Soda ash: why it forms and how to stop it

Soda ash is the dull white film that turns up on top of a loaf of cold process soap a day or two after it was poured. It is one of the commonest complaints in the craft and one of the least serious: the soap underneath is fine, and the film washes off. The useful questions are why one batch ashes when the next does not, and which of the many tips for stopping it have a reason behind them.

What soda ash is

Cold process soap is still saponifying when it goes into the mould. Most of the lye is used up within the first day, and until then a little sodium hydroxide sits in the batter that has not yet met any fat. Wherever that lye is at a surface open to the air, it reacts with carbon dioxide and becomes sodium carbonate, the chemical sold as washing soda. Those crystals are the ash.

Kevin Dunn, the chemist behind much of the published testing on handmade soap, gives two ways to recognise it. Sodium carbonate dissolves in water, so it rinses off, and it covers the whole of the surface that was open to the air rather than sitting in patches. Most ash is a thin coat on top, but it can run deeper: Soap Queen, the Bramble Berry blog, shows bars where it went down into the soap and left it crumbly. Factories knew carbonate’s effect inside soap. They added a little on purpose to stiffen cheap soap, and Simmons and Appleton’s 1908 handbook warned that too much made the soap brittle and made it bloom white on the surface.

Ash forms only where the air reached the soap: across the top of the loaf, not on the sides that were against the mould.

There is a second, rarer white coat. DeeAnna Rouse of Classic Bells describes a smooth, slightly waxy, ivory layer that looks like a coat of paint and usually sits only on the top of the bar. It is made of large crystals of soap itself, not carbonate. It is just as harmless, but steam won’t shift it and washing leaves the bar looking used; a plane takes it off cleanly.

Why some batches ash and others don’t

Ash needs free lye at the surface and air to meet it. Anything that shortens the time the lye lingers at the surface, or keeps the air off while it does, means less ash, and most of the familiar causes turn out to be one or the other.

For most of these, consistent reports with a chemical reason are as far as the evidence goes: we know of no controlled test of ash against temperature or water.

Heat matters because it speeds the reaction: a loaf that heats itself into gel uses up its lye sooner. Dunn’s lecture on time and temperature shows what decides that heat. A big loaf holds it while a cavity mould loses it, and a warmer start raises the peak. Hot process soap rarely ashes for the same reason: the cook uses up the lye before the soap ever reaches the mould. Melt and pour has no free lye to begin with.

How to prevent it

Pick the measures that suit the batch. A milk or honey soap can’t be pushed into a hot gel without scorching, for example, but it can always be covered.

1. Cover the top. Lay plastic wrap on the surface or fit a lid, and leave it until the soap has cooled to room temperature. Let the wrap settle onto the batter by itself: Alicia Grosso finds that pressing it down leaves lumps and bumps in the top. Classic Bells leaves covers on for up to two or three days. This measure has the plainest reason behind it: no air, no carbon dioxide.
2. Keep the loaf warm. Soap Queen soaps with the lye and oils at 38 °C (100 °F) or more, usually 46–49 °C (115–120 °F), then covers the mould with cardboard and a towel so it gels. In a cold room a heating pad on medium for 30 to 60 minutes, then switched off, helps. The gel phase guide covers when you would rather not gel.
3. Use less water. Classic Bells suggests a lye concentration of 33 to 40 %; Soap Queen a water discount of 5 to 10 %. The water guide explains the three ways of stating it.
4. Pour a little thicker. Medium trace rather than thin, when the design allows it.
5. Leave it in the mould. David Fisher waits 36 to 48 hours before unmoulding, so the sides stay covered while lye remains.
6. Mist the top with alcohol, lightly. Soap Queen sprays 99 % isopropyl alcohol over the top straight after pouring and again 10 to 15 minutes later; Anne-Marie Faiola uses 91 % up to three times in the first 90 minutes. Keep it to a fine mist. A drenched top dries hard over hot soap, which is one way to get a crack.

100 g of sodium hydroxide25 %300 g · 3.00 : 1slow; ash and shrinkage likelier28 %257 g · 2.57 : 130 %233 g · 2.33 : 133 %203 g · 2.03 : 1our default, the everyday middle35 %186 g · 1.86 : 140 %150 g · 1.50 : 1a water discount: faster trace45 %122 g · 1.22 : 150 %100 g · 1.00 : 1master batch; crystallises < 12 °CWater as 38 % of the oils, the old default, is about a 27 % solution for a typical recipe whose lye is 14 % of its oils.

That bar is our everyday recipe, which is written at a 33 % solution. It makes good soap either way; at 33 % it is less prone to ash, by most makers’ experience. Our classic olive, coconut and palm bar builds three of the measures above into its method: a 33 % solution, a lid on the mould and a towel so the loaf gels.

Which tips have a reason behind them

How to remove it

• Steam it. A hand-held clothes steamer held 2.5 to 13 cm (1 to 5 in) above each bar for 20 to 30 seconds clears a light coat. The carbonate dissolves and dries back as a film too thin to see.
• Rinse it. With gloves on, run the bar under cold water, rub the white areas gently with a paper towel, rinse off any lather and let it dry. Classic Bells prefers distilled water for this, on the grounds that the minerals in tap water can help set off orange spots later.
• Plane or peel it. A soap planer or a vegetable peeler takes off a thin layer. This is the one method that also removes soap-crystal ash, at the cost of a little soap.
• Leave it. It washes off the first time the bar is used.

When the white stuff isn’t soda ash

Dunn’s rule of thumb: ash covers every surface the air touched, and white dots or streaks below the surface are usually something else.