Liquid Soap
Part of Soap Making
Making potassium hydroxide based liquid soap — the easiest soap to produce from scratch using wood ash lye.
Why This Matters
Liquid soap is the most accessible soap you can make. If you can burn wood and leach ash, you have the alkali. If you have any fat — animal, vegetable, or even rendered kitchen grease — you have the other ingredient. The chemistry is identical to hard bar soap but the inputs are easier to obtain and the process forgives a wider range of lye concentrations.
For a community rebuilding from scratch, liquid soap should come before hard bars. It covers all critical hygiene applications: handwashing, dishwashing, laundry, wound cleaning, and general surface decontamination. A single large batch can supply a household for weeks. The paste can be stored concentrated and diluted as needed, extending shelf life and reducing storage volume.
Historically, soft soap made from wood ash lye was the dominant soap type everywhere wood was available. Hard bars were a later refinement requiring either industrial NaOH or more complex chemistry. Starting with soft soap is starting where every soap-making tradition started.
The Chemistry of KOH Soap
Wood ash leached in water produces potassium carbonate (K2CO3). When combined with slaked lime (Ca(OH)2), the potassium carbonate converts to potassium hydroxide (KOH) — the active alkali for soap-making.
The saponification reaction: KOH + fat acids → potassium soap (soft) + glycerin
The potassium soap molecule is identical to sodium soap except for the metal ion. Potassium soap is more soluble in water than sodium soap, which is why it remains soft or liquid rather than forming a solid bar.
KOH has a molecular weight of 56 g/mol vs NaOH at 40 g/mol. This means KOH soap requires approximately 40% more alkali by weight than equivalent NaOH soap — a critical calculation point when working with measured materials.
Saponification values for KOH (grams of pure KOH per 100g fat):
- Tallow: 19.6g KOH
- Lard: 19.4g KOH
- Olive oil: 18.8g KOH
- Coconut oil: 26.6g KOH
- Linseed oil: 18.9g KOH
These values assume 100% pure KOH. Commercial KOH is typically 90% pure; homemade wood ash lye varies and requires testing.
Producing Strong Lye from Wood Ash
The most common failure in liquid soap making is weak lye. Understrength lye produces “slime” — a greasy, non-saponified paste that smells rancid and does not clean. Lye strength determines everything.
Best ash sources for strong KOH:
- Hardwood ash (oak, hickory, beech) — highest potassium content
- Grapevine ash — traditionally prized in Mediterranean soap-making regions
- Bracken fern ash — very high potassium
- Avoid softwood (pine, fir) ash — lower potassium, higher resin content that discolors soap
Ash leaching procedure:
- Fill a wooden barrel, clay pot, or other container with clean hardwood ash. Do not pack tight — water must percolate through.
- Pour boiling water over the ash slowly — 2-3 liters per kilogram of ash. Allow to percolate through. Collect the liquid draining from the bottom.
- Pour the collected liquid back over the ash 2-3 more times to concentrate it.
- Add slaked lime (Ca(OH)2) to convert K2CO3 → KOH: approximately 100g slaked lime per liter of ash water. Stir, let settle 30 minutes, decant clear liquid.
- Test lye strength:
- Egg/potato float test: A raw egg or small potato should float with approximately a 25mm (1 inch) diameter spot above the surface. If it sinks, lye is too weak — concentrate by boiling. If the egg floats very high (more than 50mm), dilute with water.
- Feather test: Dip a feather in the lye. Strong lye dissolves the barbs quickly, leaving just the quill within 30-60 seconds.
- pH paper: Target pH 13-14 for soap-making strength lye.
Concentrating weak lye: Boil gently in an open pot (outdoors or in very good ventilation — the steam is caustic). As water evaporates, concentration rises. Do not boil dry — this destroys some alkali content and makes the lye difficult to work with. Monitor the egg float test as you concentrate.
Core Formula for Liquid Soap
This formula uses oils that are readily available and produces a gentle, all-purpose liquid soap. Adjust based on available fats.
Batch size: ~1 liter of finished liquid soap
Soap paste ingredients:
- 300g olive oil (or any vegetable oil — linseed, sunflower, hemp work well)
- 200g coconut oil (optional but improves lather; replace with more olive oil if unavailable)
- 100g KOH (pure solid) dissolved in 175ml water — OR — 350ml of tested full-strength lye solution
- If using 90% KOH, use 112g instead
Dilution water (added after paste is made):
- 400-600ml water (more for thinner soap, less for thicker)
Fat ratio rationale: Heavy olive oil (soft oil) produces a gentle, slow-lathering liquid soap suited for skin contact. Coconut oil (hard oil) speeds up lather and boosts cleaning power. For laundry or dishwashing soap, increase coconut oil to 40-50% of the fat blend. For pure skin soap, 100% olive oil works but produces very slow lather.
