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Who cares about contamination of saltpetre?
Alchemists and people whose lives depend on making things go bang, that’s who.

Regarding the alchemists, Lawrence Principe (on page 65 of his “The secrets of Alchemy”) brings to our attention a work by John of Rupescissa (or rather of Roquitaillade as he is knowin in France, which was his birthplace) that tells us how to purify saltpetre of salt. The De Confectione attributed to him has a section near the end on the use of fractional crystallisation to produce pure saltpetre. However the same text also has an added section which describes the importance of table salt and states that “The whole secret is in the salt”.

This is in the context of an important recipe at the start of the text being the distillation of a mixture of roman vitriol, saltpetre and mercury, producing a substance that is white like snow. To make this, presumably mercuric chloride, requires the element chlorine to be present with the mercury. But what confuses me is why would you give a recipe for removing the salt, then suggest obliquely in the next paragraph that you had to add the salt in again?
Certainly this would give you better control of the proportions required, it being rather hard to judge how much sodium chloride was present in the saltpetre before it was purified. In fact I think this concern with pure starting ingredients is very old; it’s just that by the time the De Confectione was written, they knew that saltpetre could be impure whereas they didn’t realise it earlier when mineral acids were invented.
(See Manget’s Bibliotheca chemica curiosa, volume 2 pages 80-83 for the De Confectione and 84-87 for the Liber Lucis. Thanks to the work of the people who scanned it and uploaded it I have a downloaded scan of it, the problem is the pagination is different and I think it’s a different edition; either way I am sure I have at least got the correct texts)

Another possibility I have thought of is that the passage on table salt was added by a later editor in order to alert the reader to something which was required to carry out the experiment. Principe also wrote this in the notes at the back of the book; since I read it over a year ago I can no longer remember if it is his idea or mine, but either way it is an important thing to consider when thinking about what is written and why.

I’m working on my Latin, so at the moment haven’t attempted a translation of the method of purification of saltpetre.

However it is important to remember that saltpetre wasn’t only part of alchemy, it was used in gunpowder and other industrial chemical practises, thus we find a concern with its purity in other works. It is useful to see how alchemy and artisanal work overlapped so much in the early modern period.
Biringuccio, on page 404- 408 of his De Pirotechnia, instructs us how to extract the saltpetre from the earth and to concentrate it by boiling the solution. He then instructs us:
“… or if for some of your processes you wish to have it purer than usual and completelyh without gross earthiness, without fatness, and without salt (which it must be for making very fine gunpowder and aqua fortis for parting),….”
The first method uses water, you dissolve lime, ashes, dissolved alum, put 4 to 6 jugfuls of this into the barrel of water that you had saltpetre in (Or else some aqua fortis instead of it), add as much saltpetre as you can, and boil it all together. When a foam has risen on it, take it out of the kettle and put into a vat which has well washed coarse river sand on the bottom, let the water drain out slowly through a hole you have made in the bottom. Then you boil the filtered water almost to congelation, add some aqua fortis from time to time, expecially when it raises a foam. Then take the material from the kettle and put it to congeal, and Biringuccio thinks the crystals from this are much better and cleaner than before.
His second method is with fire, in which you take an iron or copper vessel, fill it with saltpetre, put a cover on it, melt the saltpetre, when well melted, sprinkle finely powdered sulphur on top, and he reckons the uncrousness will be burnt away, when it is left to cool the earthiness will be at the nottom and the ssaltpetre will be white and like marble.
Smith and Gnudi think this latter method wouldn’t do anything useful.

A full description of the production of saltpetre is given in Ercker’s treatise on Ores and Assaying. The purification of the crystals of saltpetre (The illustration shows spiky things in vats) is carried out by putting it into just enough water to dissolve it, and then it is heated to boiling, and when it is boiling, apparently coarse grains of salt will precipitate out, because the salt does not dissolve as easily as saltpetre. Black foam will be thrown up when it is boiling, and this should be removed. There follows a description of sampling the liquor to ensure it is of the right strength. Next, good wine vinegar is to be added, and the black foam removed, this is to be done several times, and later finely crushed burnt alum is added, which causes the saltpetre to precipitate out. Then it is to be poured into a tall, well insulated vat and left for a couple of hours, during which a yellow sediment will fall to the bottom. Finally, when it is still hot, it should be poured into wooden pans or copper tubs which are sunk into the ground, for the final crystallisation.

What immediately jumps out are the numerous steps for removing different impurities. But what are they, I wonder? The salt is clear enough, but black foam? A yellow precipitate? The usual impurities that I know of, such as sodium carbonate, are white.
Ercker mentions the use of lime, but isn’t so much for it, and there are other little tricks that people use.

The original start to this post was my decision, wisely as it turned out, to test for salt in the green vitriol and saltpetre that I have, as part of my preparation for carrying out the aforementioned production of mercuric chloride. The green vitriol solution didn’t change colour or have anything happen in it, except that the excess nitric acid dissolved some of the insoluble muck at the bottom of the flask.
The saltpetre though gave this reaction:

silver chloride in knitrate solution

a fine white precipitate, formed of silver chloride, because I poured silver nitrate into the solution of saltpetre. Therefore there is salt in it. Exposure to light breaks down the silver chloride into colloidal silver, i.e. very small bits of silver, which then gives it a purple colour.
silver nitrate purple
This is an old analytical chemistry method for testing for the presence of chloride ions, and is also in use, with modern electrodes and conductivity testing for determining the amount of silver in a sample of metal which has been dissolved.

I think I’ll try removing the salt from the saltpetre, the question is, which method to use?

Silly question – all of them!
I shall report on my findings as I complete the experiments.