What is blister copper and how does it contain silver?

Blister copper is a product of the copper smelting and converting process, typically containing between 98.5 and 99.5 percent copper. It is named for the appearance of its surface, which results from the release of gases during the cooling of the molten metal.1

Blister copper contains silver because silver is naturally present in many copper sulfide ores. During the smelting and converting stages, these ores are processed to remove impurities like iron and sulfur. Because silver is often associated with copper in these mineral deposits, it remains with the copper throughout these metallurgical steps. Consequently, blister copper typically retains 97 to 99 percent of the silver that was originally present in the copper concentrate.23

This silver is later recovered as a valuable by-product during the electrolytic refining of the copper, where it accumulates in the insoluble residues known as slimes.2