Hard Bar Soap

Part of Soap Making

How to make solid sodium hydroxide soap — and how to produce NaOH from scratch when commercial chemicals are unavailable.

Why This Matters

Hard bar soap outlasts soft soap in almost every practical dimension. A cured bar can be wrapped in cloth and stored for years without spoiling. It travels without spilling. It sits by a washbasin and lasts weeks with daily use. It can be rationed, traded, and distributed. When you are supplying soap to a community of any size, hard bars are the practical format.

The challenge is that producing hard bar soap requires sodium hydroxide (NaOH), not the potassium hydroxide (KOH) that wood ash lye naturally provides. KOH produces a soft, pasty soap. NaOH produces a hard, solid bar. If you only have wood ash lye, you are working with KOH by default — but there are methods to either convert that KOH soap to hard bars or to produce NaOH directly from available materials.

Understanding the distinction between NaOH and KOH soap, and the pathways to obtain NaOH without industrial supply, is essential knowledge for any soap maker aiming to produce durable solid bars at community scale.

The NaOH vs KOH Problem

Both sodium hydroxide and potassium hydroxide are strong alkalis that saponify fats. The difference is in the metal ion:

• Sodium (Na) produces sodium soap — dense, crystalline structure, solid at room temperature
• Potassium (K) produces potassium soap — more soluble, softer, pasty or liquid

Wood ash lye is potassium carbonate (K2CO3) dissolved in water, which converts to KOH in the presence of slaked lime. This is the alkali available from the simplest ash-leaching process. The soap it makes is inherently soft.

To make hard bars from scratch, you need one of:

1. Salt-out method — convert KOH soap to NaOH soap using table salt (NaCl)
2. Soda ash route — produce sodium carbonate (Na2CO3) from specific plant ashes, then convert to NaOH with lime
3. Lime-soda process — react soda ash with slaked lime to produce NaOH directly

Each method is viable at different scales and with different resource availability.

Method 1: The Salt-Out Conversion

The salt-out method is the most accessible path to hard bars if you already know how to make soft soap. It exploits the low solubility of sodium soap in saturated salt solution.

The chemistry: When you dissolve salt (NaCl) in a soft KOH soap solution, sodium ions displace potassium ions in the soap molecule. Sodium soap is much less soluble in water than potassium soap, so it precipitates — falls out of solution — as a solid curd. The potassium chloride (KCl) and glycerin remain dissolved in the water, which is drained off.

Step-by-step salt-out procedure:

1. Make soft soap first — complete a standard KOH soft soap batch (wood ash lye + fat, hot process, fully saponified). You need a thin liquid soap paste, not a thick gel. Add extra water if needed — target consistency is pourable.

2. Prepare saturated brine — dissolve as much table salt (non-iodized preferred) in hot water as it will accept. At boiling, approximately 360g salt per liter of water dissolves. You will need roughly 200-300g salt per kilogram of finished soft soap.

3. Combine — pour the liquid soft soap into a large pot. Add the hot saturated brine gradually while stirring. The soap will begin to curdle — white lumps forming and separating from the liquid. This is the sodium soap precipitating.

4. Simmer and separate — heat the mixture gently (do not boil vigorously) while stirring. The soap curds will clump together. After 15-20 minutes, remove from heat and let cool. The soap floats as a solid layer on top; the liquid below (called “lye water” or “spent lye”) contains glycerin, KCl, and excess water.

5. Drain — ladle or pour off the solid soap layer. The liquid can be discarded or processed for glycerin recovery if desired.

6. Re-melt and reform — the precipitated soap is granular and rough. Melt it in a double boiler with a small amount of water (10-15% by soap weight) to create a smooth paste. Add any desired fragrance or additives at this stage.

7. Pour and cure — pour into molds. The soap will harden as it cools. Unmold after 24-48 hours. Cure 4-6 weeks before use — this completes saponification and hardens the bar.

Limitations of salt-out: The conversion is never 100% efficient — some potassium soap remains in the bar, making it slightly softer than soap made with pure NaOH. The bars will be serviceable but may not have the rock-hard quality of commercial NaOH soap. Multiple salt-out passes improve conversion.

