Soda ash on soap: what it is, why it is formed and how to avoid it

TL;DR: Soda ash is a white dusty coating on soap that is formed by the chemical reaction of the surface with CO₂ from the air. It is not a safety hazard - purely an aesthetic problem. Prevention: cover the mold, gel phase, lower water. Remedy: mating, trimming.

Soda ash is a white or grey powdery coating that forms on the surface of CP soap during the first hours or days after filling the moulds. It is the most common cosmetic defect in CP soap - one that virtually every aspiring soapmaker will encounter.

The good news: soda ash is purely an aesthetic issue - the soap is completely safe to use and its quality is not affected in any way. It's all about appearance.

Why soda ash is formed chemically

Soda ash is formed when the surface of fresh CP soap comes into contact with airborne CO₂. Unsaponified or semi-saponified alkalis on the surface react with carbon dioxide from the air to form sodium carbonate (Na₂CO₃) or sodium bicarbonate (NaHCO₃).

The result is a white crystalline coating on the surface - the same sodium carbonate that is added to washing powders.

Soda ash ≠ unconsumed lye. People sometimes think that the white layer is leftover sodium hydroxide - this is not true. NaOH is a strong base that would cause burning. Soda ash is a weak salt that is perfectly safe.

Why soda ash arises more in some recipes

The risk of soda ash increase:

High oleic acid (olein) content: Olive oil, sunflower oil, sesame oil - recipes with a high proportion of olein are more susceptible. Olein slows trace and prolongs the time the soap surface is wet and alkaline.

Lower production temperatures: Cold production at oil/arc temperatures below 35 °C increases the risk.

Insufficient covering of the moulds: Surfaces exposed to air are significantly more susceptible.

Absence of gel phase: Soap that has not gone through the gel phase (CPOP) has slower saponification and longer air exposure time.

High water content: More water in the recipe = longer surface evaporation = more opportunity for soda ash.

How to prevent soda ash

1. Cover the mould

The simplest and most effective measure. Cover the filled mould with a tight-fitting plastic film (food film) or lid. The foil prevents the surface from coming into contact with CO₂ from the air.

Technique: Press the foil directly onto the surface of the soap mass - no air pocket between the foil and the surface.

2. Support gel phase

During the gel phase, the soap will go through the alkaline process faster and the surface will close sooner. Heat the mold in a warm (not hot) room or wrap in towels.

Note: The gel phase causes a darker, transparent "window“ in the cross-section of the soap - this is normal and desirable. For milk or honey formulas, check the gel phase to prevent overheating.

3. Powdered lavender or cornstarch on the surface

Spray or sprinkle the surface of freshly poured soap with a fine powder (lavender, cornstarch). The powder absorbs moisture on the surface and reduces contact with CO₂.

Aesthetic effect: The powder also creates a decorative texture or visual effect.

4. Work with higher production temperature

Mix the oils and the leaching solution at 40-45 °C instead of cold production at 30 °C. Higher temperatures will accelerate trace and reduce the alkaline exposure time of the surface.

5. Reduce water content (water discount)

Instead of the standard 38% water to oil ratio, try 30-33%. Less water = faster cure and shorter window for soda ash. Caution: water discount also speeds up trace, which can complicate swirl designs.

6. Add sodium lactate

Sodium lactate (1 tsp per 500 g of oils, in the leaching solution) accelerates solidification - shorter time before surface closure.

How to remove soda ash when it has already formed

If the soda ash still arose, you have three options:

Steaming

The gentlest method. Use a handheld steamer (for clothes, hair) or a pot with steam:

1. Hold the soap over the steam for 10-20 seconds on the affected area.
2. Soda ash will dissolve - the white coating will disappear.
3. The surface is aligned and smoothed.
4. Allow to dry and harden.

Advantage: Preserves the decoration on the surface, does not remove the paint.

Trimming (planing)

Cut off the thin top layer (1-2 mm) with a soap scraper or knife. Suitable for loaf moulds where the top surface is not decorative.

Advantage: Fast, reliable. Disadvantage: Loss of soap part, can not be used for cupcake or decorated soaps.

Washing (rinsing)

Rinse the surface of the soap with warm water and wipe with a damp cloth. The soda ash will dissolve in the water.

Disadvantage: The surface will be dull and less attractive after drying. Suitable only for "working“ soaps without aesthetic requirements.

When soda ash CANNOT be completely eliminated

Some formulas are inherently susceptible and soda ash will return even after treatment:

• Castile soap (100% olive oil) - Soda ash forms on the surface virtually every time
• Very simple recipes without saturated fat
• Soap made in a cold environment (winter workshop)

In these cases, adopt soda ash as a characteristic feature of natural soap or plan to trim.

Soda ash vs. other white coatings

Not every white coating on soap is soda ash. They can be:

Titanium dioxide migration: If you have used too much TiO₂ or it has not been mixed enough, it may form white streaks or spots on the surface. It differs from soda ash in that it is more compact and does not decompose with steam.

Stearic spots: White dots or spots from unsaponified stearic acid or palm oil. See DOS and other spots.

Haze: For transparent soap - normal condensation of moisture on the surface.

Frequently asked questions

Is soda ash on soap really safe? Yes, perfectly safe. Soda ash (Na₂CO₃) is a weak alkaline species - the same as in washing powders. It's not unconsumed NaOH.

How long does it take for soda ash to appear? Usually within the first 24-72 hours. If the surface is covered or covered, the risk decreases significantly after the first 24 hours.

Can I prevent soda ash altogether? For some recipes (Castilian soap) this is almost impossible. For others, the prevention of these 6 measures is effective in 80-90% of cases.

Does soda ash change the quality of soap? Not at all. The soap is functionally identical. Purely aesthetic defect.

Can soda ash be fixed without steaming? Yes - by trimming 1-2 mm of the top layer. Not ideal for designer soaps, but it works.

How much water is the max. content for the minimum soda ash? 30-33% water to oil significantly reduces the risk of soda ash.

See also:

• Troubleshooting encyclopedia - overview of all CP soap problems
• DOS spots - another type of surface defect
• Soft soap - problems with solidification
• What is cold process - basic chemistry of saponification