https://www.thevespiary.org/rhodium/Rhodium/chemistry/psychedelicchemistry/chapter8.html (https://www.thevespiary.org/rhodium/Rhodium/chemistry/psychedelicchemistry/chapter8.html)
5. Burning test. The powdered cocaine is placed on aluminum foil and held over a low flame or match. The cocaine will burn clear. A sugar cut will darken and burn a dark brown or black therefore the larger the cut, the darker the burn. Crystallized speed or methamphetamine will pop when burned. Salts do not burn and remain as residue (cuts such as procaine or quinine also burn fairly pure although it is alleged that procaine can be detected by a bubbling of the substance before it burns clear).