Method 2: Soda Ash from Plant Ashes

Certain plants are exceptionally high in sodium compounds rather than potassium. Burning them produces ash rich in sodium carbonate (Na2CO3, soda ash) instead of potassium carbonate.

High-sodium ash sources:

• Seaweed (kelp, wrack) — historically the primary source of soda ash, called “kelp ash” or “barilla” in Spain. Burned dry seaweed produces 2-5% Na2CO3 by ash weight. This was a major industry in coastal Scotland and Spain before the Leblanc process.
• Glasswort / saltwort (Salicornia species) — salt marsh plants with very high sodium content. Burned ash called “barilla” was the premium soda ash source in pre-industrial Europe.
• Chenopodiaceae family — chenopods (goosefoot, orache) grown in saline soils accumulate sodium. Not as concentrated as coastal plants but useful inland.
• Certain grasses from saline soils — test local species by ash taste (salty = sodium-rich, bitter = potassium-rich).

Producing soda ash:

1. Dry plant material thoroughly
2. Burn completely in a contained fire — collect all ash, minimize loss to wind
3. Leach ash in water (same process as wood ash lye) to produce a sodium carbonate solution
4. Evaporate the solution to dryness — the white crystalline residue is crude soda ash (Na2CO3 mixed with NaCl and other salts)
5. The crude soda ash can be used directly in the lime-soda process

Yield: Expect 15-25kg of dried seaweed to produce 1kg of workable soda ash. High volume process but no special inputs required beyond coastal or saline plant access.

Method 3: The Lime-Soda Process

The lime-soda process converts soda ash (Na2CO3) to sodium hydroxide (NaOH) using slaked lime (Ca(OH)2). This is the historic “causticization” reaction:

Na2CO3 + Ca(OH)2 → 2 NaOH + CaCO3

Soda ash reacts with slaked lime to produce sodium hydroxide (dissolved in water) and calcium carbonate (insoluble precipitate that settles out).

Step-by-step:

1. Prepare slaked lime — burn limestone or seashells to produce quicklime (CaO), then add water slowly (it will steam and heat violently) to produce slaked lime (Ca(OH)2) powder or paste.

2. Dissolve soda ash — dissolve your crude soda ash in hot water. Concentration: approximately 200-250g soda ash per liter of water. Filter to remove insoluble ash residue.

3. Add slaked lime — for every 106g of pure Na2CO3 in solution, add 74g of Ca(OH)2 (slaked lime). In practice with impure materials, add excess lime (120-130% of theoretical) to drive the reaction to completion.

4. React — combine the soda ash solution and slaked lime in a pot. Heat to just below boiling, stirring continuously. The calcium carbonate precipitate will form as a white sludge. Continue heating and stirring for 1-2 hours.

5. Settle and decant — remove from heat. Allow to settle for several hours. The CaCO3 sludge sinks to the bottom. The clear liquid above is sodium hydroxide solution (lye).

6. Decant carefully — pour or siphon off the NaOH solution, leaving the sludge behind. Filter through cloth to remove remaining CaCO3 particles.

7. Test strength — float a raw egg or potato in the lye. If it floats with a surface roughly the size of a quarter coin (~25mm) above the surface, the lye is at approximately 25-30% NaOH, suitable for soap making. Weaker lye will not make good soap; concentrate by gentle evaporation.

8. Use immediately — NaOH solution is corrosive and hygroscopic (absorbs water from air). Use within a few days or concentrate further for storage. Do not store in metal containers — use clay, glass, or hardwood.

Making the Soap: Cold Process vs Hot Process

Once you have NaOH lye (solution or solid), soap-making proceeds by one of two methods:

Cold Process (CP):

• Combine lye solution and melted fats at approximately the same temperature (40-50°C / 100-120°F)
• Mix until “trace” — the mixture thickens to a pudding-like consistency where a drizzle on the surface leaves a visible trace before sinking
• Pour into molds, insulate (wrap in blankets) for 24-48 hours to complete saponification via residual heat
• Unmold and cut after 48 hours; cure 4-6 weeks
• Advantage: preserves more glycerin in the bar; requires less active time
• Risk: incomplete saponification if temperatures drop too fast, leaving caustic lye pockets

Hot Process (HP):

