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Don’t Mix Epsom Salts with Pure-Castile Soap

Dr. Bronner's Castile Soap and Epsom salts don't mix

Don’t mix Epsom salts with any true soap, including Dr. Bronner’s Pure-Castile. I’m stating this bluntly up front in case that’s all you have time to read.

Combining these two is a recipe for creating copious amounts of soap scum. I mean that in the most literal sense because the reaction between these two molecules produces an insoluble magnesium salt that we commonly call soap scum.

I have advised against combining soap and Epsom salts in various other places in my writing, but always in the context of a broader topic. Lately, I’ve been seeing more and more suggestions to combine Epsom salts and our Castile Soap, so I want to address the combo at greater length and explain why it’s a problem.

Dr. Bronner’s Pure-Castile Soap and Epsom salts (so called because they were found in abundance in the mineral springs near the town of Epsom in England) are both excellent bath soaks when enjoyed separately. A warm soak with a squirt of Dr. Bronner’s Peppermint or Lavender Pure-Castile are definitely my go-tos, depending on whether I want to clear my head or relax my psyche. A soak in a warm tub with Epsom salts also has many reputed therapeutic benefits. However, if you try to enjoy the benefits of both at once, chemistry gets in the way.

I have a couple theories for why it is so common to want to mix Epsom salts and Castile. The first is that both are useful in personal care, so the thinking goes that if they are good separately, they’d be even better together. They’re not, though, because chemistry happens.

There is salt, and then there are salts

Another likely culprit for this misplaced desire is confusion about salts. It is fine to mix table salt (sodium chloride) with soap. Why would Epsom salts be different?

Although we commonly use the word “salt” to mean sodium chloride—aka table salt, and also the main component of sea salt, Kosher salt, Himalayan salt, and more—there is a far broader realm of salts beyond sodium chloride. Chemically speaking, a salt is the combination of a metal cation (positively charged ion) and a non-metal anion (negatively charged ion), connected by an ionic bond, forming an electrically neutral molecule. Don’t worry if that just lost you. Keep reading.

There are many, many different salts with many, many different properties and uses. Many of these will sound familiar to you: sodium chloride (table salt), sodium bicarbonate (baking soda), magnesium sulfate (Epsom salts), sodium hypochlorite (chlorine bleach), sodium tetraborate (borax), potassium bitartrate (cream of tartar), and one that I particularly love: Pure-Castile Soap. Yep. Soap is the salt of a fatty acid, produced by reacting fatty acids (anions) from fats or oils with the metal cations from a strong alkali like sodium hydroxide or potassium hydroxide.

All that to say, just because one kind of salt is ok to mix with Castile Soap, it does not mean all salts are ok to mix with Castile Soap.

Why do Epsom salts and Castile soap react?

There’s no way around it: to explain why and how Epsom salts and Castile soap react, I have to explain the chemistry. Hold on to your hats.

There’s an acid/base reaction

Chemically speaking, Epsom salts are magnesium sulfate. This is an acidic salt, which means when it dissolves in water, it creates a slightly acidic solution. Conversely, any true soap, such as Dr. Bronner’s Pure-Castile, is alkaline (or “basic”). This means, on the simplest level, combining Epsom salts and soap is an acid/base reaction (along the lines of vinegar & baking soda, or vinegar & Castile soap, the latter of which is another green cleaning no-no).

But how does it make soap scum?

Let’s look at what the Castile soap molecule is made of.

Dr. Bronner’s Castile Soap starts with a blend of oils: coconut, olive, palm kernel, hemp, and jojoba. These oils are reacted with either potassium hydroxide to make liquid soap or sodium hydroxide to make bar soap. To simplify this explanation (too late!), I’m going to discuss liquid soap, but you can swap out the word “potassium” for “sodium” in any of this and the concepts will still hold true.

The reaction between the potassium hydroxide and the blend of oils produces a variety of soap molecules which include potassium cocoate (from the coconut oil), potassium kernelate (from the palm kernel oil), and potassium olivate (from the olive oil). Again, for simplicity (way too late!), I’ll stick with the largest component by volume, which is the potassium cocoate, but everything will hold true for the kernelate and olivate.

Fatty acids from soap like magnesium more than potassium

When Epsom salts and Castile soap combine, the magnesium from the Epsom salts bumps the potassium off of the fatty acid of the soap molecule. The magnesium bonds with the fatty acid, forming magnesium salt (yes, another salt) of a fatty acid, which in this case is magnesium cocoate.

