Borax and lime are used as flux agents in the cupellation process to separate silver and gold from base metals and other impurities. The process is carried out at temperatures between 800 and 900°C, i.e. lower than the melting point of the noble metals, and even below the temperature at which the slag becomes totally molten. Base metals are oxidized and removed with the slag, while gold and silver, which do not oxidize, sink and build up, leading to doré bars for further partitioning and refining. This process would only be economical if silver and/or gold are pre-concentrated first, as it is the case in anode slimes from copper electrolytic refining.