The Art and Science of Soap Making
From wood-ash lye to formulating your own bar
A Brief History
Soap is one of the oldest manufactured chemicals. The earliest direct evidence comes from Babylonian clay tablets dated to around 2800 BCE, describing a mixture of animal fat and wood ash. An Egyptian papyrus from about 1550 BCE (the Ebers Papyrus) records mixing animal and vegetable fats with alkaline salts to make a soap-like substance used both for cleaning and treating skin conditions. Pliny the Elder, writing in the first century CE, described two varieties of soap made by Germanic tribes from tallow and ash — a hard variety (using soda ash from coastal plants) and a soft one (using potash from inland wood ash).
For most of human history, soap making was household work. Every spring, wood ash accumulated over winter was leached with water to extract lye. The resulting liquid was boiled with rendered animal fat — mostly tallow from cattle or sheep — and the result, after days of cooking and testing, was soft potash soap, a semi-liquid brown paste stored in crocks. The chemistry was unknown; what mattered was hard-won empirical knowledge: that the lye should be strong enough to float an egg; that the fat should be clean and free of water; that undercooked soap would separate; that overcooked soap would be harsh.
Hard bar soap — the kind that could be stored, traded, and shipped — required soda instead of potash as the alkali. Soda (sodium carbonate, Na₂CO₃) occurred naturally in dry lakebeds and was also obtained by burning coastal plants such as glasswort (Salsola). Spanish barilla ash, rich in sodium salts from these plants, was traded across Europe. Marseille (France) and Castile (Spain) became centers of quality olive oil soap production as early as the 13th century. Aleppo soap, made from olive oil and laurel berry oil, is one of the oldest soap types still in continuous production.
The chemistry of saponification was established by the French chemist Michel Eugène Chevreul between 1813 and 1823. He showed that fats are esters of glycerol and fatty acids, and that the alkali cleaves these bonds rather than simply mixing with the fat. This was foundational work in organic chemistry. Industrial soap production was transformed shortly after by the Leblanc process (1791) and later the Solvay process (1861), which made soda ash and sodium hydroxide cheap commodities for the first time. Soap went from a luxury to an everyday item within decades.
The Chemistry
Saponification
A fat or oil is a triglyceride — one glycerol molecule esterified with three fatty acids. Sodium hydroxide (NaOH) or potassium hydroxide (KOH) in water cleaves all three ester bonds simultaneously:
\[\underbrace{(RCOO)_3C_3H_5}_{\text{triglyceride}} + 3\,\text{NaOH} \;\rightarrow\; \underbrace{3\,RCOONa}_{\text{soap}} + \underbrace{C_3H_5(OH)_3}_{\text{glycerol}}\]
The products are soap (sodium or potassium salts of fatty acids) and glycerol. Commercial manufacturers remove and sell the glycerol separately, which is why commercial bar soap tends to be drying. Handmade soap retains all its glycerol, which stays in the bar and makes it noticeably more moisturizing.
Fatty Acids and Soap Properties
The \(R\) in \(RCOO^-\) is the fatty acid chain — a hydrocarbon of 8 to 22 carbons. The chain length, saturation, and any functional groups on it determine almost everything about how the soap behaves. This is why oil selection is the central skill of soap making.
| Fatty acid | Carbons | Found in | Effect on soap |
|---|---|---|---|
| Caprylic / capric (C8–C10) | 8–10 | Coconut, palm kernel | Very soluble, quick lather; bars feel dry; rarely used above 5% |
| Lauric (C12) | 12 | Coconut (~47%), palm kernel | Hard bar; fluffy, abundant lather; strongly cleansing — can be drying |
| Myristic (C14) | 14 | Coconut (~18%), palm kernel, butter | Hard bar; bubbly lather; cleansing |
| Palmitic (C16) | 16 | Palm (~44%), lard, tallow, cocoa butter | Hard, long-lasting bar; stable creamy lather |
| Stearic (C18:0) | 18 | Tallow (~21%), lard, cocoa butter, shea | Very hard bar; dense, stable lather |
| Oleic (C18:1) | 18 | Olive (~75%), lard (~46%), almond | Conditioning; soft, creamy lather; slow to trace |
| Linoleic (C18:2) | 18 | Sunflower (~65%), hemp, safflower | Skin-conditioning; soft bar; shortens shelf life |
| Ricinoleic (C18:1-OH) | 18 | Castor oil (~90%) | Powerfully boosts lather and bubbles; conditioning; used at 5–10% |
Shorter, saturated chains (lauric, myristic) make hard, lathery, cleansing soaps. Longer, unsaturated chains (oleic, linoleic) make conditioning, gentle soaps that lather less dramatically. Most good soap recipes balance these two tendencies.
