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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 of your soap, effectively covering up your lovely swirl job.

What the heck is that white film on your soap? And what can you do about it?

Hello, soda ash

That weird, white film that develops on the top surface of a soap bar is called soda ash. It generally appears during the first few days after making your soap, either while it's setting in the mold or just after you've cut and during cure.

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

Soda ash is harmless

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.

You can use soap with soda ash. In fact, the white film will wash away during the first use or two

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

What causes soda ash on soap?

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 soda ash only appears on the surface of your soap bar, or any edges exposed to air.

So, in a perfect world, if you protect the surface of your soap from air during the entire time of saponification it should prevent soda ash.

The funky thing about soda ash though, 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 in the same way a few weeks later and have it ash.

In my experience, temperature also plays a 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, very rarely 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. Gelling the soap seems to prevent ashing.

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

Preventing soda ash on soap

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. Try mixing your lye water and oils together when they're both between 110 to 120 degrees.

If you make milk soap, though, you don't want your mixture that hot. Instead, blend at about 90 to 95 degrees. This works for me, but you'll want to experiment with your recipe.

Keep your soap nice and warm during the saponification process. Insulate your soap mold 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, protect your soap from air during the entire saponification process. This is the most effective way to prevent soda ash, in my opinion.

Put a lid on your mold before you insulate. My "lid" is just a thick piece of plywood I place over my mold. Make sure the lid isn't actually touching the surface of the soap, though.

In cooler weather, or in soap recipes that I've had problems with ashing before, I leave soap in the mold for three entire days before uncovering. 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.

Things that don't work (in my experience)

These soda ash fixes are commonly recommended and, while they may work for other soap makers, they were big duds for me. I don't recommend:

Spraying the surface of the soap with rubbing alcohol right after pouring. This never made a lick of difference.

Covering the surface with plastic wrap. Yeah, no. It's incredibly messy and ruins the pretty surface of the soap.

Adding a touch of beeswax to the recipe. I never had a worse case of soda ash as the soap I made with beeswax.
After treating for soda ash, you can see the swirl patterns.  Not perfect, but much better than before.

How to remove 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, 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 also had good results with steaming the ash away. Quickly run the ash-ridden surface through the steam from a tea kettle. Be careful you don't burn yourself, though.

Embrace the ash!

Instead of getting bummed over soda ash, you can just learn to embrace it. I used to hate soda ash, but my perception is changing.

This change of heart came after I saw a soap maker at a craft fair who used soda ash to her advantage, creating the loveliest soft purple bars with textured and ashed tops that were to die for. It made me look at soda ash in a different way. Although I don't always love it, I can appreciate it as part of the charm of handcrafted soap.

If your soap has ashed, try to enjoy the rustic beauty of your bars even if they didn't turn out exactly like you envisioned. Because you made soap. And that's pretty awesome, ash and all.

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