First, a saturated solution of potassium nitrate (KNO3) was prepared. Using a glass rod, a shape was drawn on a sheet of paper using the solution. Next, the paper was dried. Water evaporated, leaving solid potassium nitrate on paper fibres.
The paper was touched with a burning lath at the point where it was impregnated with the potassium nitrate solution. The paper started to burn along the impregnated trail.
Heating of potassium nitrate releases oxygen:
2KNO3(l) → 2KNO2(l) + O2(g)
The released oxygen supports the combustion of paper (main component cellulose) primarily in those spots where it was impregnated with the KNO3 solution:
(C6H10O5)n(s) + 6nO2(g) → 6nCO2(g) + 5nH2O(g)