foxtail

A foxtail is the final batch of cinders or dross removed during the fining process in iron and copper work. It represents the last oxidized impurities brought to the surface as the bath approaches completion, and it sits at the bottom of the fining pot once the bulk of usable metal has been cast off. The material is largely waste, containing iron oxide, silicates, and unrecoverable base metals, so it is typically returned to the furnace or discarded rather than refined further.

The fining process itself is a controlled oxidation stage that follows smelting. Molten metal is held at temperature while air or oxygen is blown through, burning off carbon, sulfur, and other impurities. As these elements oxidize, they float to the surface as slag. Early in fining, this slag is relatively rich and may be skimmed and returned to the charge. By the end, however, the remaining cinders are lean and heavily contaminated with the slag-forming oxides of the gangue minerals. This late-stage material is the foxtail.

The name likely derives from the appearance of the final cinder mass when cooled and broken up: a fibrous, ragged texture resembling a fox's tail, or possibly from the notion of the last brush or trailing end of the process. Usage is most common in non-ferrous metallurgy, particularly in historical and traditional copper refining, though the term appears in iron foundry work as well.

Foxtail distinguishes itself from other refining cinders by its point in the sequence and its composition. It is far poorer in recoverable metal than early skimmings and higher in silicate and aluminate phases, making it economically marginal for reprocessing. In modern operations with continuous fining or more intensive oxidation control, the distinction between foxtail and general fining slag has blurred, but the term persists in foundry terminology and technical literature as a marker for exhausted fining waste.