Mercury(II) fulminate, or fulminated mercury is an explosive substance with the chemical formula Hg(CNO)2. The substance was used as an example of an explosive chemical by Walter White during chemistry class in the episode "Crazy Handful of Nothin'" (season 1 episode 6). A bagful of it looks like crystal meth. Later in that episode, he uses a finger sized crystal of it to blow up Tuco Salamanca's headquarters, and uses a bagful of it to acquire $50,000 to cover the stolen pound of meth ($35,000) and injuries to Jesse Pinkman, and negotiate a sales agreement.
Mercury(II) fulminate is prepared by dissolving mercury in nitric acid and adding ethanol to the solution.
Silver fulminate can be prepared in a similar way, but this salt is even more unstable than mercury fulminate; it can even explode under water and is impossible to accumulate in large amounts because it detonates under its own weight.
The thermal decomposition of mercury(II) fulminate can begin at temperatures as low as 100°C, though it proceeds at a much higher rate with increasing temperature.
A possible reaction for the decomposition of mercury(II) fulminate yields carbon dioxide gas, nitrogen gas, and a combination of relatively stable mercury salts: