People often ask how to make lye from wood ash and then how to make soap from this lye.
What is wood ash lye?
To make wood ash lye, ashes of wood or other plant material are soaked in water to dissolve water-soluble alkaline compounds in the ash. The liquid that is collected from this process is wood ash lye or "potash" solution.
If the water in this liquid is evaporated, the solid crystals of impure potash can be heated and further purified to "pearl ash." Potash and pearl ash have been valuable raw materials for many industries, including agriculture, glass making, textile production, and soap making. (16)
People often have some misconceptions about potash -- what we soap makers often call "wood ash lye" --
Potash is not sodium hydroxide (NaOH) nor potassium hydroxide (KOH), the two main alkalis normally used in modern soap making. The chemicals in potash are mainly the carbonate alkalis -- mostly potassium carbonate (K2CO3) and some sodium carbonate (Na2CO3) -- along with calcium salts and smaller amounts of other water-soluble chemicals. (7,13,20)
Soap made with wood ash lye will not be a hard, solid bar like the soap made from sodium hydroxide. Wood-ash soap will be a relatively soft potassium-based soap.
People are skeptical about these points, because modern-day tutorials and articles on the internet say otherwise. Soap making manuals and instructions written in the 1700s and 1800s tell an entirely different story. (1,2)
Sources of materials high in carbonate alkalis
The oldest form of intentional soap making used a "ley" (an old word for "lye") made from the ashes of various plants or from natron, a mined mineral.
Natron, one of the materials used by the Egyptians for mummification, was a cheaper but less satisfactory alternative to plant ash. Natron is a mixture of soda ash (sodium carbonate, Na2CO3), sodium bicarbonate (NaHCO3), and other chemicals.
Ash from select marine or seacoast plants contain the highest levels of sodium carbonate, so lye made from these plants, historically called "barilla" (6), makes the firmest type of paste soap. The historically important soap making regions of the world have typically been seacoast communities for this reason -- they had access to sodium-rich plants that produced higher quality soap.
Ash from inland plants, including wood, grasses, and wastes from crops, can also be used to make potash. The amount of sodium is lower in ashes from inland plants, so lye made from this ash produces a softer soap compared to lye made from ashes of marine/seacoast plants.
The ash of hardwoods, grasses, and agricultural crop wastes can produce more potash compared with ash from softwoods or coal. Ash from grasses and ag wastes can be a good source of potash, as good or better than wood ash. (11, 15) But people in temperate climates usually burn wood for home heating, so wood ash is often the most convenient and abundant source of potash available to the average person.
Wood produces roughly 0.5% to 1.5% ash by weight when completely burned under controlled laboratory conditions (7,10), but the amount of ash produced in a typical woodstove or fireplace may be as much as 6% to 10% by weight due to incomplete combustion. (8)
Chemical composition of ash from inland woods
The chemical composition of wood ash and the purity and concentration of the potash solution will vary depending on the species of wood, growing conditions, temperature of the fire, completeness of combustion, how the ash is stored, etc.
Ignoring carbon (C), calcium (Ca) is the most abundant element in the ash of inland woods. Potassium (K) and magnesium (Mg) are the next most common components. Other elements are present in smaller amounts -- typically less than 2% by weight. These minor components include sulfur (S), phosphorous (P), manganese (Mn), zinc (Zn), iron (Fe), aluminum (Al), sodium (Na), silicon (Si), boron (B), and copper (Cu). (7,8,9)
Of these elements, potassium and sodium are of most interest to soap makers. The potassium content in inland wood ash averaged 2.6% with a range of 0.1% to 13% by weight in samples tested. (8) The potassium in this ash is mostly potassium carbonate (K2CO3) with some potassium sulfate (K2SO4) and traces of other potassium based minerals. (7)
Wood ash contains sodium in much smaller amounts, with an average of 0.19% and a range of 0% to 0.54% by weight in the same samples tested. (8)
Producing the best wood ash for lye making
Use hardwoods rather than softwoods if possible. Hardwoods tend to produce more ash by weight, although the numbers vary a lot. (7,10) In addition, hardwood ash tends to have more potassium by weight compared with softwood ash. (12) For these reasons, hardwood ash will yield a stronger potash mixture.
If all you have is softwood ash, however, by all means use it. Just understand you may have to do more to concentrate the lye until it is strong enough for soap making.
Burn the wood to a "white" ash that has the fewest impurities. Avoid dark gray or black ash or ash that still contains charcoal or partly unburned wood.
