Furnace residue is the material that is left behind after burning fuel in a furnace. The three crossword solutions for this term are CLINKER (7 letters), ASHES (5 letters), and CALX (4 letters). CLINKER is a hard, black, glass-like material that is formed when coal or other fuels are burned at high temperatures. ASHES are the powdery remains of burned material, such as wood or coal. CALX is a white powdery substance that is formed when certain metals are heated to high temperatures.