Recently I spent some time tracing the very early history of gunpowder or Huo Yao (China, ca 850 AD). It turns out that the earliest clear description of a gunpowder-like composition was described in a document produced during the Tang Dynasty. A document titled “Classified Essentials of the Mysterious Tao of the True Origins of Things” contained a list of particularly dangerous elixirs. A comprehensive history of Chinese science can be found in Science and Civilisation in China, Volume 5: Chemistry and Chemical Technology, Part 4, Spagyrical Discovery and Invention: Apparatus, Theories and Gifts” by Joseph Needham. Within this list of hazardous compositions, a warning was offered citing the dangers of mixing and heating together realgar, salt peter, sulfur, and honey.  The document tells of alchemists mixing this combination and heating it resulting in a deflagration leading to burnt beards, faces, and hands as well as the loss of the structure to fire. This mixture has been translated as “fire-drug”.

There are earlier references to admixtures that could produce a violent effect, but the compositions are not disclosed. The information in the 850 AD document clearly describes the components of classic gunpowder- a nitrate oxidizer, sulfur for low ignition temperature, and a carbohydrate reducing agent- honey. What is notable about gunpowder is that is a self-contained redox system containing two sides of the fire triangle– fuel and oxidizer in intimate contact. All that is needed for an exothermic reaction is initiation with some kind of stimulus.

A couple of thoughts on the realgar present in the mix. First, alchemists were commonly in the  apothecary trade and made their living preparing medicaments. It is not unreasonable to suppose that the composition was intended for some medicinal effect. Realgar is tetraarsenic tetrasulfide (As4S4) and may have been a common apothecary ingredient of the time. Crystalline realgar is a ruby-like, eye-catching substance and it is not surprising that it captured the fancy of alchemists.

Second, realgar is found in hydrothermal deposits as are copper, gold, silver, and mercury sulfides (sulphuret). Back when roasting ore was widely practiced (and legal), it was common for miners to heap sulphuretted ores onto a wood pile and set it alight directly or oxidize it in a reverberatory furnace. This process would actually ignite the sulphureted ore and in the case of gold and mercury, release the native metal.  The point is that sulphuretted arsenic would be expected to contribute in the deflagration process as a nitrate reductant or as a spectator fuel.

Updated 3/2/18