What is blister copper and how pure is it?

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Molten copper pours from a large industrial furnace into rectangular molds at a copper smelting facility, with glowing metal and industrial equipment visible. — copper processing Molten copper being made into plates.

Blister copper is a form of metallic copper produced during the converting stage of copper extraction. It is named for the characteristic blistered appearance of its surface, which results from the escape of gases during the cooling process. Also known as Bessemer copper, this material is an intermediate product that requires further fire refining to reach the purity levels necessary for electrolytic refining.12

Regarding its composition, blister copper is typically between 98.5 and 99.5 percent pure copper. In addition to the copper content, it may contain up to 0.8 percent oxygen, along with residual impurities such as sulfur and various precious metals, including silver, that were present in the original concentrate.123