the furnace. A similar process was described by Agricola in book IX of his De Re Metallica and has been replicated experimentally.