• Same initial steps as cold process
• After trace, continue cooking in a double boiler or slow cooker (low heat) for 1-2 hours
• Soap goes through distinct stages: applesauce, tapioca, mashed potato — when it looks like glossy mashed potato, it is done
• pH-test: touch a tiny amount to the tongue — should taste slightly soapy, not sharp and burning (which indicates uncompleted saponification)
• Pack into molds immediately while still warm and pliable
• Can be used within 1-2 weeks (no lye remaining); still benefits from 4-week cure for hardness
• Advantage: no raw lye in finished soap; faster usability
• For community production, hot process is safer and more reliable

Fat Selection for Maximum Hardness

Hard bars require fats with high saturated fatty acid content. Saturated fats produce harder soap; unsaturated fats (olive, linseed) produce softer soap.

Hardness ranking (harder to softer):

1. Coconut oil — hardest, fastest curing, but drying at >30% of blend
2. Palm kernel oil — similar to coconut
3. Tallow (beef/mutton fat) — excellent hardness, mild lather, long-lasting
4. Lard (pork fat) — slightly softer than tallow, creamy lather
5. Palm oil — good hardness, orange color from carotenoids
6. Olive oil — soft bars, very slow cure (up to 12 months for pure castile)
7. Linseed/flaxseed oil — very soft, not suitable as primary fat

Recommended starter formula (makes approximately 8-10 bars):

• 500g tallow
• 150g coconut oil
• 91g NaOH (solid, 100% purity) or adjust for solution strength
• 190ml water

NaOH quantities assume 100% pure NaOH. If using homemade lye solution of unknown strength, start with a test batch and adjust. Commercial NaOH is sold as “drain cleaner” (must be 100% NaOH, no additives — check label).

Superfatting: Most formulas use 5-8% excess fat (lye is calculated to saponify only 92-95% of fat). This leaves free conditioning oils in the finished bar and provides a safety margin against lye-heavy soap. Never reduce below 5% superfat for personal use soap.

Hardening and Curing

Freshly made soap is soft, somewhat caustic, and high in water content. Curing transforms it:

• Week 1-2: Saponification completes. pH drops from ~12 to ~9-10. Bar firms significantly.
• Week 3-4: Water evaporates. Bar continues to harden and whiten.
• Month 2-6: Bar reaches maximum hardness. Lather becomes creamier. Mild scent develops.

Curing conditions: Single-layer on wooden boards or wire racks in a dry, ventilated area. Do not stack until fully cured (6+ weeks). Avoid direct sunlight (causes rancidity in superfat oils) and high humidity (slows curing, promotes DOS — dreaded orange spots from fat oxidation).

Accelerating hardness: Adding 1 teaspoon of sodium lactate or 1 tablespoon of fine salt (dissolved in the lye water) accelerates hardening and allows unmolding in 24 hours rather than 48.

Common Mistakes

• Using iodized salt for salt-out — iodine discolors the soap; use non-iodized salt
• Skipping the pH test after hot process and assuming “done” by visual appearance alone
• Pouring cold process soap into metal molds — sodium soap reacts with some metals; use silicone, wood, or plastic
• Adding fragrance oils before trace — they can accelerate trace to unusable thickness in seconds
• Cutting bars too early — bars that are still soft will deform and develop uneven surfaces
• Using lye solution that is too weak — soap will not saponify fully, leaving sticky, greasy bars
• Failing to protect skin and eyes during lye handling — NaOH is more immediately dangerous than KOH; wear gloves and eye protection at all times

Key Takeaways

• Hard bar soap requires NaOH; wood ash lye produces KOH and makes soft soap by default
• The salt-out method converts KOH soft soap to hard bars using table salt — accessible but imperfect
• Soda ash from seaweed or salt-tolerant plants provides a sodium-based alkali for NaOH production
• The lime-soda causticization converts soda ash + slaked lime to NaOH solution ready for soap-making
• High saturated fat content (tallow, coconut oil) produces the hardest, longest-lasting bars
• Hot process is safer for community production — no raw lye in finished product
• Cure bars 4-6 weeks minimum for hardness and mildness
• Superfat at 5-8% to ensure no caustic lye remains in personal use soap