The electric arc method of fullerene preparation leaves about 90% of the carbon as insoluble soot. Unprotected storage of C60 also leads to insoluble carbonaceous material. This "fullerene black" has been characterised with respect to its structural, electron spectroscopic and chemical properties. Comparison of the results with data obtained from industrial carbon black shows that the insoluble residue has nothing in common with conventional soot which is composed of small graphitic structural units. Fullerene black as well as pure C60 and C70 were all found to exhibit high reactivities in combustion reactions under realistic conditions.