Smut
Smut turns up in 2 fields: powder coating, mining. Here is what it means to the people in each one.
Smut in powder coating
A loose dark film left on metal after alkaline etching or acid pickling that must come off before coating.
On aluminum, alkaline etching leaves alloying elements such as copper, silicon and magnesium behind as a gray-black dusting; on steel, pickling can leave carbon smut. Smut wipes off on a white rag and ruins adhesion if powder goes over it. A deoxidizer or desmut stage, often acid-based, removes it before the conversion coating.
"Wipe test came back black on the 6061 extrusions, the desmut tank needs to be checked."
Smut in mining
Soft, low-quality coal contaminated with earthy matter.
A friable, degraded form of coal that is heavily mixed with clay, dirt, or other earthy materials. It is often found near the surface where weathering occurs, or adjacent to geological faults where tectonic movement has crushed the seam and introduced impurities. Because of its high ash content and low calorific value, it is generally considered waste.
"The seam starts turning to smut as you get closer to the fault line."