If you’ve ever made cold process soap and noticed a white coating or powdery layer on the surface, you’re not alone. This common issue is usually called soda ash. Soda ash on soap resembles the soft white rind on Camembert cheese or a white powder.

In the previous chapter, we explored possible causes soda ash on cold process soap and how to control and remove soda ash. Although, this coating is harmless, it can be frustrating — especially for soapmakers who sell soap online or at markets.

Soda ash is a term for sodium carbonate. It’s readily available in the stores sold as Washing Soda. In your soap making lab, soda ash forms each time sodium hydroxide (lye) comes in contact with air. It reacts with carbon dioxide in the air.

Is Soda Ash on Soap Really Soda Ash?

So, let’s dig a little deeper and test whether the white residue on handmade soap is truly sodium carbonate. I conducted a simple vinegar test on my soap to find out.

Soda ash on soap is commonly believed to be sodium carbonate that is formed when sodium from lye (NaOH) reacts with carbon dioxide in the air:

NaOH + CO₂ → Na₂CO₃ + H₂O

But here’s the thing — it might not be soda ash. At least not on my soaps.

The Vinegar Test: Is it Soda Ash or Something Else?

You probably know that mixing vinegar (acetic acid) and washing soda (sodium bicarbonate) produces bubbles.

Just like vinegar and baking soda, vinegar and washing soda produce fizzing and bubbles. Likewise, if the white coating on soap really is sodium carbonate, it should fizz when vinegar is added. Here’s the chemical reaction:

2 CH₃COOH (acetic acid) + Na₂CO₃ (sodium carbonate) → 2 CH₃COONa (sodium acetate) + H₂O (water) + CO₂ (gas bubbles!)

Testing Soda Ash vs. Soap Coating

Step 1. Demonstration Sodium Carbonate and Vinegar Reaction Producing Gas Bubbles

For demonstration purposes, I ran a test reaction between and soda ash. I took vinegar and washing soda from my pantry.

As you can see in the pictures, vinegar and soda ash reaction produced bubbled, just as expected.

Step 2: Vinegar Soda Ash on Soap Directly

Next, I poured vinegar directly onto my soap’s white coating. It ran off before I could snap a photo — but there were no bubbles. As you can see in this picture, there is no fizz or bubbles in the residual vinegar puddle.

Step 3: Mixing White Film Shavings in Vinegar

So I scraped off some of the white layer from my soap.

I added the shavings to the jar with vinegar. Again — no fizz. No bubbles. No reaction.

Step 4. Thorough Mixing Created a Suspension of Soap in Vinegar

After some shaking, I got a cloudy suspension, but no signs of carbon dioxide release.So I scraped off some of the white layer and placed it it into the vinegar in my clean spice jar. Again — no fizz. No bubbles. No reaction.

Step 5. Optional and Not Recommended

Just to be thorough — I tasted the result (not recommended, by the way). It tasted like vinegar. Not sweet or neutral like sodium acetate and water, which would form if soda ash were present.

Conclusion: the white coating on my handmade cold process soap in not soda ash (sodium carbonate) at least in detectable amounts.

What Is the White Film on Cold Process Soap, Then?

The white residue on my soap doesn’t behave like soda ash. It doesn’t fizz with vinegar, and it feels and looks like soap itself. My opinion? It’s probably recrystallized soap molecules, as described by Susan Miller Cavitch in The Soapmaker’s Companion.

In 1945, M. J. Buerger et all at MIT, conducted an experiment on commercial soap using specialized X-ray procession photographs to demonstrate that soap is composed crystalline molecules arranged in a lattice structure. (American Mineralogist (1945) 30 (9-10): 551–571.)

This white layer may form when soap absorbs moisture from the air, dissolves slightly, then dries again — leaving behind a film of soap crystals. It’s different from stearic spots or lye pockets, and it is harmless, but may not look nice for product listings.

Final Thoughts

If you’re troubleshooting your cold process soap, try a vinegar test for soda ash to see what’s really on your bars. You may find, as I did, that the white layer isn’t soda ash after all.

Have you tried it? Let me know if your soap bubbles when vinegar hits the “ash.” No need to taste it like I did! 😅