Put on safety goggles and gloves to ensure the highest level of safety. Put on a long-sleeve shirt. Due to the highly explosive nature of the reaction, you should cover all parts of your body.
Place the potassium nitrate in the flask and place it on gentle heat. The potassium nitrate will not ignite by itself as it is an oxidizer and only provides an oxygen-rich environment so other compounds can ignite.
Add a reducing agent to the flask. Examples of reducing agents include sulfur or charcoal-based compounds. Once heated, the potassium nitrate will produce enough oxygen for the reducing agent to react.
Increase heat until a reaction takes place. The flame should burn a purple or lilac color due to the presence of potassium. The reaction may be explosive, so it is necessary to be cautious and aware of the reaction as it occurs.
Wait for the reaction to stop. Once the reaction is finished, turn off the heat source and wait for the flask to cool down. Test the temperature of the flask by quickly touching it for a second. If it is not hot, remove the flask and dispose of the remaining chemicals properly.