Making soap with potash

The first jar of soft soap I ever made looked like cloudy apple juice and smelled faintly of a chemistry lab. I nearly poured it away. Three days later it had settled into something clear and golden, and I’ve been making potash soap ever since — it’s the oldest kind there is, and the most misunderstood.

Inside My Potash Soap Practice

My education began with a terminology mistake that cost me a whole batch. I read that potash makes soap, bought a bag labeled potassium hydroxide, ran my recipe through a calculator set to 100% purity, and ended up with a greasy, separating mess that never went clear.

The bag was 90% pure. Nobody had told me that mattered. Around 900 grams of oils, gone — and a lesson I’d now put in bold letters on the front of every beginner’s notebook.

Commercial KOH is typically only 85–95% pure — assume 90% unless your supplier says otherwise, and enter that into your calculator. If you calculate as though it were 100% pure, you build a hidden 10% superfat into the recipe. That much excess fat will not saponify, will not dissolve, and will separate out after dilution.

What Potash Soap Really Is

Here’s the thing that trips almost everyone up. Potash and caustic potash are not the same substance, and the words get used interchangeably by people who should know better.

True potash is potassium carbonate — the residue you get from leaching wood ashes in a pot, which is exactly where the name comes from. It’s mostly potassium carbonate with some sodium carbonate, calcium salts, and whatever else the tree happened to contain.

Caustic potash is potassium hydroxide, or KOH. Historically it was made by treating that ash solution with slaked lime, which converts the carbonate into the far stronger hydroxide. This is what you buy in a bag today.

Wood-ash lye will make soap. What it will not make is a hard bar. Soap made with a potassium alkali is a soft, soluble, paste-like soap — and every soapmaking manual from the 1700s and 1800s says so plainly. The internet tutorials promising rustic hard bars from ashes are contradicting three centuries of documented practice.

Why does the alkali determine the texture? Because sodium builds tight, rigid molecular structures and potassium builds loose, soluble ones. Sodium soaps set hard. Potassium soaps stay soft and dissolve readily in water.

Ever wonder why liquid soap and soft soap came first historically? Because potash was what people had. Hard bar soap only became the norm once industrial-scale sodium hydroxide production arrived in the 18th and 19th centuries. Liquid soap is the older technology.

So why don’t more people make their own ash lye? Because you cannot measure its strength. A batch leached from oak differs from one leached from pine, and neither can be run through a recipe calculator. Traditional soapers dealt with this by boiling and adjusting by feel — an art, not a recipe. I’ve done it once, for the experience, and I’d never build a formulation on it.

Who’s this for? Someone comfortable with cold process who wants to go further. The chemistry is the same, but the process is hot, long, and considerably fussier. This is not a first project.

Materials and Tools You’ll Actually Need

Never use aluminum. KOH attacks it and releases hydrogen gas, and this applies to your cooking pot as much as your mixing jug. A non-aluminum heavy-bottomed pot or a ceramic slow cooker insert is what you want.

Key Techniques and Skills

• Correcting for KOH purity — if a calculator asks for 90 g of pure KOH and yours is 90%, weigh out 100 g.
• Keeping superfat low. Zero to 3% for liquid soap; anything higher clouds the finished product and encourages separation.
• Choosing a lye concentration around 25% (roughly 3 parts water to 1 part KOH), which keeps the paste soft and easy to dilute.
• Dissolving KOH into cool distilled water, never the reverse, and standing back from the fumes.
• Heating oils to around 140–158°F (60–70°C) — hotter than cold process wants.
• Blending to trace, which with KOH arrives in two to five minutes rather than the leisurely wait of cold process.
• Cooking the soap paste for 2 to 3 hours in a slow cooker on low, stirring every half hour, until it turns translucent.
• Running the clarity test: dissolve a spoonful of paste in a few ounces of boiling distilled water and look at it. Clear means done; cloudy means more cooking or a formulation problem.
• Zap-testing the paste for free alkali before you dilute.
• Diluting with near-boiling distilled water, stirring gently to avoid foam, then covering and leaving it overnight.
• Adjusting consistency afterward — hot water in 2 to 4 ounce increments if too thick; lid off to evaporate if too thin.
• Resting the diluted soap 24 to 72 hours before judging its final clarity.

