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 that produces a large mass of coiling, serpent-like solid is started.
 An inconspicuous flame, which is often blue but can also be yellow/orange, accompanies the combustion.
The resulting solid can range from dark graphite gray to light tan in color with the inside generally much darker than the outside.

The reaction has several stages as follows:  Igniting mercury thiocyanate causes it to form an insoluble brown mass that is primarily carbon nitride, C₃N₄. Mercury sulfide and carbon disulfide are also produced.
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 that produces a large mass of coiling, serpent-like solid is started.  An inconspicuous flame, which is often blue but can also be yellow/orange, accompanies the combustion. The resulting solid can range from dark graphite gray to light tan in color with the inside generally much darker than the outside.
The reaction has several stages as follows:  Igniting mercury thiocyanate causes it to form an insoluble brown mass that is primarily carbon nitride, C₃N₄. Mercury sulfide and carbon disulfide are also produced.
Mercury(II) thiocyanate