[Note: This is an updated version of an earlier post.]
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 or sulfide 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 energy 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, not so much searching for the philosopher’s stone. It is not unreasonable to suppose that the composition was intended for some medicinal effect. Realgar is red tetraarsenic tetrasulfide (As4S4), possibly with some amount of yellow Orpiment (As2S3), 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 and orpiment are found in hydrothermal deposits as are copper, gold, silver, and mercury sulfides (metal sulfides as a group were referred to then by the obsolete term sulphuret). Back when roasting ore was widely practiced (and legal), it was common for miners in American lode gold districts to heap sulphuretted ores onto a wood pile and set it alight directly or air 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 to the combustion process as a reducer of nitrate or just as a spectator fuel.
My understanding of sulfur’s role in gunpowder is that of a low-melting, combustible substance which, when ignited, maintains intimate thermal contact with the solid nitrate and carbon elements, transferring heat to trigger and sustain the deflagration or explosive redox reaction.
In medieval times, before blasting with gunpowder was available, it was common in hard rock mining to fracture rock by creating a large fire adjacent to a rock face to get it hot, then water was splashed on it in an attempt to fracture the hot rock by thermal shock. [In my lonely voice squeaking out from under my rock along the riverbank, I would offer that this activity might have presented the opportunity to discover that some (sulphuretted) minerals were combustible. Alternatively, building a ring of sulfuretted rock around a campfire may have led to the same discovery.]