Burn the wood at the lowest temperature that still produces white ash. You would think a very hot fire is the best way to produce "white" ash for making lye, but high combustion temperatures cause the potassium carbonate (K2CO3) in the ash to decompose into potassium oxide (K2O) and carbon dioxide (CO2) gases. (7)
One study reported "...A significant decrease in the potassium concentration is observed at temperatures greater than 900 C [1650 F]...." (7)
Another study stated "...When the temperature was increased from 500 to 1300 C [930 to 2370 F] the loss ... was due to the decrease in ... K, S, B, Na and Cu [potassium, sulfur, boron, sodium and copper]..." (9)
Avoid heating the ash any longer than necessary, again to prevent the loss of valuable potassium and sodium from the ash. To collect the best ash for soap making, consider removing the white ash from the stove or fireplace after every fire.
Store ash intended for soap making in an airtight, water-tight, alkali-resistant container. For best results, protect ash from the carbon dioxide and moisture in fresh air until it is time to make the potash solution.
Leaching wood ash
I recommend reading the article Waste Not: Wood Ashes by Dan Hettinger. (20)
Concentrating wood ash lye
The potash solution needs to be fairly concentrated to make soap. It can be concentrated in two ways.
A weak potash solution can be concentrated by reusing it to leach the ash several times rather than by using fresh water each time.
Another way to concentrate the lye is to boil the potash solution to remove excess water. The potash solution can even be boiled to dryness. (2) Dry potash is convenient for storage and transport and is necessary when purifying potash to pearl ash (20), but dry potash is is not strictly necessary for soap making.
Testing the lye concentration
Soap makers historically did not measure the concentration of the lye directly; they checked it indirectly by measuring the density of the solution.
Hydrometer test. More fortunate soap makers used a hydrometer to measure the density. This is the same glass gadget that beer and wine makers use to estimate the alcohol content of their beverages.
Low-tech density tests. A low-tech time-honored alternative to the hydrometer is to see how a a freshly laid egg floats in the lye. A concentrated lye is more dense (heavier) than water or weak lye, so buoyant things such as eggs and hydrometers float higher in a concentrated lye and sink deeper in a weak lye.
Unfortunately, most people do not have access to freshly laid eggs; most grocery store eggs can be as much as several weeks old, and the density of eggs decreases as the eggs age. But a grocery store egg of unknown age can still be used if it is calibrated with a solution of table salt and water.
The specific gravity of a freshly laid chicken egg is about 1.03. (17,18) A solution of 5% sodium chloride (NaCl, table salt) by weight in water also has a specific gravity of 1.03. (19)
A freshly laid egg should float in a 5% NaCl solution with little or no shell showing above the liquid. Older eggs become less dense, so more of the shell will show above the liquid.
To calibrate an egg, mix 5 grams of sodium chloride in enough room-temperature water to make a total of 100 grams of solution. Float the egg in the salt solution. Mark the boundary where the egg emerges above the liquid. Make a second mark about halfway between the first mark and the bottom of the egg.
Test the potash solution with your "calibrated" egg. If you put this egg into a lye solution and the first mark you drew is below the surface of the liquid, the solution is too weak and not suitable for soap making. If the boundary is above the surface of the liquid, the lye solution is stronger.
Many sources say the lye is sufficiently strong for making soap if a freshly laid egg floats about half above water. If your calibrated egg floats high enough so the second mark is near the surface of the liquid, the lye solution is strong enough. "Eyeballing" the strength is good enough -- there is no benefit to being more accurate than this.
Other low tech methods. Other low-tech methods of evaluating the strength of the lye include floating a potato in the lye or seing if the lye solution can dissolve a poultry feather or a piece of paper.
pH test. The alkali concentration cannot be accurately measured by checking pH alone. For example, a sodium hydroxide solution at 5% concentration by weight will have a pH of about 13.7. The pH of an NaOH solution at 50% concentration will be about 14. This is a tiny 0.3 unit change in pH for a drastic change in concentration. This situation is also true for other alkalies such as potassium hydroxide, sodium carbonate, and potassium carbonate -- a large change in concentration results in only a small change in pH. On top of that, no pH test strip on the market is accurate enough to measure such small differences in pH. Even lab-quality pH test meters are not reliable in the concentrated alkali solutions required for soap making.
Titration test. The alkali concentration can be measured directly by adding an acid of known concentration, drop by drop, to a sample of an alkali solution until the pH of the mixture reaches a given pH reading, often 8.2. This procedure is called titration (tie-TRAY-shun). The concentration of the alkali solution is calculated based on the weight of acid used to reach that pH, not on the pH itself.
Making soap with carbonate lye -- wood ash only method
When the carbonate lye was ready to make soap, it was mixed with a portion of the fat to be saponified. The mixture was simmered and stirred over a fire for many hours. Because the lye concentration varied from batch to batch despite one's best efforts, this "boiled" soap making process was a trial and error method. Lye was gradually added, the mixture cooked and stirred, more fat and more lye were added as needed, and the mixture was stirred and cooked further until it formed a crude paste soap.