Be patient with dilution. This is where beginners give up. Pour hot water over the paste, cover it, and walk away for eight to twelve hours. Stir once or twice. Break stubborn chunks with a potato masher if you must. It looks hopeless at hour two and perfectly fine by morning.

Skill Level and Time Investment

Advantages and Challenges

Potash soap gives you something a bar simply cannot: a soap you can pour, pump, foam, dilute, and adjust to any strength you like. That flexibility is the whole appeal.

• One batch of paste yields hand soap, body wash, shampoo, and dish soap — just dilute differently.
• The paste stores beautifully. I’ve kept it a year in an airtight tub and diluted it as needed.
• No cure. Once the paste is cooked and diluted, the soap is finished.
• Complete formulation control, which no store-bought liquid soap offers.
• Historically genuine — this is how soap was made for most of its existence.
• Excellent for shave soap, where the potassium softness is a virtue rather than a defect.
• Diluting is a lever: thicker for dish soap, thinner for a foaming pump.

The frustrations are real. Clarity is a moving target and cloudiness has half a dozen possible causes. Liquid soap has an inherent slightly chemical smell that some people can never stop noticing. And the dilution stage is slow enough that you will convince yourself, more than once, that the batch has failed when it simply hasn’t finished dissolving.

• KOH purity corrections trip up nearly every beginner, and the error only shows up hours later.
• The process takes a full evening plus an overnight rest, versus an hour for a cold process loaf.
• You cannot make a proper hard bar with potassium alkali. Trying is fighting chemistry.
• Diluted soap contains a lot of water and benefits from a preservative, which purists resist.
• Wood-ash lye cannot be measured, which makes truly traditional potash soap an experiment rather than a recipe.

Real Applications and Working Numbers

A workable starting formulation: 50% coconut oil, 30% olive oil, 15% sunflower, 5% castor, at a 2% superfat and a 25% lye concentration. Roughly, that means KOH at around 17% of your oil weight and water at around 34%.

For 1000 g of oils that’s about 170 g of KOH by the calculator — but if your KOH is 90% pure, you actually weigh out closer to 190 g. That correction is the single most important number in the whole recipe.

The paste is the real prize, and it’s why I make this at all. One cook produces a tub I can draw on for months. A tablespoon in a foaming pump with distilled water gives hand soap. A heavier dilution gives a body wash. Thicker still, and it’s dish soap. Three products, one evening’s work, no cure.

Cook the paste until it turns from opaque and mashed-potato-like into something translucent and amber, roughly two to three hours in a slow cooker on low, stirring every half hour. Then test: a spoonful into a few ounces of boiling distilled water.

Clear? You’re done. Cloudy with no zap? You have too much superfat — the free oil is scattering the light. Cloudy and zappy? Saponification isn’t complete; keep cooking.

Cloudiness has more causes than people expect. Excess superfat is the usual culprit, but jojoba clouds soap reliably, and so do high-stearic fats like butters, lard, palm, and tallow. So does non-distilled water. If your paste is stubbornly cloudy, look at the oil list before you blame the cook.

For dilution, remember that your paste weighs roughly the sum of oils plus KOH plus initial water, minus a little evaporation. Add near-boiling distilled water, stir gently — vigorous stirring just makes foam — cover, and leave it overnight.

Lisa Mandel

Then wait again. Diluted soap wants 24 to 72 hours of rest before it settles into its final clarity. My first batch looked like a failure at hour six and was crystal clear by the third day.

Beyond hand soap, potash soap has a long working history: shave soap, shampoo, and agricultural insecticidal soaps all rely on the softness of potassium salts. If you want something closer to a firm bar, you can push it with soy wax and a hard-oil-heavy formulation — but you’re working against the chemistry, and it will never match a sodium bar.