When a solid substance forms from a chemical reaction and will not dissolve back into the solution, it’s called a precipitate. It is insoluble, which means it does not dissolve in water. You cannot rinse it off. It can only be scoured off or dissolved with an acid. (Read more on how to do this in my article, “Scum, Scum, Go Away.”)

All soaps react with magnesium

This Epsom salts/soap reaction is not unique to Dr. Bronner’s Castile Soap. It would occur with any true soap. On the other hand, detergents (which go by names like shower gel, body wash, and bubble bath) do not react in this way with Epsom salts. This is one way to tell whether you have a true soap or a syndet (industry term for synthetic detergent).

The more Epsom salts and soap you combine, the greater the amount of insoluble soap scum you create. If left unattended, it will build up in pipes and possibly clog them. This is why you should not combine Epsom salts and Dr. Bronner’s Castile (or any true) Soap.

Soap scum made with Castile Soap and Epsom salts
I combined Epsom salts and Castile in a cup and then poured it through my fingers. These waxy flakes are what was created by the combination. Don’t put this down the drain.

Other causes of soap scum

I’m anticipating a question you might ask here: How come even when I haven’t combined Epsom salts and soap I still get some soap scum?

Under regular use, we might find small amounts of magnesium cocoate on our bathtubs and sinks and shower walls. It can look like a white film or like the sink or tub’s surface has been dulled.

Soap scum is also created when hard water reacts with a true soap. This is because the minerals that make water hard are…you guessed it…magnesium and calcium. (Calcium reacts with soap in the same way as magnesium.)

However, the amount of soap scum created from hard water minerals is nowhere near the amount of soap scum created from a whole scoop of Epsom salts added to a Castile bath. During normal use, hard water soap scum is easily cleared by regular cleaning.

Help! I’ve already combined them!

If this explanation has come too late, and you have already combined Epsom salts and Castile soap, there are a few steps you can take to combat the residue. While magnesium salts of fatty acids do not dissolve in water, they do dissolve in acids, such as vinegar. Carefully combine vinegar and very hot water 1:1 in a pitcher and pour it down your drain. After letting that sit for 10 minutes or so, if you happen to have a plumber’s snake, use that to clear the loosened residue from the pipes.

For the residue on your tubs, scour it with my GIY Soft Scrub and a sturdy brush. You can also use vinegar, but the acid should not sit on the surfaces too long as it could dull them.

Magnesium cocoate has uses – just not in the tub

On its own magnesium cocoate is not considered dangerous. In fact, it is widely used in cosmetics, foods, and pharmaceuticals as an anti-caking agent, bulking agent, and slip modifier (aiding the flow of a powdered product), among other things. The Environmental Working Group’s Skin Deep Database gives magnesium cocoate a “1,” the lowest hazard rating on its scale. However, there are more refined ways of producing magnesium cocoate for these purposes than scraping it off a bathtub.  

Make this combo yourself – under control

I think it is more compelling if you try this for yourself in a controlled environment (not the bathtub) so you can see and feel first-hand what I am talking about. I included this as one of the Lab Coat Moments in my book, Soap & Soul: A Practical Guide to Minding Your Home, Your Body, and Your Spirit with Dr. Bronner’s Magic Soaps, so that readers can experience the chemistry and be thoroughly convinced not to mix these in the tub. Watch my demonstration in this video and then try it for yourself.

Once you’ve combined the Epsom salts and soap and observed the “frozen soap” in the solution, strain the solution through a paper towel. Feel the waxy solid that is the magnesium cocoate. It is harmless, but you can feel why you do not want this solution in your drains. Do not pour it down the sink – dispose of this in a trashcan.

And perhaps if you have a science fair coming up, you can use this reaction to demonstrate the concept of a precipitate.

DIY recipes, store displays, and more

I absolutely understand that people mean well. They have the best of intentions. But I cringe whenever I come across recommendations for bath soaks that involve Epsom salts and soap, and especially when it involves Dr. Bronner’s Pure-Castile Soap. I don’t want them to clog their drains and I certainly don’t want them to blame the soap when they experience this intense residue!

Similarly with gorgeous endcaps in stores that showcase the full Dr. Bronner’s Castile rainbow, beautifully arranged side by side with bags of Epsom salts. Customers will not be happy if they follow the implied recommendation to use the two products together.

Please pull this following sentence out and share it with your friends: Epsom salts and Castile soap should not be mixed. This is right up there with why you shouldn’t mix Castile Soap and vinegar. It’s not dangerous, just messy and problem-causing. The rationale is the same for both mixtures: Chemistry happens.

Further reading:

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About Lisa Bronner

My grandfather was Dr. Bronner, my family makes soap, and I share ways to use it plus tips on greener living.

Learn about my book, Soap & Soul!

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