It is a misconception that renaissance alchemists were only concerned with the philosophers stone. Hermetic alchemy was an overlap of alchemical practice within a mystical or spiritual framework. This branch of alchemy and its practitioners are perhaps better known in the popular literature than the alchemists who might be regarded as more pragmatic experimentalists.

Many alchemists over history were very practical and quite occupied with their trade in medicaments, tinctures, distillates, and elixirs or with metallurgical and compounding endeavors.  Paracelsus is regarded as an early practitioner of iatrochemical work, but within a hermetical framework.  Agricola and Biringuccio were 16th century chroniclers of metallurgy that had a basis in earlier alchemical progress.

Consider an entry from a translation of The Laboratory, or School of Arts; in which are faithfully exhibited and fully explain’d, I. A variety of curious and valuable experiments in refining … VI. A dissertation on the nature and growth of saltpeter; … Translated from the German, by Godfrey Smith, published 1738.  In this volume, available from ECCO, Eighteenth Century Collections Online, contains a passage under the heading of “To prepare Aurum Fulmina__s” (two letters obscured). I have retained the archaic character “f” in place of “s” for the reader to enjoy.

To prepare Aurum Fulmina__s

Take Gold that is refin’d with Antimony, beat it to thin Plates, put it into a Phial or Matrafs, pour Aqua Regis upon it, then fet the Phial or Mastrafs upon warm sand, till the Aqua regis is diffolv’d as much of the gold as it is able to contain, which you will knw when you fee the Ebullitions ceafe, pour your Solution by Inclination into another Glafs, and if you fee there remains any Gold in the Matrafs, dissolve it as before with a little frefh Aqua regis, mix your Diffolution, and pour to it five times as much common Water, afterwards drop into this Mixture, by Degrees, the Volatile Spirit of Sal Ammoniac, or Oil of Tartar, and you will see the Gold precipitate to the Bottom of the Glafs, let it reft a good while for the Gold to settle, then pour off the Water by Inclination, wafh your powder with warm Water, till grows infipid, dry it to the Substance of a Pafte, then form it in little round Corns, the Bignefs of a Hempfeed, dry them by the Sun, if you put one of them into a Fire, it will fly and difperfe with a terrible Noife, and beat about with great Violence. [Emphasis mine]

It seems likely that the worker is trying to refine the gold by dissolution of the Sb/Au blend by complete dissolution in aqua regia, followed by what we would now regard as a reduction of the gold solution. Quenching the aqua regia would be expected to cause the gold to reduce and fall out as the native metal. But gold chemistry is not what is interesting in this account.

The Spirit of Sal Ammoniac, meaning either ammonia itself or ammonium chloride, would do as follows: the ammonium would ion pair with nitrate and, upon drying, leave a residue of ammonium nitrate, which is an explosive. Simple open burning of  small kernels material enriched in ammonium nitrate might be expected to deflagrate or pop, as indicated in the end of the description.

The Oil of Tartar, however, might have an altogether different fate when dissolved in aqua regia. Oil of Tartar is a concentrated aqueous solution of potassium (or Na) tartrate.  In solution with aqua regia, one would reasonably expect the two hydroxy groups of tartaric acid to form the dinitro ester if appropriate nitrating species are present. A nitrate ester group is a common explosophore and consists of O2N-O-C comprising an oxygen linkage between NO2 and carbon. This linkage is sensitive to low levels of stimulus, making compounds with such linkages susceptible to rapid or explosive decomposition. The nitrite ester is listed as an explosophore as well.

The nitration of tartaric acid is described in US patent 1,506,728. This patent teaches the use of the standard H2SO4 catalyzed HNO3 nitration of the tartaric acid diol functionality to form a dinitro ester via the standard nitronium ion formation. In the case of aqua regia, the presence of NO2(+) is questionable. Aqua regia is known to produce nitrosyl chloride, ClNO which dissociates to Cl2 and NO.  Literature on the nitration of alcohols to nitro esters in aqua regia is non-existant in Chemical Abstracts. There are a few citations describing aromatic nitration by aqua regia, but no clear description of nitro ester formation.  Indeed, there are many descriptions of direct extraction of gold from aqua regia using isoamyl alcohol with no warnings of explosive or nitro formation.

There are, however, reports of the use of ClNO to produce organonitrites when reacted with a monohydroxy alcohol (Journal of the American Pharmaceutical Association (1912-1977) (1932), 21, 125-8). It is possible that a tartaric nitrite was formed which may be energetic to some extent.

But perhaps the application of Occams Razor is needed. Potassium or sodium tartrate would be mildly basic and upon addition to a mineral acid solution, it would neutralize the acid in sufficient quantities, affording potassium or sodium nitrate (saltpeter). On evaporation of water, the saltpeter residues would be comingled with tartaric acid, comprising a fuel/oxidizer mixture.

Small quantities of crude nitrate esters, nitrite esters, or nitrate salts could have been present in the dried paste, giving the pyrotechnic effect described. The formation of energetic materials was not the primary purpose of the procedure, although the observed behavior of the residues was apparently compelling enough to document.