Step-by-Step Production: Hot Process
Hot process is strongly recommended for liquid soap. Cold process liquid soap is prone to false trace and unreliable saponification; hot process cooks it to completion and allows you to verify it is done.
Equipment needed:
- Large stainless steel or enamel pot (not aluminum — NaOH and KOH react with aluminum)
- Wooden or stainless steel spoon for stirring
- Smaller container for dissolving lye
- Immersion blender if available (speeds up trace dramatically)
- Scale for measuring (or calibrated volume measures if no scale)
- Gloves and eye protection — mandatory
Procedure:
Step 1 — Dissolve the lye: Add KOH to water (never water to KOH — adding water to concentrated alkali causes violent spattering). The solution heats dramatically — this is normal. Stir until fully dissolved. Allow to cool to 60-70°C (140-160°F) before use, or use immediately if working with pre-made lye solution.
Step 2 — Melt the fats: If using coconut oil or tallow, melt gently in the pot. Liquid oils need no treatment. Target fat temperature: 60-70°C (140-160°F) — same range as lye solution.
Step 3 — Combine: Pour the lye solution into the fats slowly, stirring constantly. Do not pour fats into lye — the soap pot should contain fats first, lye added to it.
Step 4 — Blend to trace: Stir vigorously by hand or use an immersion blender in short bursts (5-10 seconds on, stir by hand between pulses). The mixture will lighten in color and thicken. “Trace” for liquid soap is lighter than for bar soap — a thin applesauce consistency where the surface holds a drizzle for 2-3 seconds before sinking. Do not over-blend — liquid soap can seize into a thick, unworkable mass if pushed past light trace.
Step 5 — Cook: Place the pot on low heat (double boiler or very low direct heat — target 70-80°C / 160-175°F). Cover with a lid. Cook for 2-3 hours, stirring every 20-30 minutes. The soap will pass through stages:
- Applesauce stage (30-60 min): separated, curdled appearance — normal
- Tapioca stage (60-90 min): translucent lumps in liquid — keep cooking
- Vaseline/petroleum jelly stage (90-150 min): semi-translucent, homogeneous paste — almost done
- Finished paste (150-180 min): fully translucent, thick, glossy — like petroleum jelly. Sticks to the back of a spoon and holds shape.
Step 6 — Zap test: Touch a tiny amount of paste (barely a fingertip smear) to the tip of your tongue. A finished batch tastes soapy — like soap, not caustic. Raw lye causes an immediate sharp “zap” sensation, like touching a 9V battery to your tongue. If you get the zap, cook 30 more minutes and test again.
Step 7 — Rest: Turn off heat. Allow paste to cool undisturbed. The paste can be stored in this concentrated form indefinitely (months to years in a sealed container). It will harden somewhat as it cools — this is normal.
Diluting the Paste to Liquid Soap
The soap paste must be diluted with water before use. Dilution ratio controls final viscosity and strength.
Dilution procedure:
- Weigh or measure your soap paste — 100g paste is a useful working unit
- Heat distilled or clean water to 60-70°C (boiling then cooled slightly is fine)
- Add hot water to paste in the ratio you need:
- Thin handwashing soap: 1 part paste : 3-4 parts water (by weight)
- Medium all-purpose soap: 1 part paste : 2 parts water
- Thick dish soap / laundry concentrate: 1 part paste : 1 part water
- Stir until paste fully dissolves. This may take 20-30 minutes for a cold paste. Gentle heat speeds dissolution.
- Allow to cool before assessing final consistency — liquid soap thickens as it cools.
Cloudiness: Freshly diluted liquid soap is often cloudy. This clears on its own over 24-48 hours as the soap molecules fully hydrate. Filtering through cloth removes remaining particulates.
Fragrance: Add essential oils after dilution when the soap has cooled below 40°C (104°F). Essential oils evaporate above this temperature. Typical usage: 1-2% by total soap weight (5-10ml per liter of finished soap).
Thickening Liquid Soap
Plain diluted KOH soap is often thinner than expected — more like water with some viscosity than the thick gel of commercial liquid soap. Several methods increase thickness:
Salt thickening (most accessible): Dissolve table salt (NaCl) in a small amount of water to make a saturated brine. Add to finished diluted soap slowly, a teaspoon at a time, stirring after each addition. Liquid soap thickens dramatically with salt — but only up to a point, then it suddenly thins again if you add too much. Add salt in very small increments and stop when desired thickness is reached.
