For most people, “lye” means one thing: a white, factory-made chemical in a bottle or bag.
But historically, lye was not something you bought. It was something you made.
Before modern chemical suppliers, soap makers created alkaline solutions from wood ash, water, and pickling lime. This was not superstition. It was practical chemistry. The ash contained potassium salts. Water pulled those salts into solution. Pickling lime converted that carbonate solution into potassium hydroxide that could react with oils and fats to make soap.
That is the foundation behind traditional ash lye soap.
At Real Lye Co., I make soap using handcrafted lye because I want to preserve the original system of soap making: ash, alkali, oil, heat, time, and chemistry.
This article explains how potassium hydroxide, or KOH, can be made from wood ash, what is actually happening chemically, and why this process is more complicated than simply mixing ash and water.
First: What Is KOH?
KOH stands for potassium hydroxide.
It is a strong alkali used in soap making, especially for soft soap, paste soap, and liquid soap. Unlike sodium hydroxide, which usually makes harder bar soap, potassium hydroxide tends to produce softer, more soluble soaps.
Both sodium hydroxide and potassium hydroxide are commonly called “lye,” but they are not the same chemical.
Sodium hydroxide = NaOH
Potassium hydroxide = KOH
Both are highly caustic and can burn skin and eyes. Potassium hydroxide is officially classified as corrosive and can cause severe burns and eye damage, so this process requires chemical-resistant gloves, eye protection, ventilation, and serious respect.
The Big Misconception: Ash and Water Alone Do Not Usually Make True KOH
A lot of people think wood ash and water instantly make potassium hydroxide.
That is only partly true, and it is where most online explanations get sloppy.
When I boil hardwood ash in water, I am not magically creating pure KOH right away. I am extracting the water-soluble potassium salts from the ash. The most important one is usually potassium carbonate, not pure potassium hydroxide.
Boiling helps this extraction because potassium carbonate becomes more soluble as water gets hotter. In other words, hot water can pull more of those alkaline potassium salts out of the ash faster than cold water alone.
That boiled ash water can be strongly alkaline. It can feel slippery. It can test very high on pH.
But high pH does not automatically mean you have potassium hydroxide.
If the goal is to make a stronger lye, then boiling ash water is only the first major step. To push potassium carbonate toward potassium hydroxide, you need calcium hydroxide, also known as slaked lime or pickling lime.
This second step is called causticization.
A simple way to say it is:
Boiling ash in water extracts potassium carbonate. Adding pickling lime converts that carbonate into potassium hydroxide.
That distinction matters because it separates real traditional chemistry from oversimplified internet folklore.
The Basic Chemistry
The simplified reaction looks like this:
Potassium carbonate + calcium hydroxide → potassium hydroxide + calcium carbonate
Or chemically:
K₂CO₃ + Ca(OH)₂ → 2KOH + CaCO₃
In plain English:
You start with potassium carbonate dissolved from wood ash.
You add calcium hydroxide (Pickling Lime).
The calcium grabs the carbonate.
That forms calcium carbonate, which falls out as raw chalk.
The potassium stays in the water as potassium hydroxide.
That chalky sludge at the bottom is not waste in the chemical sense. It is evidence that the reaction happened.
This is the old-world refinement step that turns crude ash water into a stronger hydroxide-type lye solution. Soap makers were using versions of this lime-refined ash lye process centuries ago.
Materials Used in the Traditional Process
The basic materials:
Ash
Water
Calcium hydroxide / pickling lime
A non-aluminum container
A filter setup
A way to test strength
Protective gloves and eye protection
Hardwood ash has been historically preferred because it tends to produce stronger ash than other plants, and it was readily available for our ancestors that lived inland. The exact strength depends on the type of plant, how completely it burned, and how much soluble potassium remains.
Not all ash is equal. That is why testing matters.
