I think it is fair to say that most chemists are familiar with the fact that mercury fulminate, Hg(CNO)2, is a pressure sensitive explosive material. But because only a few of us actually handle such materials, myself not included, thankfully, the history and actual boundaries of safe handling practice are probably somewhat indistinct. Mercury, as the fulminate or the metal, has been applied to the extraction of gold and silver from ore. The former as a primary explosive for blasting compositions, and the latter as a solvent (and possibly a reductant).

In the course of my ongoing studies in historical metallurgy, I have been searching the very earliest history of chemicals and processes related to the extraction of gold and silver. The threads between these two metals in history are closely interwoven and include an extensive list of civilizations, scholars, monarchs, banking institutions, viceroys, scientists, engineers, and chemical technology.

One fascinating thread in the metallurgy of gold and silver is the role of quicksilver. The discovery of native mercury occured independently in Asia, the Iberian penninsula, central Europe, and the American CordilleraCinnabar has been used as a pigment by aboriginal peoples for adornment and decorative purposes back into prehistory.  There is no documentation in written or other form of the sudden discovery of native mercury. The earliest references to metallic mercury are from Pliny, who mentions some curious properties of the substance in relation to gold, namely, that gold was the only substance known to sink in quicksilver, leaving behind the mineral components of the ore floating on the surface.

The invention comprising the use of quicksilver in the refinement of silver is usually attributed to Spanish merchant Bartolomé de Medina in the part of New Spain comprising what is now Mexico. According to the story, Medina was approached by a German known only as “Maestro Lorenzo” who described a process by which ore was treated with sodium chloride (sea salt water) and quicksilver. Medina travels to Mexico and develops what will be come to be known as the Patio process.

The Patio process proves to be a substantial improvement over smelting processes known in Europe at the time and this fact leads to a long term demand for quicksilver in the Americas. According to records from New Spain, for every quintal (100 lbs) of silver extracted, two quintales of quicksilver were consumed in the Patio process. The primary quicksilver mines in operation by the close of the 16th century were Almaden in Spain, Idria in Slovenia, and Huancavelica in Peru.

Over time mercury was used to produce explosives, Fahrenheit‘s thermometer, and antimicrobial preparations. The discovery of mercury fulminate was crucial to the production of detonating caps for mining and bullet cartridges. Unlike NI3, mercury fulminate, Hg(NCO)2, could be isolated and handled, albeit with great care.

The shelves of a chemist (or apothecary) of the late 18th century would have certainly have contained sulfuric acid, nitric acid, numerous salts, sulfur, lime, various extracts and elixirs, caustics, etc.  It was inevitable that one day someone would combine nitric acid, ethanol (“hydrated ethylene”), and a metal or its salt.  This particular admixture of nitric acid and ethanol, to which a metal oxide or other compound was added would produce a mixture whose vigorous ebullition with the evolution of vapors and smoke would be referred to as fulmination. A residuum or precipitate recovered from the mixture came to be known as a fulminate.  The treatment of red mercury oxide with nitric acid and ethanol produced a mercury fulminate. 

Mercury fulminate was discovered by Edward Howard around the year 1800. The details of his work were published in Philosophical Transactions of the Royal Society of London, vol 90 (1800), pp. 204-238.  The paper can be found at jstor.org and is worth a read. In it Howard describes an experiment wherein he detonates a small quantity of mercury fulminate in a thick glass vessel and notes the relatively small volume of gas produced in the explosive reaction. He also notes the presence of finely divided mercury on the vessel walls.  The reader will notice that Howard fabricated a rudimentary electrical resistance heater as an initiator to stimulate the fulminate into decomposition.

Howard’s attempts to evaluate this fulminate as a new type of gunpowder are also detailed. Howard’s experiments show that the fulminate reliably burst the breech assemby of all of the guns tried, but strangely did not have the ability to propel a ball with the energy of an equivalent quantity of gunpowder. What he learned was that great sensitivity does not necessarily confer high explosive energy.

As an interesting aside, it was later determined by Gay-Lussac, Liebig, and Wöhler that silver fulminate had the same composition as silver cyanate. After much debate, Berzelius was able to introduce the idea of isomerism to settle the matter.

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