Setting things on fire is usually fun (except when it creeps over into the “arson” category), but the decomposition of Mercury(II) thiocyanate (Hg(SCN)₂) is definitely the stuff of nightmares. 

Mercury thiocyanate is a white powder that looks unassuming enough. The introduction of heat creates an incredible exothermic reaction producing a mostly blue flame and a growth of solid snake-like mass. The reaction is a trumped up version of the one seen in “black snake” fireworks, which generally use baking soda, linseed oil, and hydrocarbons. However, the mercury thiocyanate reaction is toxic, which is why it isn’t used in fireworks anymore.