At some point in the medieval period, people, perhaps alchemists, came up with a recipe for a new, shiny white crystal substance. Potassium hydroxide, which is highly caustic.
It’s all about turning the potassium carbonate from tree and plant ashes into potassium hydroxide, by adding calcium oxide, quicklime. (That is, KCO3 to KOH by the addition of CaO, all done in H2O)
I don’t know when this was first invented, perhaps in the middle east in the 8th or 9th century? Or in Spain? This is just conjectural, it is surprisingly hard to find good information. Either way recipes occur in a couple of European texts which are part of the same family of works which trace their origin back to those of Rhazi, the 9th century AD Arabic alchemist. Anyway, I can speculate about it’s origins, given the alchemical interest in white stuff and the use of ashes for making glass and soap and the like in the middle east at that time. What could be better than taking one white powdery substance which withstood fire, (The ashes) dissolving it in water and adding a different, fiery white stuff, to give you something that was like the ashes, only stronger?
One of the texts a recipe appears in is the “Of the Invention of Verity, or perfection”, a pseudo-Geberian work probably written in the 14th century. Unlike the Summa Perfectionis of Geber(The Latin geber, not Jabir), of which it is an extended discussion and elucidation, it spends some time specifying what salts are and how they are made, in a manner similar to that of al-Rhazi’s “Book of the Secret of Secrets”.
The recipe is:
5lbs of ashes of heartsease, 1lb of quicklime, boil it together in water, which is filtered and congealed; repeated this once and it is prepared.
Heartsease itself is a plant that grows in meadows, and is the progenitor of the modern cultivated pansy. Why that would be so good for making ashes I don’t know, it doesn’t even seem to live beside the sea and be a good source of sodium unlike some plants specified for industrial purposes. But I don’t have any of it growing near me, and as I don’t think I need to get that accurate in following the recipes I won’t bother with this one.
Coincidentally, just before this recipe, it says that true sal-alkali is made from dissolved soda that is dissolved, filtered and boiled away. This will surely be sodium carbonate, a potent alkali, but not quite as strong as sodium hydroxide would be. In fact nowadays NaOH was made from sodium carbonate by the use of lime in an industrial process in the 19th century and early 20th, and a similar process was used for potassium hydroxide. The most famous kind of ‘soda’ is that called natron, which was used by the ancient Egyptians for mummification and was rich in sodium carbonate. It is interesting to find it thousands of miles from home in a European recipe, but then the author was probably drawing upon older Arabic sources. Although, if you’ll excuse my hypothesising, the fact it doesn’t specify an origin for the soda suggests it is a14th century or later recipe rather than one of the earlier type, where substances were often named with geographical designations, such as ‘alum of Aleppo’.
But anyway, the actual recipe I have used comes from a different but related series of recipes to the one mentioned above. This is the “Libellus de Alkimia” by pseudo-Albertus Magnus, the Virginia Heines translation. It says:
“Sal Alkali is important in this art, and, when it has been well prepared, frees all the calxes of bodies as a solid mass. By nature it is warm and moist.”
So far, so good; even the “Summa Perfectionis” refers to the use of salts for softening of calxes, and that is one use of them in the “Secret of Secrets” of Rhazi. It continues:
“It is prepared in this manner: take a large quantity of putrid oaken ashes, or better clavellated (apparently these are burnt lees or dregs of wine) ashes, which are used for washing garments, grind very finely, add a sixth part of quicklime, mix once and put a closely woven cloth over a tina ( A vessel for holding wine) and upon it as much of the ashes mixed with the calx as it will hold, and pour hot water over the whole from above. Then filter into the lye until all the bitterness has been extracted. Remove this solution and replace it by a fresh one, and repeat as before. Put all the filtrates into the same vessel until morning, and then distill through a filter. Heat in a small cauldron until the solution evaporates and does not fume. Allow to cool and a hard stone will remain which is called alkali, that is, dregs of bitterness.”
The instructions make me wonder – there are oaks in southern Europe, but does the attachement of the name of Albertus Magnus suggest these were compiled more in Northern Europe, where there are plenty of oak trees? And of course wine lees are available all over the place. Just to add to the fun, there are many copies of this family of recipes, and not all are attributed to Albertus. Their basis is the “Summa Perfectionis” of Geber, but in the 14th century the recipes were chopped and changed and altered, a common feature of alchemical texts as the readers made their own recipe books upout of whatever sourcestook their fancy. Nevertheless it is interesting how two different sets of recipes appear with a similar inspirational source.
