Pharaoh’s serpents, or pharaoh’s snakes,are a type of small firework in which a lighted tablet exudes smoke and ash in a growing column which resembles a serpent.  They are toxic so now this firework is only produced as a chemistry demonstration.  The reaction is a result of the combustion of mercury (II) thiocyanate (Hg(SCN)₂).

Igniting mercury(II) thiocyanate causes it to decompose into an insoluble brown mass that is primarily carbon nitride, C₃N₄. Mercury(II) sulfide and carbon disulfide are also produced.

2Hg(SCN)₂ → 2HgS + CS₂ + C₃N₄

Flammable carbon disulfide combusts to carbon(IV) oxide and sulfur(IV) oxide:

CS₂ + 3O₂ → CO₂ + 2SO₂

The heated C₃N₄ partially breaks down to form nitrogen gas and dicyan:

2C₃N₄ → 3(CN)₂ + N₂

Mercury(II) sulfide reacts with oxygen to form mercury vapor and sulfur dioxide. If the reaction is performed inside a container, you will be able to observe a gray mercury film coating its interior surface.

HgS + O₂ → Hg + SO₂