At last, the experiment you have all been waiting for. Purifying saltpetre.
As we all know, it is one of two critically important ingredients for gunpowder, the other being charcoal. The latter is easy enough to make, but saltpetre is a lot harder. The medieval folks at the Danish Mittelalter centre tried for several years before 2010 to make it and didn’t succeed. More recently, Haileigh Robertson has been working on a PhD related to gunpowder, and has made crude saltpetre at the centre, but there are major problems with the impurities, which render it of very low quality.
Saltpetre is a comparatively recent arrival in Europe. It was almost certainly imported from the middle east in the 12/13th centuries, and in the early 14th we have the evidence for its use in making gunpowder for cannon, but it was very expensive. However over the next century the price dropped greatly, making it much easier to buy and use, helping fuel the use of cannon and bombards, the latter having calibres of a foot and more and requiring several kilograms of blackpowder to fire.
Like anything else though, saltpetre needs to be pretty pure to be useful. So I thought I’d have a go at purifying it myself.
I have few instructions on how to purify saltpetre, the best seem to be post medieval. This post describes the trial of a method of purification given in Lazarus Ercker’s treatise on Ores and Assaying, my copy is the Sisco and Smith translation of 1951 (It seems to be a first edition) from the 1580 German translation, page 303.
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, involving taking a sample and putting it in a copper bowl which is then put into cold water so as to suddenly freeze the crystals by lowering the temperature, if the solution is sufficiently concentrated. Next, good wine vinegar is to be added, and the resulting black foam removed, this is to be done several times, and later finely crushed burnt alum is added, which “This will cause the saltpetre to form beautiful long shoots and can do it no harm”, although I didn’t see that actually happen. 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 by their reaction with vinegar and whatever else is in wine. 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. Without authentically made saltpetre I cannot use experiments to probe what these impurities are, and am loth to make guesses without much information, but suspect they are various forms of organic compound from the dung and soil used in the manufacture of saltpetre.
Ercker mentions the use of lime in purification, but isn’t so keen on it.
I became interested in the purification of saltpetre because I was concerned about it’s purity for upcoming alchemical experiments. So I tested for salt in the green vitriol and saltpetre that I have. 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:
a fine white precipitate, formed of silver chloride, because I poured silver nitrate into the solution of saltpetre. Therefore there is salt in it. This is an old analytical chemistry method for testing for the presence of chloride ions, and is atill in use, with modern electrodes and conductivity testing for determining the amount of silver in a sample of metal which has been dissolved.
When we look again at Ercker, a couple of things spring out. One, he thinks/ knows that NaCl is less soluble in a saturated saltpetre solution than the saltpetre. Two, he uses a copper vessel to check how saturated the solution is. This implies that he knows that glass and ceramic vessels can be broken by sudden changes in temperature. And perhaps that copper conducts heat well. Both of which can be verified and learnt by personal experience.
Another obvious point looking at Erckers description is the amount of specifically designed equipment to use.
Some pictures of it can be seen in this Mittelalter Centre report from 2004 on their early attempts at saltpetre making:
I suspect Erecker may not be exactly correct though in identifying the new, white crystals as salt. Some will surely be there, but the problem is that there is a possibility it could be calcium carbonate, which is precipitated from solution by boiling which breaks the calcium bicarbonate up. Whether much calcium carbonate is present will depend on the hardness of the water and how much is carried over in the crude saltpetre from the manufacturing process (which involves taking calcium nitrate and treating it to make potassium nitrate). That he says to use well water doesn’t say much either way about whether he realises there are potential contamination issues in the choice of water to be used. In my case there might be some there because of impurities in the saltpetre which aren’t in that made using the old fashioned method.
I decided to replicate Ercker’s method as much as I could, but could only start from modern mass produced saltpetre, which is already a lot purer than that which he uses. But it still contains salt, the removal of which would be useful from my point of view, as well as desirable in a historical context since I have read that salt in gunpowder helps absorb more moisture making it less effective.
Given the limitations of substances and equipment, I started with dissolving about 40g of it in 700ml of tapwater until it didn’t dissolve any more. Some white undissolved stuff as well as particles of what looked like sand did fall down to the bottom, obviously residues from the manufacturing processes. I did heat it just to make sure that it had all dissolved, and when cooled some undissoluble stuff had appeared at the bottom, lots of little white crystals, which might be salt as Ercker says.
I then poured off the liquid, leaving the crystals behind, into an old pan, boiled it, added some acetic acid, distilled wine vinegar and some old, oxidised red wine (Yes, I often keep such things around in case they come in useful in the future) in order to mimic what Ercker said to add. No scum formed, there was a thin oily layer on the surface but that was about it. I wouldn’t really expect the scum anyway given the inorganic manufacturing methods we use nowadays. (Described in wikipedia: http://en.wikipedia.org/wiki/Potassium_nitrate )
It did in fact turn yellow!
On cooling, yellow precipitates formed and clumped together:
The particle size is so small that they are just not falling to the bottom of the flask.
At this stage in my analytical capabilities, it is somewhat hard to identify, but I might be able to get something done about finding out what they are.
I then distilled the solution by undyed wollen cloth filter,
some of the yellow colour remained, and as an experiment I added some sodium hydroxide, but nothing precipitated out or changed in any way.
The next step was to leave it to crystallise, which produces these lovely crystals:
When dissolved in water and tested with silver nitrate solution they give this result, the purified on the right, unpurified on the left:
Irritatingly there is still a little salt present, even with my rinsing things out with the kind of battery topping up de-ionised water you can buy from Halfords. A simple visual comparison with a similar amount of the purified saltpetre suggests that it has however got a lot less salt than before. Just to add to the fun though it seems I added some Chlorine with the tap water used to dissolve the potassium nitrate, since it is testing positive for Chlorine, I think I need to have a word with Scottish Water. On the other hand the internet suggests that you can get a similar reaction from other metal ions too, so more bookwork needed to check things out. Either way there’s still stuff in my potassium nitrate that shouldn’t really be there.
One partial reason might be that I didn’t rinse the crystals with clean water so some of the salt water I took them from was on the surface and would contribute to contamination. However it is a rather inefficient process overall. Since I was trying it for the first time I didn’t keep a proper note of mass in and out, but it is clear that you will lose some at all stages of the process, although his method using proper equipment would be more efficient than mine.
It seems clear to me that Erckers method is effective and useful. As such it is an improvement over earlier methods, such as the 14th century one in the “Of the invention of verity” by pseudo-Geber, which simply says to dissolve it in fountain water and distil by filter, which would remove some solids and muck but leave all sorts of salts in solution. The invention and spread of methods of saltpetre purification would be an interesting area to research. By myself I cannot do it, since it is likely that much of the evidence will be in continental libraries, in German, Latin, Italian etc, manuscripts of the late medieval period.
However, as with any skilled work that produced a valuable product, it is also likely that methods were not written down at the time they were invented. I suspect some will be found in the alchemical literature, given the importance of saltpetre for making nitric acid, but others will have been passed down by word of mouth from master saltpetre makers to their workers. Given the kerfuffles over saltpetre collection in England in the Elizabethan era, it is clear that the knowledge was still not as widely spread as it could have been even 200 years after nitre beds came into use.