On the evidence available so far, mineral acids (so called because they were made from minerals like vitriol) such as nitric and sulphuric acid were first made in Europe, in the first half of the 14th century. This is important because they are a central part of late medieval and post-medieval alchemy, used for dissolving metals and their oxides, and the alchemists gifted them to chemists who use them even today.
But how do we know they were invented then?
By comparing alchemical texts from before, during and after this time, we find firstly no production of acid, then the production of acid.
What makes this distinction difficult is the profusion of Arabic alchemical recipes involving distillation of substances, many includiong vitriols or potassium nitrate. Whilst conscious use of potassium nitrate seems to have been one of their innovations, Arabic alchemists do not appear to have invented acids.
The reason I say this is because when I tried a recipe touted as producing nitric acid, I got an alkaline solution instead:
There might be more recipes out there from the period which haven’t survived, but at this moment in time, I have to go with what we have, and I haven’t read of or found any Arabic recipes which result in mineral acids.
There’s also some suggestion of acid damage to early 14th century alchemical vessels found in Oxford:
So, onto the recipes. Possibly the earliest comes from “Of the Invention of Verity of Perfection”, a pseudo-Geberian work, written as a commentary on and elaboration of the Summa Perfectionis of pseudo-Geber. Newman apparently dates this to the early 1300’s, the appearance being perhaps a little sudden but given how few sources have survived, this would not be surprising. The same set of recipes also include the traditional Arabic ones involving the distillation of sal ammoniac, or verdigris, or other substances, similar to those seen in Arabic works like “The book of Alums and Salts”.
The recipe is simply 1lb of vitriol of Cyprus, 2lb of salt petre, and a quarter lb of alum. It says “Extract the water with redness of the alembick”, which one author embarassingly suggests means the Alembic, i.e. the entire setup, should glow red, which would be an impossibly high temperature given that the operation is pretty much complete at 500C and at that temperature glass would soften and start to melt, and pottery would need 300 degrees higher before it started to glow. Instead, from my experience, I can state categorically that using a glass alembic means you see red vapour filling it. This starts orange, becomes red over time, and is down to nitric oxides forming.
Perhaps it is mentioned to distinguish the competion from the earlier white water vapour given off by the minerals as they are heated. This proves to me that the author had a good idea what they were talking about; you can see this also by the fact the recipe then adds a quarter of a pound of sal ammoniac, so that it can dissolve silver and gold and sulphur.
Now this is the first recipe generally realised to produce nitric acid, with some sulphuric in there too. What probably led to it was the use of saltpetre in various distillations and the resulting production of orange/ red nitric oxides as mentioned previously. Even although the liquid produced was alkaline, the alchemists responsible may have focused on this red colour, because of its importance in alchemy, and someone found that leaving out the sal ammoniac produced an even better solvent water.
So someone worked out this recipe, and knowledge of it spread, but you know what alchemists are like, always fiddling with things, perhaps to make them fit better into their conception of how the world worked.
Tracing the production of nitric and such acids is made more difficult by the practise, popular in pseudo-Arnald of Villanova works (inherited from the Arabic alchemy), of subliming Mercury with Vitriol and saltpetre and/ or salt.
If salt is present, this would produce mercuric chloride, a white substance also produced by Arabic alchemists. If no salt was present, well, maybe nothing was produced, or, I wonder if mercuric sulphide might be produced, but that’s another experiment.
Anyway, what we can see happening during the late medieval period, amongst the practical recipes that are comprehensible, is a regularisation of the use of vitriol and saltpetre for various stages in the production of the stone, with or without the use of mercury in the sublimation or distillation.
There is a spurt of practical alchemical recipes in the second half of the 14th century, all using saltpetre and vitriol to make strong waters of one sort or another. The De Confectione, perhaps written by John of Rupescissa, talks about subliming mercury with saltpetre and vitriol, as does the Liber Lucis and the De Consderatione Quintessentia, definitely by Rupescissa, says to make a strong water by grinding together 2 ounces of cinnabar, half a pound of nitre, one pound of Roman vitriol, and distill them over a gentle fire.
More usefully, the Testamentum of pseudo-lull, dated to probably the 1330’s, talks about joining the green water of a lion with a metal, which water comes from vitriol. This is one of the earlier metnions of vitriol as the green lion, we would think of it now as iron sulphate.
Less famously, a Rosarius Minor mentions 3 herbs found in a rose garden, which turns out to mean to take 2lbs of green vitriol, 2 of saltpetre and a pound of feathery alum, distill them together, the first water being collected until the alembic turns yellow, then a second vessel put to collect the drops and the alembic heated until it turns red. Which is a very good recipe and procedure.
Another work, a Practica vera Alkimia” by Ortolanus says to use saltpere, vitriol and cinnabar; this is obviously still common in the 14th century, but we already have a good recipe. I wonder when the simpler, non-mercurial recipe won out? And more importantly, why? Unfortunately we can’t answer that one, but was the cinnabar or mercury dropped because it didn’t fit with the alchemists idea of how the 4 elements had to be separated, or it got in the way of reducing the metals to their own mercury? Or maybe the end result was just a lot better. I suppose I shall have to try this distillation to find out…
Anyway, by the end of the medieval period, or else in the 16th century, it isn’t clear when, we find a work, “The Compound of Compounds”, allegedly by Albertus Magnus. This starts by using the old vitriol/ salt/ Mercury method to make Mercuric Chloride, which is to be added to a dissolving water.
This water is made by distilling 2lbs of roman vitriol, 2 of saltpere, and, crucially, 1lb of calcined alum. The text insists that this is the sulphur of the philosophers, an insivible sulphur, the spirit of Roman Vitriol, as already stated by earlier authors.
Compared to the earlier recipe, it uses more vitriol, more alum, but strangely the alum has been calcined. Modern knowledge indicates that use of calcined alum, which has no water of crystallisation in it, leads to a stronger acid through lack of water from the alum. (Of course there’s sulphuric acid from the vitriol too) Maybe it is following the Geberian insistence that substances sublime better when mixed with stuff that they agree with, although this isn’t a sublimation it does make me think that it is an alchemist originated recipe. But I am still unsure how much it is a 15th or a 16th century one, which is an important point. It also says that the liquid produced turns the skin orange when poured onto it, which strong nitric acid does, as I know from personal experience. This suggests that the author had some practical experience.
The dating of the texts is perhaps a little iffy; there must have been a great deal of travel of the recipes during the first half of the 14th century. Perhaps “On the Invention of Verity” was written much closer to 1350?
Either way, there was, over 200 years or so, a revolution in the manufacture and use of mineral acids in Europe, and further work is required in the study of manuscripts to tease out the changes in recipe and technique.