NaOH vs KOH
Both alkalis saponify fats, but produce different products:
- Sodium hydroxide (NaOH) → sodium soaps. Sodium fatty acid salts pack tightly into a crystal lattice at room temperature → hard bar soap.
- Potassium hydroxide (KOH) → potassium soaps. Potassium fatty acid salts are more hygroscopic and do not pack as tightly → soft paste or liquid soap.
Traditional wood-ash lye was potassium-based (potassium carbonate from inland ash), which is why traditional soap was soft. Hard Castile and Marseille soap required soda, obtained from coastal ash or imported barilla. Today, NaOH and KOH are both cheap and pure.
Saponification Value (SAP Value)
Different oils have different fatty acid compositions and average molecular weights, so each requires a specific amount of NaOH (or KOH) to fully saponify. The SAP value is expressed as grams of NaOH per gram of oil.
| Oil / fat | NaOH SAP | KOH SAP | Key fatty acids | Notes |
|---|---|---|---|---|
| Coconut oil | 0.190 | 0.267 | Lauric 47%, myristic 18% | Very cleansing; superfat at 15–20% to reduce harshness |
| Palm kernel oil | 0.156 | 0.219 | Lauric 48%, myristic 16% | Similar to coconut; slightly milder |
| Palm oil | 0.141 | 0.198 | Palmitic 44%, oleic 38% | Hard bar, creamy lather; controversial sourcing |
| Tallow (beef) | 0.140 | 0.197 | Palmitic 26%, stearic 21%, oleic 44% | Excellent all-round soap; very hard, long-lasting |
| Lard (pork) | 0.138 | 0.194 | Palmitic 25%, oleic 46%, stearic 14% | Creamy lather; slightly softer than tallow; traditional |
| Cocoa butter | 0.137 | 0.192 | Stearic 35%, palmitic 25% | Hard, conditioning; brittle alone — blend at 10–20% |
| Olive oil | 0.134 | 0.188 | Oleic 70–80% | Gentle, conditioning; slow to trace; classic Castile |
| Sunflower oil (regular) | 0.134 | 0.188 | Linoleic 65%, oleic 25% | Conditioning; soft bar; prone to rancidity |
| Sunflower oil (high-oleic) | 0.134 | 0.188 | Oleic 80% | More stable than regular; closer to olive in character |
| Sweet almond oil | 0.136 | 0.191 | Oleic 68%, linoleic 25% | Conditioning; similar to olive but faster trace |
| Shea butter | 0.128 | 0.180 | Stearic 40%, oleic 45% | Conditioning and hardening; rich feel |
| Castor oil | 0.128 | 0.180 | Ricinoleic 90% | Lather booster; conditioning; use at 5–10% max |
| Canola / rapeseed oil | 0.124 | 0.174 | Oleic 60%, linoleic 20% | Conditioning; softer bar; good filler oil |
To calculate NaOH for a blend: multiply each oil’s weight by its SAP value, sum the results.
Getting Started
For your first batch, try one of these recipes rather than inventing your own blend. Once you understand how the soap behaves you will have the intuition to start adjusting.
For the hands-on procedure (lye safety, temperatures, trace, moulding, and curing), see the basic saponification experiment. The notes here focus on formulation rather than technique.
Superfat
A superfat (or lye discount) is a deliberate reduction in the amount of NaOH, leaving a small percentage of oil unsaponified in the final bar. Superfat 5% is the standard starting point: multiply the calculated NaOH amount by 0.95.
