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Why Does My Soap Have a White Film on Top?

You've made the most beautiful cold process soap, complete with killer swirls and an amazing scent.  You can't wait to take it out of the mold, cut it into bars, and admire your handiwork.

After letting it set a day or two, you take the cover off the mold and your beautiful soap is not quite so beautiful anymore.  It's got a layer of white film across the surface, effectively covering up your lovely swirl job.

What the heck is that white stuff?  And what can you do about it?

Soda ash is that white powdery substance that forms on the surface of some soap batches.


Hello, soda ash.

That weird, white powdery stuff that develops on the top surface of the bar is called soda ash.  It generally appears during the first day or two as the soap is in the mold.

Ash happens to nearly every cold process soap maker at some point in time.    

The good news is soda ash doesn't affect the safety of your soap or how your soap works.  It's completely harmless.  Ugly as heck, but harmless.

What causes soda ash?

The consensus is that soda ash is sodium carbonate, which is formed when the lye reacts to carbon dioxide in the air during saponification.  That explains why it happens on the surface of your soap bar, or any edges exposed to air.

The funky thing about soda ash, and the reason it plagues soap makers so, is it's so sneaky.  You can make a batch and it turns out perfectly, but make the same recipe a few weeks later and have it develop soda ash.

In my experience, temperature plays the biggest role in soda ash development.  Soaping at cooler temperatures (under 85 degrees) seems to trigger it.  So do big fluctuations in temps during the saponification process.  I have more problems with soda ash during the winter -- and never, ever in the warmer months.

Soap that is poured at very thin trace seems to develop soda ash more than soap that is poured at a thicker trace.  It's my hypothesis that soap poured at a thicker trace is able to generate more heat and temps are more stable after it has been poured into the mold versus soap poured at thin trace.

Some fragrance oils trigger soda ash, but this is probably the least common trigger, at least for me.

Preventing soda ash

Although soda ash may still pop up every now and again (it is sneaky after all!) there are a few things you can do to prevent it.  Here's what works for me, hopefully this works for you too!

Soap at warm temperatures.  If you make milk soap, like I do, you'll want to take care your mixture doesn't get too hot.  About 90 to 95 degrees works for me, but you'll want to experiment with your recipe.

Make sure that your soap stays nice and warm during the saponification process.  Insulate your soap well, with a few towels or a thick blanket, especially during the wintertime.

Pour at a medium to thick trace, if possible.  You're looking for thick gravy to nearly pudding consistency.  This really makes a HUGE difference for me!

After pouring, you want to protect your soap from air.  Put a lid on your mold before you insulate.  My "lid" is just a thick piece of plywood I place over my mold.

Then, I leave my soap in the mold for three entire days before I uncover.  The batches which I rush and open the lid earlier come out of the mold beautiful only to develop ash hours later.  Again, this happens mostly in the winter months so I speculate it's a temperature fluctuation thing.

Here's what doesn't work for me: spraying the surface of the soap with rubbing alcohol right after pouring, covering the surface with plastic wrap (messy and ruins the pretty surface of the soap), and adding beeswax to the recipe (never had a worse case of soda ash as the soap I made with beeswax.)  I know these are common soda ash fixes for other soap makers, but they were big duds for me.

After treating for soda ash, you can see the swirl patterns.  Not perfect, but much better than before.

Removing soda ash from your bars.

Even with careful soaping, there will be times that your bars ash.  If it hasn't happened to you yet, I guarantee it will one day!

Since soda ash is purely cosmetic, you can use those bars as-is -- ash and all.  You won't notice a difference.

But there are a few things you can do to remove ash from your bars and get them looking all pretty again.

My go-to treatment: a bit of rubbing alcohol on a lint-free paper towel.  Rub gently over the surface of the soap and the ash will come away.  Rub in one direction to keep the surface of the bar looking smooth.

I've had good results with steaming the ash away.  I quickly run my bars through the steam from a tea kettle.  You can also use old panty hose to buff away ash.

Hope this helps answer some of your soda ash questions.  Share your tips and tricks for dealing with soda ash!

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