Typical effective range: 1-3% salt by total soap weight. Start with 1% and add more carefully.
Glycerin addition: Commercial glycerin thickens and enriches the soap while adding a conditioning quality. Add 1-3% by weight. Source from soap-making suppliers or recover from soap production waste water.
Gum thickeners: Xanthan gum, guar gum, or locust bean gum thicken liquid soap effectively. Mix the dry gum with a small amount of glycerin before adding to soap (prevents clumping). Use 0.2-0.5% by total weight.
Increase soap paste concentration: Simply dilute to a 1:1 or 1.5:1 paste-to-water ratio rather than 1:3. The soap itself provides viscosity.
Uses and Applications
Handwashing: Dilute to medium consistency (1:2-3 paste:water). Dispense from a repurposed bottle or clay pot with a narrow pour spout. Even thin liquid soap is dramatically more effective at disease prevention than plain water.
Laundry: Concentrated paste (1:1 dilution) or even direct paste addition to wash water. For a full laundry tub: 50-100ml of medium soap per 5 liters of wash water. Agitate fabric for 5-10 minutes, then rinse thoroughly. KOH soap rinses cleanly and does not leave residue if used correctly.
Dishes: Medium to concentrated soap works well for dishwashing. The high glycerin content in homemade soap (not removed as it is in commercial soap) leaves hands feeling less raw after washing up.
Wound cleaning: Diluted soap (1:4 or 1:5 paste:water) for wound irrigation. The soap removes surface bacteria and debris. Rinse thoroughly after washing — do not leave soap in contact with open wounds. Medicated versions (with calendula infusion in the fat) provide additional benefit.
General surface cleaning: Concentrated soap for floors, tables, tools. Dilute 1:10 for general mopping and surface wipes.
Hair washing: KOH soap with high olive oil content works as a shampoo. The alkalinity may leave hair feeling rough initially — rinse with dilute vinegar (1 tablespoon per cup of water) as a conditioning rinse to lower pH and close the hair cuticle.
Storage and Shelf Life
Soap paste (undiluted): Stores 1-2 years or longer in a sealed non-reactive container. Clay jars, glass, food-grade plastic. Keep cool and dark. High salt or salt-water environments promote rancidity in the free glycerin content — less of an issue in dry storage.
Diluted liquid soap: 2-6 months in a sealed container. Water activity supports microbial growth over time; the alkalinity of the soap inhibits most pathogens but yeasts and molds can colonize at pH below 9. Keep containers clean between refills. Adding a small amount of preservative (tea tree essential oil at 0.5%, or rosemary extract) extends shelf life.
Signs of spoilage: Rancid smell (not just “soap smell” but distinctly unpleasant, oily-rotten), significant color change to brown/orange (DOS — dreaded orange spots indicate fat oxidation), or separation that does not re-emulsify on stirring. Spoiled soap is not harmful to use but is less effective and unpleasant.
Common Mistakes
- Adding water to KOH instead of KOH to water — causes violent spattering of caustic solution
- Using weak lye from a single pass through ash — test before use, concentrate if needed
- Stopping the cook at the applesauce stage and assuming the soap is done — continue to the glossy paste stage
- Diluting hot paste with cold water — causes shocking and clumping; use hot water
- Adding too much salt at once when thickening — you can overshoot the sweet spot and thin the soap irreversibly
- Storing in aluminum or zinc containers — KOH reacts with these metals, producing hydrogen gas and contaminating the soap
- Not stirring enough during the cook — heat stratification at the bottom denatures the soap; keep it moving
- Expecting commercial-grade foam — homemade KOH soap from soft oils has modest lather; it still cleans effectively
Key Takeaways
- Liquid soap is made from KOH (potassium hydroxide) — the natural product of wood ash lye leaching
- Hot process cooking to the glossy paste stage ensures complete saponification — the zap test confirms no raw lye remains
- The paste is a concentrate: dilute 1:2 to 1:4 with hot water for ready-to-use soap
- Thicken with salt (1-3% NaCl) in small increments; over-salting thins the soap again
- Store paste for up to 2 years; diluted soap for 2-6 months in sealed containers
- Covers all core cleaning needs: handwashing, laundry, dishes, wound care, general cleaning
- Soft oils (olive, linseed) for skin soap; hard oils (coconut) increase lather and cleaning power for laundry/dishes