The soap was finished when all of the fat for the batch was added to the simmering soap and the soap remained "somewhat sharp" to the tongue. In other words, the soap had a mild zap, meaning it contained a slight excess of alkali. This endpoint ensured the fat was fully saponified.
Yes, this meant the soap was slightly lye heavy.
Commercial soap makers through the late 1800s and early 1900s routinely made and sold soap for laundry, household, and everyday bathing that was lye heavy. This was done to reduce the chance that the soap would become rancid, a process that is aggravated by any excess fat in the soap (aka superfat).
Only more expensive soaps specifically intended for toiletry purposes were made with more attention to minimizing the excess alkalinity. Some of the old manuals I have read talked about the chapped and reddened faces of folks who didn't have the money to buy this fancy toilette soap for bathing. Can you imagine the terribly irritated hands of the ladies who hand-washed laundry for a living?
Even most modern-day commercial soaps are made with zero excess fat (no superfat) to only a very slight superfat (under 1%). Rancidity in the soap is further discouraged by adding antioxidants and chelators.
Making soap with hydroxide lye -- Wood ash and lime method
In a refinement known and used by many soap makers as early as the 1700s (1), the carbonate lye from ash was then mixed with slaked lime (calcium hydroxide, Ca(OH)2). The lye and lime reacted to form a hydroxide lye solution -- a mixture of potassium hydroxide and sodium hydroxide in water -- and a sludge of calcium carbonate and other solid impurities.
Chemistry comment -- Because slaked lime is only slightly soluble in plain water, it might appear at first glance this reaction does not work, but it does. The lime that dissolves is consumed by the reaction with the sodium and potassium carbonates. This reaction forms solid calcium carbonate (CaCO3) and sodium and potassium hydroxides. The disappearance of the dissolved lime allows more solid lime to dissolve and continue its reaction with the sodium and potassium carbonates.
This reaction creates solid particles of calcium carbonate (CaCO3) that are undesirable in soap. These particles were allowed to settle out of the lye solution, and only the clear hydroxide lye was used for soap making.
Several solutions of lye could be made from one charge of slaked lime. The first lye was the strongest and the final (third or fourth) lye was the weakest. Soap makers stored these different strengths in separate tanks.
The soap maker usually mixed the weakest lye with fresh fats, because weak lye and pure fat have similar densities. It is a lot easier to keep two liquids mixed with gentle hand stirring if their densities are close. In the days before stick blenders, when all soap was made by hours of patient hand stirring, this was a very useful trick to know. The strongest and most dense lye was saved for use at the end of soap making when the soap itself helped the lye and fats remain mixed.
The hydroxide lye made from lime and ash still created a soft paste soap. The same "boiled" process was still used. But there were distinct advantages to using a hydroxide lye. The soap was much faster and somewhat easier to make, and the finished soap contained fewer impurities for better appearance and longer life.
People also discovered that adding plain salt (sodium chloride, NaCl) to this soft soap near the end of the boiling process would firm the soap somewhat. The increase in firmness happens when sodium from the salt replaced some of the potassium in the soap. Sodium soap is firmer than potassium soap.
The potassium in the soap was not entirely replaced with sodium in this "salting-out" process, so the resulting soap remained more water soluble and not as firm as the pure sodium soap we make nowadays.
Making soap with hydroxide lye -- Soda ash and lime method
In my great-great- and great-grandmother's time, people on the frontier were still making soap with ash alone or lime and ash, but an easier and better alternative was gradually becoming common in more settled regions in the mid 1800s as pure, commercial soda ash (sodium carbonate, Na2CO3) became more widely available to consumers. (3, 4) Soda ash is also called washing soda, since it can be used for laundry and general cleaning.
Although a soda ash lye can be used to make soap directly, most people converted this carbonate lye into a hydroxide lye as described above. The resulting clear liquid was a relatively pure sodium hydroxide (NaOH) lye, and it was used make a firm sodium soap just like we make today.
Making soap without lye?
Some soap recipes are touted as ways to make "soap without lye." These recipes simmer washing soda (soda ash) with lime and water exactly as explained above to make a sodium hydroxide solution and this solution is used to make the soap. This "no lye" boast is a falsehood.
Making soap with hydroxide lye -- Commercial NaOH
Pure sodium hydroxide (NaOH) in a dry form became commercially available to home consumers in the early 1900s. Most housewives of this era who made soap at home quickly turned to this store-bought NaOH as a much easier alternative to the soda ash and lime method.
Large soap makers had access to pure NaOH some decades earlier, and these companies had gradually converted over from the soda ash & lime process to using commercial NaOH quite some years before our foremothers were able to do the same. (5)
References, additional reading, and notes
(1) Geoffroy, C.J. Letter entitled Monsieur Claud. Joseph Geoffroy, F. R. S. to David Hartley, M. A. F. R. S. containing his Method of making Soap-lees and Hard Soap, for Medicinal Uses. 1742.
