Making lye from charcoal (more precisely, from hardwood ash) is a straightforward leaching process: you slowly filter water through wood ash, and the water dissolves the potassium compounds in the ash to produce a caustic alkaline solution. This potassium hydroxide solution, with a pH around 13, is the same lye that people have used for centuries to make soap, clean surfaces, and process food. The whole process takes a day or two and requires no specialized equipment.
Why It Works
When hardwood burns completely, the organic material converts to gases and escapes, leaving behind mineral-rich ash. The key mineral is potassium carbonate, sometimes called potash. When you run water through this ash, it dissolves the potassium carbonate. In solution, that compound reacts with water to form potassium hydroxide, a strong alkali. This is why the resulting liquid is caustic enough to burn skin and dissolve grease.
The critical word here is “hardwood.” Softwoods like pine and spruce contain resins that contaminate the ash and produce much less potassium. Hardwoods burn cleaner and leave white or light gray ash with significantly higher mineral concentrations. The best performers are white oak, sugar maple, American beech, black walnut, and hickory. Mixed hardwood ash can contain around 8.9% potassium, while oak alone runs about 4.5%. Black walnut has notably high mineral content. Whatever you use, you want ash that burned completely to a fine, white or pale gray powder. Dark, chunite chunks mean incomplete combustion and weaker lye.
What You Need
- A barrel or bucket with drainage holes: A small wooden barrel or a food-grade plastic bucket works. Drill several holes in the bottom before you start.
- Gravel and straw: These act as a filter layer to keep ash particles out of your lye water.
- Hardwood ash: Collected from fires that burned only untreated hardwood. No charcoal briquettes, no painted or pressure-treated wood, no paper with colored ink.
- Soft water: Rainwater is ideal. Tap water with high mineral content (hard water) can interfere with the chemical process.
- Collection containers: Glass, plastic, or stainless steel. Lye reacts with aluminum, zinc, tin, and lead, producing hydrogen gas, which is flammable. Never use aluminum pots or containers.
The Leaching Process
Set your drilled barrel up on blocks or bricks so a collection container fits underneath. Line the bottom with a layer of gravel over the holes, then lay straw or small sticks on top of the gravel. This two-layer filter keeps ash from clogging the holes and sediment from clouding your lye.
Fill the rest of the barrel with hardwood ash, leaving a couple of inches of space at the top. Pack it down gently but don’t compress it into a solid mass. You want water to move through slowly, not pool on top.
Pour rainwater into the barrel. Add enough to saturate the ash thoroughly, but stop before the ash is swimming or floating. Then wait. The water needs to soak through the ash and begin dripping out the bottom, which can take four hours or longer. Many people let it sit overnight for a stronger first batch.
The liquid that drips out will be brown and slippery to the touch (that slippery feeling is the alkalinity breaking down oils on your skin, which is exactly why you should be wearing gloves). Collect this first runoff, then pour it back through the ash a second time. This double pass increases the concentration of potassium hydroxide in your solution.
After collecting the strong first batch, you can run additional water through the ash in smaller quantities, two to three pints at a time, collecting each batch separately. Each successive pass will be weaker. When the liquid coming out is no longer brown and no longer feels slippery, the ash is spent.
Testing the Strength
The traditional test is simple: drop a fresh egg into your lye water. If the egg floats so that a coin-sized area of shell breaks the surface, the concentration is about right for soap making. If the egg sinks, the lye is too weak. If it bobs on top and barely submerges at all, you have a very concentrated solution.
You can also use pH strips if you want a number. Properly leached wood ash lye typically lands around pH 13. Anything below pH 10 is too weak for soap and will need to be concentrated.
Concentrating Weak Lye
If your egg sinks, you have two options: pour the solution through a fresh batch of ash, or boil off excess water. Boiling is effective but requires caution. Use a stainless steel or iron pot outdoors or in a very well-ventilated area. The steam carries caustic particles, so standing over a boiling lye pot without ventilation is a bad idea.
The reduction ratio can be dramatic. Some home producers report reducing 20 gallons of weak lye water down to a single gallon of concentrated solution. At very high concentrations, potassium hydroxide crystals will actually begin to precipitate out and settle as fine sediment at the bottom of your container. At that point you have an extremely caustic product that needs to be handled with serious care.
Safety Essentials
Wood ash lye at pH 13 is a strong base that causes chemical burns on contact with skin and can permanently damage eyes. Treat it with the same respect you would give commercial drain cleaner.
Wear chemical-resistant gloves and long sleeves during every step of the process, including handling the ash itself (dry ash is irritating on its own). Wear splash-proof goggles, not just glasses. Have clean water nearby at all times. If lye contacts your skin, rinse immediately with large amounts of water. If it gets in your eyes, flush continuously for at least 15 minutes.
When boiling lye to concentrate it, work outdoors. The fumes irritate the lungs and mucous membranes. If you must work in an enclosed space, use breathing protection and ensure strong ventilation.
For storage, use glass jars with tight lids or heavy-duty plastic containers. Label them clearly. Keep lye away from acids (the reaction is violent and exothermic), away from food, and away from any container made of aluminum, zinc, or tin. The reaction between lye and these metals generates hydrogen gas, which is explosive.
Using Homemade Lye
The most common use for wood ash lye is soap making. When potassium hydroxide reacts with animal fat or plant oils, it produces a soft soap through a process called saponification. Homemade lye makes liquid or soft soap rather than hard bar soap, because potassium hydroxide (from wood ash) behaves differently than sodium hydroxide (the commercial lye used for solid bars).
The tricky part is that homemade lye varies in concentration from batch to batch, unlike commercial potassium hydroxide, which is standardized at around 90% purity. This means you can’t simply follow a recipe designed for store-bought lye and expect consistent results. You’ll need to test each batch with the egg float method or pH strips and adjust your fat-to-lye ratio accordingly. Starting with more lye than you think you need and testing small batches is the safest approach to avoid wasting large quantities of oil.
Beyond soap, wood ash lye has traditional uses in food preparation (nixtamalization of corn, curing olives), cleaning, and as a garden amendment. For any application where consistency matters, testing every batch is essential since no two leachings produce identical concentrations.