The interesting part from wiki answers:

Mercury thiocyanate was formerly used in pyrotechnics causing an effect known as the Pharaoh's serpent or Pharaoh's snake. When the compound is in the presence of a strong enough heat source, a rapid exothermic reaction is started which produces a large mass of coiling serpent-like solid. An inconspicuous flame which is often blue but can also occur in yellow/orange accompanies the combustion. The resulting solid can range from dark graphite grey to light tan in color with the inside generally much darker than the outside.[2]

This property was discovered soon after the first synthesis of mercury thiocyanate by Wöhler in 1821: "winding out from itself at the same time worm-like processes, to many times its former bulk, a very light material the color of graphite...". For some time, a firework product called "Pharaoschlangen" was available to the public in Germany, but was eventually banned when the toxic properties of the product were discovered through the death of several children mistakenly eating the resulting solid.[2]

A similar, though less extreme, effect to the Pharaoh's serpent can be achieved using a firework known as a black snake. These are generally benign products, usually consisting of sodium bicarbonate or a mixture of linseed oil and napthalenes.