Pipipi,
If you suspect your Potassium nitrate is impure, attempting to burn it will not teach you anything because Potassium nitrate does not burn, and the impurities are very unlikely to burn, and you will get the same result with 99.9997% Analytical KNO3 as 50/50 KNO3/Impurity (unless the impurity is a fuel).
Where did you get your Potassium nitrate from?
Did you ball mill your tigertail?
Is it in stars? Are they FULLY dry?
Have you tried making it into stars and testing them in the air?
Tigertail often burns pathetically, if at all, as a loose powder on the ground, or as stars which are not quite dry. Try ramming a few grams into a tube, it should burn better then, or if you have it made into stars, let them dry a bit longer.
Good luck
The monkey leaped off it's sunny perch and flew off into the night sky.