This is the first genuinely hazardous craft in the app, and the hazard is not exotic: it is a clear liquid that looks like water, feels slippery rather than painful at first, and blinds people.
The process itself is simple and has been done in kitchens for centuries. The care around it is the reason this page is mostly warnings.
What you are actually making
Water leached through wood ash dissolves the potassium salts in it and comes out as a solution of potassium hydroxide, historically called potash lye.
This is not the same as shop-bought caustic soda, which is sodium hydroxide. Both are strong alkalis and both are dangerous, and they behave differently in soap: potassium hydroxide makes a soft or liquid soap, sodium hydroxide makes a hard bar. See soap from lye and fat.
If you can buy sodium hydroxide, buy it. It comes at a known strength, which means you can calculate a soap recipe properly and get a reliable, safe result. Ash lye is what you make when buying has stopped being an option, and its great weakness is that you never know exactly how strong it is.
Safety, which is the page
The ash
Hardwood, and only hardwood. Oak, ash, beech, hornbeam, apple, hawthorn. Hardwood ash is far richer in the potassium salts you want; softwood ash gives a weak solution and a lot of resin.
Clean ash only. Ash from a fire that has burnt treated timber, painted wood, plywood, chipboard, MDF, pallets of unknown origin, coal, or household rubbish is contaminated, and you are going to put the product on skin. Burn clean wood for this.
White ash, fully burnt. Grey or black ash with charcoal in it is under-burnt and weaker. Sieve out the lumps and charcoal.
Keep it dry until you use it. Ash that has been rained on has already been leached and has little left to give.
You need a lot. A bucket of ash gives a modest quantity of usable lye, and it is why this is a by-product of a household that already burns wood rather than a reason to burn it.
Leaching it
The traditional arrangement is a barrel with a hole in the bottom, and it works exactly as it sounds.
- Set up the vessel. A plastic barrel, a bucket with holes drilled low, or a wooden tub. Raise it on blocks so a collecting container fits underneath. Stainless, glass or plastic for everything.
- Line the bottom with straw, small twigs and then a layer of gravel or coarse sand, to act as a filter and stop the ash washing straight through.
- Fill with ash, packed down firmly, leaving room at the top.
- Pour soft water slowly over the top. Rainwater is ideal; hard tap water works less well because the dissolved minerals interfere. Warm water works faster than cold.
- Let it percolate. Slowly. Collect what comes out.
- Run it through again, and again, through fresh ash if you have it, until it tests strong enough.
The first run is usually weak. Passing the same liquid back through the ash two or three times concentrates it considerably.
It comes out brown, the colour of tea or darker, and it feels soapy or slippery between the fingers. That slipperiness is the alkali dissolving the surface of your skin, which is exactly why you are wearing gloves.
Testing the strength
This is where honesty matters, because the traditional tests are rough.
The float test. A fresh egg or a small potato floated in the solution should sit with a circle of shell about the size of a small coin, roughly 2 cm, above the surface. Floating high means too strong; sinking means too weak.
What that test actually tells you is density, and density is affected by everything dissolved in the liquid, not only the hydroxide. It is an indication, not a measurement.
A feather or a chicken quill dipped in strong lye begins to dissolve, which is the other traditional test and tells you the same thing with the same imprecision.
pH paper or a pH meter is far better if you have one, and pH strips store for years and weigh nothing. Strong lye solution reads at the very top of the scale, 13 or 14.
The consequence of imprecision is on the soap page, and it is the reason ash-lye soap must be made by a method that can be corrected rather than by a calculated recipe.
Concentrating it
Boil it down, in a stainless or enamel pan, outdoors, to drive off water and concentrate the solution.
Storing and labelling
- Glass or heavy plastic, with a tight, well-fitting lid. Never metal, and never anything that has held food or drink.
- Label it in large clear letters: CAUSTIC LYE, DO NOT DRINK, with the date.
- Store it locked, or at minimum high up and out of reach, away from food, away from any acid, and away from children and animals.
- Tell everyone in the house what it is and where it is. An unlabelled bottle of clear brown liquid in a shed is exactly the accident this page exists to prevent.
- It absorbs carbon dioxide from the air and slowly weakens, so a well-sealed container matters and old lye should be retested.
What it is honestly good for
- Soap, which is the main reason to make it. See soap from lye and fat.
- Heavy cleaning and degreasing, diluted, with gloves.
- Historically, nixtamalising maize and processing certain foods, both of which need precision this page cannot give you and neither of which should be attempted from an untested home solution.
It is not a disinfectant for water and must never be used anywhere near drinking water. For that see shock chlorination and judging raw water.