Gunpowder

Scientists were experimenting with potassium nitrate (aka saltpeter). One individual decided to mix it with sulfur and charcoal. In a text dated from the mid-ninth century, observers remarked that the scientists’ hands and faces were burned after experimenting with the black powder. It had resulted in smoke and flames and even burned down the house in which they were working.

Gunpowder is considered to be a “low” explosive that rapidly burns in a tightly packed, confined space.