Mercury thiocyanate thermal decomposition. Footage of the snake-like growth of the structures formed during the thermal decomposition of a sample of mercury (II) thiocyanate (Hg(SCN)2). The heat is supplied by a blowtorch. This effect is sometimes known as 'Pharaoh's serpent', and was used for entertainment value in the 19th century before it was discovered that the compound is toxic. This type of reaction, where a substance swells on exposure to heat, is known as an intumescent reaction.