in solvent extraction
A deposit or emulsion at the interface between two partially settled phases.
  1. The phenomenon of crud formation arises from many causes and this definition does not imply any single one.
  2. Other terms -- some unprintable -- have been used but crud is the generally accepted term.
PAC, 1993, 65, 2373 (Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993)) on page 2387