But my comments on oaks and wine etc are perhaps not so relevant, since trade was so widespread that I’m sure an alchemist could have gotten whatever they really needed, if they had the money for it. Yet I am intrigued by the fact that potassium rich wood ash and potassium rich lees of wine are suggested as starting substances, suggesting that the author knew that they gave similar results (perhaps based upon similarities in crystals made?), almost foreshadowing the later knowledge that they had potassium in common.
This is different from the “Invention of Verity” recipe because of the different substances used and the narrower concentration on what we now know of as potassium containing compounds. And there is no boilding stage after the water and ashes have been mixed, but one of my more modern chemistry textbooks says that is necessary to ensure full reaction of the calcium with the carbonate.
This turns out to be somewhat irrelevant, because when carrying out the process, slaking the lime with boiling water leads to a vigourous reaction, which I am sure would help drive things along. So the liquid we end up with probably has some calcium hydroxide and potassium carbonate as well as the desired potassium hydroxide in it, especially since there will be variable amounts of KCO3 in the ash to begin with.
Ideally I aught to put it in the furnace to finish it off, but I need to make another crucible to avoid contamination from the others which have been used for metals, bone ash and the like.
So what is it used for? The libellus is a little unclear on that, despite claiming it is of great importance. An added chapter, perhaps not of the same vintage as the recipe above, says it can be used with calxes to fix mercury, which makes sense but is a little different from the earlier idea of salts being used to soften calxes. A further ‘additional chapter’, perhaps of post-medieval date, claims that metallic arsenicum is made by part arsenicum with two parts of white soap. Wikipedia suggests that this was first done by Roger Bacon, turning the arsenic sulphide into the silver metal, their reference being:
Emsley, John (2001). Nature’s Building Blocks: An A-Z Guide to the Elements. Oxford: Oxford University Press. pp. 43, 513, 529. ISBN 0-19-850341-5.
Now Partington’s magisterial “Inorganic Chemistry” mentions Roger Bacon, but rather in connection with the making of the colourless, clear amorphous form of arsenic trioxide. No soap involved, indeed, a few pages further on it says that arsenic trisulphide, that is, orpiment, (although in reality the historical alchemists probably used mixes of the two sulphides) dissolves in caustic potash, soda or ammonia. In the series of equations that are then discussed, no arsenic is produced at all, rather thioarsenates such as K3AsS3.
Thus, pending further information, the Bacon and soap and arsenic thing is probably complete rubbish and another example of how general surveys pollute the public sources of information through their lack of depth.
The same process is also repeatedly attributed to Jabir, as a glance at some online sources will show, but there isn’t much evidence for that either.
Anyway, the final problem with all this is that what we think of the Libellus de Alchimia is a 19th century printed version drawn from 17th century sources which in turn got stuff from one or many medieval or 16th century manuscripts, leaving me confused and uncertain over what methods and recipes were in use where and when. Not to mention the fact that people often copied out only what they were interested in and ignored the rest, leading to a fragmentation of the series of recipes. Once again, further research is required to better understand the use of potassium hydroxide in alchemy and society at large in the medieval period.
As an aside, other period uses for lye include soap making, using plant ashes. It dates back over 2,000 years:
In fact the Palestinian town of Nablus has been famous for its soap manufacture for over a thousand years:
They seem to use sodium compounds of some sort though, perhaps related to the true soda, the natron of the region, especially from Egypt, or the soda rich plants that grew in salty soil beside the sea and were important in glassmaking.
The use of alkali for cleaning is another topic again, although I am planning a post of the bleaching of linen using it.
There is a final possible use for caustic solutions which I haven’t really come across in the period recipes, and that is the stibnite and caustic giving red kermes mineral, as I show here.
So, the end result is a substance that is impure, and of doubtful utility. This seems rather appropriate given what happened to alchemy…
(Not that they knew that at the time; the alchemists were doing their best to understand the world and how it worked and manipulate it for their own purposes)