Superfat serves two purposes: it provides a safety margin against any NaOH remaining in the bar, and the free oils add conditioning. High superfat (8–12%) suits gentle facial soaps. Very low superfat (2–3%) suits shaving soaps, where excess oil would break down lather.
Water Amount
Water is just the vehicle for dissolving the lye — it evaporates during cure. A standard starting point is 33% of the oil weight as water. Less water (28%) gives a harder bar faster and reduces the chance of soda ash on the surface; more water (38%) makes mixing easier and gives a smoother pour but requires a longer cure.
Starter Recipes
Castile soap — gentle and classic; requires patience (cure 6–8 weeks minimum before the bar firms up fully):
| Ingredient | Amount |
|---|---|
| Olive oil | 500 g |
| NaOH (5% superfat) | 64 g |
| Distilled water | 165 g |
Lard soap — hard bar, creamy stable lather, economical, traditional:
| Ingredient | Amount |
|---|---|
| Lard | 500 g |
| NaOH (5% superfat) | 66 g |
| Distilled water | 165 g |
Balanced beginner bar — hard bar, good fluffy-to-creamy lather, mild; a reliable first recipe:
| Ingredient | Amount |
|---|---|
| Coconut oil | 250 g (50%) |
| Olive oil | 200 g (40%) |
| Castor oil | 50 g (10%) |
| NaOH (5% superfat) | 71 g |
| Distilled water | 165 g |
NaOH calculation for the balanced bar: (250 × 0.190 + 200 × 0.134 + 50 × 0.128) × 0.95 = (47.5 + 26.8 + 6.4) × 0.95 = 80.7 × 0.95 = 76.7 → 77 g. This matches common recipe calculators to within rounding — always verify with a lye calculator before making changes.
Tuning Soap Properties
Hardness
Hardness comes from saturated fatty acids (palmitic, stearic, lauric, myristic) and from the crystalline structure they form when soap dries.
- Increase hardness: use more coconut oil, palm, tallow, lard, cocoa butter, or shea butter; cure longer
- Salt hardening: dissolve 1 teaspoon of table salt (NaCl) per 500 g oils in the water before adding lye. Sodium ions promote crystal formation and make bars unmold faster and firm up earlier. Salt does not affect the saponification chemistry.
- Sodium lactate: a commercial additive (typically 1–3% of water weight) used for the same purpose; less effective than salt but leaves no visible texture
Lather Type
There are two distinct lather types, each driven by different fatty acids:
Fluffy, large-bubble lather — the classic shampoo-type foam — comes from lauric and myristic acids. Increase by using more coconut or palm kernel oil. This type of lather rinses off easily. At very high coconut content (above ~60%) the soap becomes stripping and harsh unless the superfat is raised to 15–20%.
Dense, stable, creamy lather — the kind that holds up in a shaving mug — comes from palmitic and stearic acids. Increase by using tallow, lard, palm oil, or cocoa butter. This lather cushions well and does not collapse.
Lather booster: castor oil at 5–10% of the recipe reliably increases both bubble count and lather persistence in any formulation, because ricinoleic acid is unusually soluble and surface-active.
Sugar boost: a small amount of dissolved sugar (a teaspoon of cane sugar, honey, or milk solids per 500 g oils, added to the water before lye) increases lather. The sugars create a slightly accelerated trace and produce a denser foam.
Conditioning
Conditioning — how much moisture the soap leaves on skin rather than stripping away — is driven by unsaturated fatty acids (oleic, linoleic) and by the glycerol that stays in the bar.
- Increase by raising olive oil, almond oil, sunflower oil, or avocado oil in the recipe
- Raise the superfat percentage (8–10% for facial or dry-skin soaps)
- The retained glycerol in any handmade soap is already a significant conditioner compared to commercial soap
Very high unsaturated content (above ~70% oleic/linoleic) produces a soft bar that wears down quickly in the shower and takes months to cure to full hardness. Olive oil Castile is the extreme case: a freshly unmolded Castile bar is almost rubbery; after four to six months it is hard and long-lasting.