Safety Warning Before You Start
This process creates a caustic alkaline solution. It can burn skin, damage eyes, and react with certain metals. Potassium hydroxide solutions are corrosive, and chemical safety references warn against skin and eye contact.
Do not use aluminum. Strong alkalis can attack aluminum and some other metals. NOAA’s chemical database notes that potassium hydroxide readily forms caustic solution and attacks aluminum and zinc.
Use:
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Chemical-resistant gloves
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Safety goggles
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Long sleeves
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Good ventilation
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Plastic, stainless steel, glass, or appropriate chemical-safe containers
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A vinegar rinse nearby only for surfaces, not as a substitute for proper first aid
- Clean running water available in case of skin contact
For skin or eye exposure, water flushing is the priority. Do not only rely on “neutralizing” your skin with vinegar.
Step 1: Make Clean Wood Ash
Start with clean hardwood ash.
Do not use ash from:
Painted wood
Pressure-treated wood
Plywood
Trash fires
Glossy paper
Charcoal briquettes with additives
Unknown construction scraps
You want clean, fully burned wood ash. The whiter and finer the ash, the better. Black charcoal pieces are mostly unburned carbon and do not contribute much soluble alkali.
Sift the ash to remove nails, charcoal chunks, bark pieces, and debris.
Step 2: Leach the Ash with Water
Instead of only pouring cold water through the ash, I personally boil the ash in water.
The reason is simple chemistry: the valuable alkaline material in wood ash is mostly water-soluble potassium salts, especially potassium carbonate. Like many salts, potassium carbonate becomes more soluble as the water gets hotter. That means hot water can pull more of the soluble alkali out of the ash faster than cold water alone.
This boiling step does not automatically make pure potassium hydroxide. At this stage, the liquid is still better understood as a strong alkaline ash-water extract, rich in potassium carbonate and other soluble minerals.
After boiling, I let the ash settle and then filter the liquid to remove the solids. The filtered liquid is the ash water we will be concentrating in the next step.
Step 3: Concentrate the Ash Water
Once you have filtered ash water, you can gently boil or evaporate it down to concentrate the potassium salts.
This step makes the solution stronger by removing water.
But there is a balance. Boiling too aggressively can make a messy, mineral-heavy solution, and leaving alkaline solutions exposed to air for long periods can allow them to absorb carbon dioxide. Potassium hydroxide also reacts with CO₂ from air, which slowly converts some hydroxide back toward carbonate.
For traditional soap making, this does not automatically ruin the solution, but it does matter if you are trying to calculate strength accurately.
Step 4: Add Pickling Lime to Convert Carbonate into Hydroxide
This is the step many people skip or do not understand.
To make KOH from ash water, you add calcium hydroxide.
Pickling lime is calcium hydroxide. Historically, people could make lime by heating limestone or shells to make quicklime, then carefully slaking it with water to make calcium hydroxide.
When calcium hydroxide is added to potassium carbonate solution, it reacts and forms calcium carbonate, which appears as a white chalky solid. The potassium hydroxide remains dissolved in the water.
This step is what turns the solution from mostly carbonate-style ash lye into a more hydroxide-style lye.
The reaction is not always perfect. Real ash water contains impurities. Pickling lime may vary in purity. Mixing, temperature, concentration, and settling time all affect the result.
That is why I do not treat handmade lye like a fixed chemical from a supplier. I test it.
Step 5: Let the Chalk Settle
After adding calcium hydroxide, the mixture should be stirred well and then allowed to settle.
The calcium carbonate sludge will sink to the bottom. The clearer liquid above it contains the potassium hydroxide-rich solution.
This liquid is the part used for soap making.
The sludge should be separated carefully by decanting, filtering, or both. The goal is to remove as much chalk and ash sediment as possible without losing the lye solution.
At this stage, the solution is caustic.
Do not touch it bare-handed.
Step 6: Test the Strength
This is the difference between romantic old-world soap making and responsible old-world soap making.
Handmade lye must be tested.
There are a few ways to estimate strength.
