Armstrong's mixture is a highly sensitive primary explosive. Its primary ingredients are red phosphorus and strong oxidizer, such as potassium chlorate and potassium perchlorate. Sulfur is used to substitute for some or all of the phosphorus to slightly decrease sensitivity and lower costs; calcium carbonate may also be present in small proportions. Commercially, Armstrong's mixture is used in milligram quantities on the paper caps in toy cap guns and in party poppers. An improvised version can be made with match-heads, ground up into a fine powder, and mixed with another fine powder, this time made of the striker strip found on the side of match boxes.
Because of its sensitivity to shock, friction and flame, Armstrong's mixture is an extremely dangerous explosive. Only about 10 mg of it is used per item of consumer fireworks. Depending on composition, conditions and quantity, Armstrong's mixture can explode violently in an enclosed space. Due to extreme sensitivity to friction, mixing dry potassium chlorate and red phosphorus will most likely lead to an explosion, hence the ingredients are usually combined in a slurry with water, formed into the final product (for example, single drops onto paper for "paper caps") and allowed to dry.
- J. B. Calvert. "Flash! Bang! Whiz!: An introduction to propellants, explosives, pyrotechnics and fireworks". Archived from the original on 15 November 2006. Retrieved 2006-11-11.
- US patent 3973502, Charles R Olsen, "Tube primer", issued 1976-08-10
- John Donner. "Impact Firecrackers" (PDF). Retrieved 2006-11-11.