What is blister copper?

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A periodic table element box for copper shows its symbol Cu, atomic number 29, atomic weight 63.546, electron configuration, and name. Icons indicate copper is a solid, weakly basic transition metal with face-centered cubic crystal structure. A key explains the symbols and color coding. — copper Properties of copper.

Blister copper, also known as Bessemer copper, is a form of metallic copper characterized by its black, blistered surface. It is the product of the converting process, in which molten matte is treated to remove remaining iron and sulfur.12

During this stage, air or oxygen-enriched air is blown into the molten bath, converting impurities into oxides that are either removed as gas or slag. The resulting blister copper typically has a purity level of 98.5 to 99.5 percent, though it may contain up to 0.8 percent oxygen. Because this product is not yet pure enough for many industrial applications, it must undergo further fire refining to reduce oxygen and sulfur levels before it is cast into anodes for electrolytic refining.21