To heat an alloy or ore mixture just enough to melt the constituent with the lowest melting point, leaving the rest solid. The liquid fraction drains away from the solid matrix, allowing for a crude but effective physical separation of metals. It is a traditional method for extracting silver from copper using lead, or removing bismuth from lead bullion.
Heard on the job
We need to liquate the bullion slowly so the lead sweats out without melting the copper matrix.