Soft vs. Hard Bar Summary
| Goal | Increase | Decrease |
|---|---|---|
| Hard, long-lasting bar | Coconut, palm, tallow, lard, cocoa butter; salt brine; longer cure | Soft oils (olive, sunflower, canola) |
| Creamy stable lather | Tallow, lard, palm oil, cocoa butter | Coconut (reduces creamy character) |
| Fluffy abundant lather | Coconut, palm kernel; castor oil; sugar | High olive content |
| Conditioning/gentle | Olive, almond, avocado; higher superfat | High coconut content |
| Faster cure/unmold | More saturated oils; salt brine; sodium lactate | High water content; high olive content |
Liquid Soap
Liquid soap is made with potassium hydroxide (KOH) rather than NaOH. The process is the same but requires the hot-process method — cooking the soap paste until saponification is complete — because potassium soap paste is too soft to judge by cold-process trace behavior.
To convert a recipe from NaOH to KOH:
\[\text{KOH (g)} = \text{NaOH (g)} \times \frac{56.1}{40.0} = \text{NaOH (g)} \times 1.403\]
Commercial KOH is typically sold at 90% purity. Adjust:
\[\text{KOH needed} = \text{theoretical KOH} \div 0.90\]
The result after cooking is a thick, glossy paste (soap paste or neat soap). Dilute with distilled water — starting at roughly equal weights of paste and water, then adjusting — to reach the desired consistency. The paste keeps indefinitely; dilute only what you will use in a few weeks, as the diluted liquid soap does not keep as well.
Methods
Cold process (CP): mix lye solution and oils at 40–45°C, stir to trace, pour into a mold. The soap saponifies over the next 24–48 hours; unmold after two days and cure 4–6 weeks. The most common method for bar soap. Results in a smooth, consistent bar.
Hot process (HP): same initial mixing, then cook the batter in a slow cooker or double boiler at 60–80°C until saponification is complete (the batter goes through distinct stages: applesauce, mashed potato, taffy — when it stops being sticky and slightly waxy it is done). Soap can be used sooner (days rather than weeks) but has a rougher, more rustic texture and is harder to pour into detailed molds. Better suited to high-superfat recipes and recipes with sensitive additives.
Traditional ash lye (historical/demonstration): leach wood ash with water over several days, evaporate the dark liquid to concentrate it, test concentration with an egg or potato (it should float), and use in place of a NaOH solution. The result is soft potash soap. Wood ash lye is imprecise and varies widely; you cannot calculate a recipe around it in the same way. As a demonstration of historical method it is instructive; for reliable soap making, use measured NaOH or KOH.
Safety
Sodium and potassium hydroxide are severely caustic. Always wear gloves and eye protection when handling lye or fresh soap batter. Always add lye to water, never water to lye — adding water to the solid causes violent spattering. The lye solution heats to 80–90°C on mixing; set it in a safe place to cool before combining with oil.
Fresh soap batter contains unreacted lye until saponification is complete. Treat uncured soap as you would lye — keep it away from skin and out of reach of children. If skin contact occurs with lye solution or fresh batter, flush with running water for 15 minutes. Do not use vinegar — the neutralization reaction generates heat and makes the burn worse.
Use stainless steel, glass, or HDPE plastic containers. Avoid aluminium — NaOH reacts with aluminium, releasing hydrogen gas and damaging the container.
Resources
- Basic saponification experiment — hands-on cold-process procedure with safety detail, the reaction equation, and the micelle explanation
- The Science of Cleaning — how soap and other surfactants work mechanically; soap scum chemistry; why synthetic detergents replaced soap for laundry
- SoapCalc — lye calculator; enter your oils and superfat, get NaOH/KOH amounts and a predicted properties profile
- Brambleberry Soap Queen — reliable practical tutorials for cold and hot process
- Kevin Dunn, Scientific Soapmaking (2010) — the rigorous reference; covers the physical chemistry of soap in depth, with controlled experiments comparing recipes