Making wood ash lye
Can you make lye from wood ashes? And if so, then how do you make soap from wood-ash lye?
People pop onto the Soap Making Forum fairly often to ask these questions and most of them expect quick and easy instructions about making the lye solution and soap from said lye. The answer is not as straightforward and simple as most of these people want.
What wood ash lye is and what it is not
First off, I want to dispel two common misconceptions about wood ash lye and the soap made from this lye --
Lye made from wood ashes is NOT a sodium hydroxide and/or potassium hydroxide solution as is used in modern soap making.
Wood-ash lye is a solution of mostly potassium carbonate and some sodium carbonate (washing soda).
Soap made with wood-ash lye will not be a hard, solid bar like the sodium soap most people make nowadays. It is a mostly-potassium soap, which means it will have a softer paste texture.
People want to argue with me about these points, especially the first, 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, and they are the ones to learn from.
Leaching wood ashes to make ley
The oldest form of intentional soap making used a "ley" (a very old word for "lye") made from the ashes of plants or from natron, a mined product. When the ashes are leached (soaked) in water, chemicals dissolve out of the ashes into the water. They form an alkaline solution of water with potassium carbonate and varying amounts of sodium carbonate.
The ashes from select marine or seacoast plants contain the highest levels of sodium carbonate, so ley made from these plants will make the firmest type of paste soap. That's why the historically important soap making regions of the world have typically been seacoast communities -- they had access to these plants.
The ashes of inland plants can also be used to make ley, but the amount of sodium carbonate is lower in these ashes in proportion to the potassium carbonate. This means a ley from inland plants will produce a softer soap compared to soap made with a marine/seacoast ley. If you are going to use ashes from inland plants, the ashes from hardwood are preferred for soap making purposes.
The plant matter should be burned hot enough so the ashes are nearly white. Black ash contains more carbon and other contaminants, so it makes a less desirable ley.
Natron, one of the materials used by the Egyptians for mummification, was also used as a cheaper but less satisfactory alternative to ashes. This is a mined product that is a mixture of soda ash (sodium carbonate, Na2CO3), sodium bicarbonate (NaHCO3), and other chemicals.
Concentrating wood ash ley
This carbonate ley needs to be fairly concentrated to make soap. It was concentrated in two ways.
A weak ley can be made more concentrated by using it to leach the ash multiple times rather than using fresh water.
Another way to concentrate the ley is to boil the ley to remove excess water. Some people even boiled the ley to dryness, but that is not strictly necessary for soap making.
Soap makers did not measure the concentration of the ley directly; they checked it indirectly by measuring the density of the solution. 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.
A low-tech alternative to the hydrometer is to put a fresh egg in the ley and see how it floats. A concentrated ley is more dense (heavier) than water or weak ley, so buoyant things such as eggs and hydrometers will float higher in a concentrated ley.
For the most reliable results, the egg needs to be freshly laid so the air space inside the egg is as small and consistent as possible. If you have your own hens or can buy eggs direct from the farmer, this method can be useful. If you only have access to store-bought eggs, this method may not be reliable.
Other low-tech methods of evaluating the strength of the ley include floating a potato in the ley or checking if the ley is strong enough to dissolve a poultry feather.
The alkali concentration can be measured directly by adding an acid of known concentration, drop by drop, to a sample of the ley with until the pH of the mixture reaches a given reading. This procedure is called titration (tie-tray-shun).
And, no, the alkali concentration cannot be accurately determined by just measuring the pH alone. A KOH or NaOH solution of 1% or more by weight in water will have a pH of over 13. The pH of a K2CO3 solution over about 2% by weight will be about 11.
Making soap with carbonate ley
When the carbonate ley was ready to make soap, it was mixed with fat and the mixture was simmered and stirred over a fire for many hours until it formed a crude paste soap. Because the ley concentration varied from batch to batch despite one's best efforts, this "boiled" soap making process was a trial and error method. Ley was gradually added, the mixture cooked, more ley was added as needed, and the soap was stirred and cooked further.
The soap was finished when it 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 this soap is further discouraged by adding antioxidants and preservatives.
Wood ash and lime method
In a refinement known and used by many soap makers as early as the 1700s, the carbonate ley from ashes was not used directly to make soap. The wood-ash ley instead was mixed with water and slaked lime (calcium hydroxide, Ca(OH)2).
The chemical reaction of the carbonate solution with the lime formed a purer, stronger ley that was a mixture of potassium hydroxide and some sodium hydroxide. The mixture also contained solid particles of calcium carbonate (CaCO3) that were unwanted by soap makers. These calcium carbonate particles were allowed to settle out of the ley solution and the resulting clear liquid ley was saved for soap making.
Some things didn't change -- This mixed hydroxide ley made from lime and ashes still created a soft paste soap. The same "boiled" process as described above was still used. But there were distinct advantages -- 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 to this soft soap near the end of the boiling process would firm the soap somewhat. The increase in firmness was caused by the sodium in the salt replacing some of the potassium in the soft soap. Sodium soap is firmer than potassium soap.
The potassium in the soap could not be entirely replaced in this "salting-out" process, so the resulting soap remained more water soluble and not as firm as the pure NaOH soap we make nowadays.
Soda ash and lime method
In my great-great- and great-grandmother's time, people on the frontier were still making soap with ashes alone or lime and ashes, but an easier and better alternative was gradually becoming common in more settled regions. This improvement was ley made directly from pure, commercial soda ash (sodium carbonate, Na2CO3). Soda ash is also called washing soda, since it is used for laundry and general cleaning.
Although this ley can be used to make soap directly, most people converted the carbonate ley to hydroxide ley as described above. The resulting clear liquid was a relatively pure sodium hydroxide (NaOH) ley, and it was used make a firm sodium soap just like we make today.
Make soap without lye?
Some soap recipes billed as making "soap without lye" use this method of simmering soda ash with lime and water. The "no lye" boast is a falsehood. The soap made with such recipes is made with an NaOH solution just like normal. The only difference is the NaOH solution isn't made by mixing NaOH granules with water; it is created by chemically reacting soda ash with lime.
Commercial NaOH for home soap makers
Pure sodium hydroxide (NaOH) in granule form became commercially available in my grandmother's day. Most housewives who made soap at home quickly turned to this store-bought NaOH as a timesaving, easier, and better alternative to the soda ash and lime method.
Large soap makers had access to pure NaOH for some decades before this time, and these companies had gradually converted over from the soda ash & lime process to using commercial NaOH decades before our foremothers were able to do the same.