Of Potash and Lye

Potash lye is used in soap cooking, glass making and the making of gunpowder in medieval times. Wood ashes, wood ash pastes and lye solutions are also known to be used to clean the house, to clean laundry, to clean ourselves and medicinally to deter bacterial growth on wounds and the skin.

What is Potash?
The part of ash that can dissolve in water and be leached out is called potash, or potassium carbonate. But potash is not the only soluble component, and depending on soil conditions, sodium carbonate, potassium sulfates and sodium sulfates might also be present. Crystallized potash can be used as a fertilizer, to make glass or rehydrated to make lye water for soap cooking.

How does Potash become Lye?
When potassium carbonate (K2CO3) is in solution with water (H2O) it will react with the hydrogen (H) and hydroxide (OH) of the water, which creates potassium hydroxide (KOH) or potassium lye.

What Kind of Wood Ashes?
To make a usable potassium hydroxide solution hardwood ashes are ideal (“well burnt ashes from good logs”). Soft woods contain more resins and therefore burn hotter than hardwoods. Ashes from woods combusted at temperatures above 900 °C lose a significant amount of potassium to evaporation. Softwoods are known to burn at around 950 °C and hardwoods at around 700 °C, making it likely softwood ash would contain less potassium than hardwood ash.

The best hardwood ashes are light grey to white (sometimes referred to as white ashes), well heated to incinerate or break down most of the organics and combustibles (charcoal) but not over 900 °C. Sieve or sift the white ashes well to remove any partially burnt materials, especially charcoal as it is activated charcoal and when wet will draw minerals out of suspension (purify).

Grass ashes (corn cobs, straw) are known to produce more lye than hardwood ashes, but grass ashes were not generally available in abundance, unlike wood ashes.

What Kind of Water?
Use either rain water or distilled water - well water, spring water and tap water can have many other minerals in solution while still being perfectly fine to drink (like sulfides and chlorides). These minerals can dissolve together with the potassium salts and can later interfere with the soap cooking process.

Hot Water or Cold Water?
Due to the easily dissolving nature of potassium salts it does not really matter to use either cold or hot water, it will dilute either way. Same with plugging up the vessel and letting it sit, it is in suspension quick enough that as long as the ashes are well tamped down to slow the flow of water to a drip, most lye leaches out the first time around.

Add Lime or Quick Lime?
In water, lime (calcium oxide, CaO) becomes slaked lime (calcium hydroxide, Ca(OH)2), which reacts with potassium salts, such as potassium carbonate (K2CO3) to form calcium carbonate (CaCO3) and potassium hydroxide (KOH).  Ashes that contain charcoal, or carbonates, benefit from calcining with lime as the incompletely burned carbonates can still be converted to hydroxides, boosting the strength of the lye.

Do Multiple Leachings Increase Strength?
I was not successful using multiple runs to strengthen my lye solution, it comes out at about pH11 either way as checked by Litmus paper. If using a shallow sieve-like whicker basket as a lye vat, making multiple runs does make sense as the water probably leaches through pretty quick - maybe too quick to dilute all the alkaline salts available in the ashes – so in that case more runs would result in a darker, stronger lye solution.

Then Why do Multiple Leachings?
The first run of lye solution is very dark in color, in large part by organic compounds (plant lignin, or cellulose) dissolved right alongside the lye and other salts. By running the lye solution through again it became much clearer, and the less colored the lye solution the whiter the soap will be. I theorize some medieval recipes advice to run the lye through again because the solution becomes lighter, or better, for the higher priced white soap making, probably assuming it won’t hurt strength of solution either.

Is This Solution Pure Lye?
When wood ashes are leached to collect potash all soluble potassium salts are dissolved, which include potassium hydroxide (our lye) and potassium carbonate but also potassium chloride and other impurities like sodium chloride and sodium sulfate. It is important to realize this mixture of different salts when learning to cook soap the medieval way as an excess of these foreign salts could prevent the caustic salts (lye) from acting on neutral fats during soap cooking, thus preventing the saponification process.

Potash can be crystallized by evaporating by heat water from the leached alkaline solution. As potassium carbonate (147g/100ml) is highly soluble compared to sodium sulfate (11g/100ml), potassium sulfate (12g/100ml) and sodium carbonate (22g/100ml) the contaminants will crystallize out of solution first. If the alkaline solution is cooled down the less soluble compounds will crystallize out and sink to the bottom while potassium carbonate stays in solution as long as possible. Pour off this solution to leave these crystals behind (decant, or clarification) and the lye thus made will have less contamination.

What has also been done to remove impurities during soap cooking is to use multiple applications of lye solution. As most solubles will be removed the first time around (the first lye), the second and third leachings will be mostly water. During cooking at the point where no more soap curds are formed, either the curds are scooped out of the used up lye solution (the nigre) into another container, or the used up lye solution (the nigre) is poured off, and a second (and third) lye solution is added to saponify the remainder fats. Either way, the settled contaminants are separated from the soap curds.

Determine Strength of Solution?
Medieval soap cooking does not exactly measure the amounts of fats and the strength of the lye solution used. They did find it prudent to use as strong a lye solution as they could make, mostly by evaporating excess water over heat because otherwise it would need to be evaporated during soap making. But for the soap cooking technique used at that time it was not necessary to know the exact strength of lye as modern soap makers do now, as either the formed soap curds or the remainder liquid nigre is separated during soap cooking. So no need to use floating eggs or similar density or pH tests, all they needed to know was that it is caustic lye, which is tested by tasting on the tongue as a good lye solution will taste metallic and have a distinct tingle on the tip of your tongue.

Does potassium lye make the same soap as sodium lye?
Potassium is a relatively large molecule compared to sodium (check the periodic table). The larger the molecule the weaker the bond between its parts, thus potassium has a weaker crystalline (solid) structure than sodium, which is a smaller molecule. This means that potassium lye soaps tend to be softer than sodium lye soaps, especially if fats or oils are used which are not solid at room temperature. This is not necessarily a bad thing as potassium soap makes great liquid soap, something sodium soap has trouble with as it does not dilute very well, or stay in solution.

Why is table salt added to potassium lye soap?
Potash soap can be converted by the addition of sodium chloride (common table salt) into soda soap and potassium chloride. The “salting out” of soap was done by first creating a soft potassium soap which was then converted into sodium soap by repeated boiling up with table salt. Adding soda ashes (sodium carbonate) does not work, as carbonates, in contrast to chlorides, have the tendency towards the creation of potash soap and sodium carbonates, where as chlorides have the tendency towards the creation of sodium soap and potassium chlorides. Soda ashes first need to be leached with water to form sodium hydroxide, which does make sodium soap.

Conclusion:
Making medieval soap using leached lye is a truly different process from making soap with commercially available pure lye using modern techniques. Not only is early medieval soap a Hot Process soap while most modern handmade soaps are Cold Process, but because of the use of contaminated lye this hot process soap could be separated from the cooking liquid, or nigre, to remove the naturally occurring glycerin!

Intrigued? Read more about the process in my handout “To Make Black Sope”.
available at https://www.academia.edu/27755720/TO_MAKE_BLACK_SOPE

Download this class handout, including photo essay, at Academia.edu:
www.academia.edu/27755101/Of_Potash_and_Lye