(2) Morfit, Campbell. A treatise on chemistry applied to the manufacture of soap and candles. 1856.
(3) Wikipedia. Leblanc Process. Version dated 16 October 2020. https://en.wikipedia.org/wiki/Leblanc_process
(4) Wikipedia. Solvay Process. Version dated 16 October 2020. https://en.wikipedia.org/wiki/solvay_process
(5) Wikipedia. Chloralkali Process. Version dated 30 October 2020. https://en.wikipedia.org/wiki/Chloralkali_process
(6) Wikipedia. Barilla. Version dated 23 August 2019. https://en.wikipedia.org/wiki/Barilla
(7) Misra, Mahendra K., Kenneth W. Ragland, and Andrew J. Baker. Wood ash composition as a function of furnace temperature. Biomass and Bioenergy 4.2 (1993): 103-116. https://doi.org/10.1016/0961-9534(93)90032-Y URL to download full text: https://www.fpl.fs.fed.us/documnts/pdf1993/misra93a.pdf
(8) Risse, L. Mark, Gaskin, Julia W. Best Management Practices for Wood Ash as Agricultural Soil Amendment. University of Georgia Cooperative Extension Bulletin 1142. Last reviewed March 2013. https://secure.caes.uga.edu/extension/publications/files/pdf/B%201142_3.PDF
(9) Chowdhury S et al., The incorporation of wood waste ash as a partial cement replacement material for making structural grade concrete: An overview, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.11.005
(10) Owens, Eleanor, and Cooley, Sarah. Ash content of Irish woodfuel. Processing/Products No. 30. Coford, Department of Agriculture, Food, and the Marine. Dublin, Ireland. 12 November 2013. http://www.coford.ie/media/coford/content/publications/projectreports/cofordconnects/
Ash%20content%20of%20Irish%20woodfuel..pdf
(11) Taiwo OE, Osinowo FA. Evaluation of various agro-wastes for traditional black soap production. Bioresour Technol. 2001 Aug;79(1):95-7. https://doi.org/10.1016/s0960-8524(00)00188-7
(12) Berg, Marlene G. Fertilizer Guide: Using wood ashes in the home garden. FGNO 061. Revised May 1982. Oregon State University Extension Service. https://ir.library.oregonstate.edu/downloads/2j62s561s
(13) Author unknown. Potash defined. Version viewed 9 January 2021. https://www.saltworkconsultants.com/potash/
(14) Liodakis, S. & Katsigiannis, G. & Kakali, Glikeria. Ash properties of some dominant Greek forest species. Thermochimica Acta. (2005). 437. 158-167. https://doi.org/10.1016/j.tca.2005.06.041
(15) Cooper, H.P. Ash constituents of pasture grasses, their standard electrode potentials and ecological significance. Plant Physiology Apr 1930, 5 (2) 193-214; https://doi.org/10.1104/pp.5.2.193
(16) Wikipedia. Potash. Version dated 31 December 2020. https://en.wikipedia.org/wiki/Potash
(17) What is the density of an egg? Sciencing. Version viewed 25 February 2021. https://sciencing.com/density-egg-5127458.html
(18) Rahn, H., Paganelli, C.V. The initial density of avian eggs derived from the tables of Schönwetter. J Ornithol 130, 207–215 (1989). https://doi.org/10.1007/BF01649755
(19) Density of aqueous solutions of inorganic sodium salts. The Engineering Toolbox. Version viewed 25 February 2021. https://www.engineeringtoolbox.com/density-aqueous-solution-inorganic-sodium-salt-concentration-d_1957.html
(20) Hettinger, Dan. Waste Not: Wood Ashes. Version dated 4 February 2021, viewed 16 March 2022. https://livingwebfarms.org/waste-not-wood-ashes/
Additional reading
J.O. Babayemi, G.O. Adewuyi, K.T. Dauda and A.A.A. Kayode, 2011. The Ancient Alkali Production Technology and the Modern Improvement: A Review. Asian Journal of Applied Sciences, 4: 22-29. https://scialert.net/fulltext/?doi=ajaps.2011.22.29
Dunn, Kevin. A Primitive Alkali: Potash. Caveman Chemistry. Version viewed 9 January 2021. https://cavemanchemistry.com/oldcave/projects/potash/ (old version)
Dunn, Kevin. 28 Projects, from the Creation of Fire to the Production of Plastics. Caveman Chemistry. Chapter 8. Job (Alkali). Version viewed 9 January 2021. https://cavemanchemistry.com/cavebook/chpotash2.html (new version)
References 1 and 2 are readily available from various online resources including Google Books, Project Gutenberg, and/or the Internet Archive. Also see these same online resources for other soap making manuals from the 1800s and early 1900s.
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