The Learning Experience

Beginners fail in a small, predictable set of ways. Wrong purity assumption. Superfat set too high out of cold-process habit. Impatience at the dilution stage. And cloudy oils chosen without knowing they’d cloud.

My breakthrough was accepting that liquid soap wants a low superfat. In cold process I’d been trained to think of 5% as generous protection. In liquid soap, 5% is a defect — that free oil has nowhere to go, and it shows up as haze and eventually as a floating layer.

Everything you learned about being generous with your oils in bar soap, you have to unlearn here. Liquid soap wants the lye and the fat to meet almost exactly. Two percent superfat. Sometimes zero. That’s the whole secret to clarity.

Test small. My first liquid soap experiments used 500 g of oils, which is about the minimum that blends properly. Small enough to lose without grief; big enough to behave like a real batch.

The smell surprises people. Liquid soap carries a faint, slightly chemical, unmistakably soapy odor that has nothing to do with anything going wrong. Most of us just fragrance it. It’s a known quirk, not a fault.

If you’re drawn to the historical version — actual wood-ash lye — do it once, properly, and enjoy it. Leach hardwood ash, boil it down, and make a soft soap by feel the way our great-grandmothers did. It is genuinely fascinating. Just don’t expect a repeatable recipe, and don’t expect a bar.

Comparison with Related Approaches

Common Questions from Fellow Crafters

Q: Is potash the same as lye?

A: Not quite. Potash is potassium carbonate from wood ash. Caustic potash is potassium hydroxide, which is what you actually buy and cook with. Both get called “lye,” which causes endless confusion.

Q: Can I make hard bars with KOH?

A: Not properly. Potassium soaps are inherently soft and soluble. Firming them up with soy wax or hard oils is possible but it’s a workaround, not a solution. Use sodium hydroxide for bars.

Q: Why is my paste cloudy after cooking?

A: Too much superfat, incomplete saponification, cloudy oils like jojoba or heavy butters, or non-distilled water. Zap-test first: cloudy with no zap points to excess fat; cloudy and zappy means keep cooking.

Q: What KOH purity should I use in the calculator?

A: Assume 90% unless your supplier states otherwise. If a calculator only offers 100% or 90% options, pick 90% — it gets you close enough.

Q: How long does the dilution take?

A: Overnight, realistically. Add near-boiling water, cover, stir once or twice, and let it dissolve for eight to twelve hours. Rushing it is how batches get abandoned.

Q: Does liquid soap need a preservative?

A: It’s water-rich, so yes, I’d use one. Unpreserved and airtight, expect six to twelve months; preserved, twelve to eighteen.

Q: Can I really make soap from wood ashes?

A: Yes, and it will be a soft soap, not a bar. The strength can’t be measured, so it’s a historical exercise rather than a repeatable formulation. Rewarding to try once.

Q: Why does my liquid soap smell odd?

A: That faint chemical-soapy note is normal for liquid soap and not a sign of failure. Most soapers simply add a fragrance.

My Personal Results and Insights

Final Thoughts and My Recommendation

I recommend potash soap wholeheartedly, but not as anyone’s first project. Learn cold process first. Get comfortable with an alkali, a scale, and a calculator. Then come here, where the same chemistry runs hotter and longer and demands more precision.

For the beginner ready to try it: 500 g of oils, coconut-heavy, 2% superfat, a 25% lye concentration, and KOH entered at 90% purity. Cook it, test it, dilute it overnight, and let it rest three days before you judge it. That single batch teaches more than any amount of reading.

Do not chase a hard bar with potassium hydroxide. Do not skip the purity correction. And do not throw away a cloudy jar on day one — it very often clears on its own.

Is it worth the extra hours? For me, unquestionably. One tub of paste sits in my cupboard and becomes whatever I need that week, with no cure, no waiting, no bars to stack. That flexibility has changed how I think about soap entirely.

And there’s something else, harder to justify but real. When I stir a pot of amber paste that started as ashes and oil, I’m doing exactly what people were doing four hundred years ago, in the same order, for the same reason. Bars are the modern innovation. This is the old craft. It still smells faintly strange and it still works, and I find that quietly wonderful.