Mixing potassium permanganate and glycerin causes spontaneous combustion. The chemical reaction causes purple flames to form in just a few minutes. Adding water speeds up the reaction. This experiment is best for use as a demonstration in a well-ventilated space.
When adding these two chemicals, the amounts used must be limited because mixing a large amount creates a great deal of heat. The University of Washington recommends limiting the glycerin to 3 milliliters and the potassium permanganate to 15 grams. The reaction is much slower with larger crystals of potassium permanganate; if the crystals are too large, combustion does not occur.