The Egg Test
Historically, people floated an egg or potato in the lye solution. The stronger the solution, the more the object floated.
This worked as a density test, not a magical pH test. Dissolved salts make water denser. When the solution became dense enough, the egg floated higher.
The egg test can show whether the solution is concentrated, but it does not tell you the exact KOH percentage.
Titration
A more accurate method is titration.
In simple terms, titration uses a known acid, such as vinegar of known acidity, to measure how much alkali is in your lye solution.
This lets you estimate the strength of the solution and calculate how much is needed for a soap recipe.
For handmade lye soap, titration is one of the best ways to bridge traditional craft and modern chemistry.
Weight-Based Soap Testing
Another practical method is to make small controlled test batches with a known oil, such as coconut oil, and observe whether the final soap is lye-heavy, oil-heavy, or balanced.
This should not replace titration, but it can help confirm whether the lye behaves as expected.
Why Handmade KOH Is Not the Same as Store-Bought KOH
Commercial potassium hydroxide is standardized. If you buy 90% KOH flakes from a supplier, you can plug that purity into a soap calculator.
Handmade KOH is different.
It may contain:
Potassium hydroxide
Potassium carbonate
Sodium hydroxide in small amounts, depending on ash source
Other dissolved minerals
Water
Trace impurities
That does not make it fake. It makes it traditional.
But it also means you cannot blindly use a commercial soap calculator unless you know the strength of your solution.
This is the part that matters most: handmade lye is real lye, but it is not automatically standardized lye.
The craft is not just “making lye.”
The craft is making it, testing it, correcting it, and then building a soap recipe around the actual strength of the batch.
Can You Use Wood Ash KOH for Bar Soap?
Potassium hydroxide usually makes softer soap than sodium hydroxide. That is why KOH is commonly used for liquid soap, paste soap, and soft soap.
Traditional ash lye soap was often softer than modern bar soap because the lye was potassium-heavy.
For hard bars, sodium hydroxide is usually preferred. But potassium-based lye can still be part of traditional soap making, especially if the goal is a rustic soft soap, liquid soap, or heritage-style cleanser.
At Real Lye Co., the purpose is not to imitate factory soap. The purpose is to preserve the older system: making the alkali from raw materials, then using that alkali to transform oils into soap.
Why This Process Matters
Making KOH from wood ash is more than a chemistry trick.
It is a link to an older way of manufacturing, before every ingredient was hidden behind a supply chain. It shows that soap was once a local product made from fire, water, ash, lime, oil, and patience.
It also exposes a modern misconception.
A lot of products are marketed as “natural,” “traditional,” or “handmade,” but they still begin with factory-made lye. That does not make them bad products. All real soap requires alkali. But there is a difference between using purchased lye and making the lye yourself.
Handmade lye is not easier.
It is not more convenient.
It is not more predictable.
That is exactly why it matters.
It brings the soap maker back into the chemistry instead of outsourcing the most important reaction.
The Simple Version
Here is the process in plain terms:
Burn clean hardwood into ash.
Sift the ash.
Leach the ash with water.
Filter the alkaline ash water.
Concentrate the liquid if needed.
Add calcium hydroxide to convert potassium carbonate into potassium hydroxide.
Let the calcium carbonate sludge settle.
Separate the clear caustic liquid.
Test the strength.
Use the measured lye solution to make soap.
That is the foundation of handmade KOH from wood ash.
Making potassium hydroxide from wood ash is not just “ash plus water.” That is the beginning, not the whole story.
The real process is ash extraction, lime conversion, settling, filtering, testing, and soap formulation.
It is chemistry that our ancestors understood through practice long before they had modern chemical language. Today, we can honor that tradition while also using modern testing and safety standards.
That is what Real Lye Co. is about: not pretending the old way was simple, but proving that it was real.
Fire becomes ash.
Ash becomes alkali.
Alkali meets oil.
And with enough care, that becomes soap.
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