Brine Design: Salt In Soap

The week before last I made this swirled soap as part of my experiments with black wattle bark extract. You can read about those experiments here.

On a forum I mentioned that I had used brine to ‘bleach’ the white portion of this soap. Many wanted to know what I meant by that. I’ll demonstrate that shortly, but let’s first look at salt and soap and salt in soap.

Soap is salt

For the record soap IS salt. Traditional soap made with oil and lye is either sodium or potassium salt of fatty acids. A label term like ‘sodium stearate’ means ‘sodium salt of stearic acid’. But within the not-so-scientific framework of this post, I shall use the term ‘salt’ to mean sodium chloride as in common cooking salt, sea salt, Himalayan rock salt etc.

Salt has long traditions in soapmaking. Original Marseille soap was made with Mediterranean seawater which contains plenty of salt. Where soap was made with fresh water salt was often added in the process in order to ‘salt out’ the soap, i.e. to separate the soap from water, glycerine and impurities which would stay behind in the water.

Modern-day soapmakers often add small amounts of salt to soap to make the soap harden faster for quicker unmoulding. Formulas for so called ‘salt bars’ call for large quantities of salt; anything from 50% to 100% of the weight of oils is common. That kind of ‘soap’ might not have impressed out great grandfathers much. In earlier days of commercial soapmaking manufacturers added things like chalk, clay, sand and salt to soap. These ‘fillers’ were cheap additives for beefing up the weight and volume of the product. Today many ‘fillers’ are in fact regarded as adding value to soap. Few countries today require declaration of TFM (Total Fatty Matter) on labelling for cosmetic soap.

Brine soap

Brine soap, or Soleseife in German, typically contains much less salt than salt bars. Originally the salt content came from sea water used as liquid. Today brine soap is often made with grains of salt which are dissolved in the liquid before NaOH is added. Unlike salt bars where undissolved salt crystals are added to soap batter, brine soap has a very smooth texture.

Beyond these uses for salt in soap salt is often used for decorational purposes. Salt crystals are pretty, natural and readily available and can be used as decor on the surface of coldprocess soap.

Salt can also have a decorative effect inside coldprocess soap. Grains of salt added to soap batter will not dissolve. Instead they will show up in the fully saponified soap – each grain with its own light-coloured ‘halo’. You can read about my salt dapple soap here.

But what does salt actually do in soap? How does it affect the soap and the experience when using the soap?

Salt effects

As mentioned earlier soapmakers often use salt to make coldprocess soap harden up faster. Some on the internet will tell you that salt only makes soap hard initially, but that seems unlikely. Salt significantly changes the structure of soap making it hard, opaque and brittle – permanently. The more salt, the harder and more brittle the soap will be.

Soap is less soluble in saltwater than in fresh water. The salt in salt soap makes the water salty and that makes the soap less easily soluble. All else equal you get less soap off a soap bar with salt when you use it. That has an impact on the longevity of the bar. Salt bars and even brine soaps are often longer lasting than the same soaps without salt.

Lower solubility also has an effect on lather. The more salt the less the soap will lather. To make up for this salt bars are often formulated with lots of coconut oil. Coconut oil soap is highly cleansing and will lather even in salt water.. Brine soaps which contain much less salt typically have lather with a lotion-like quality and very fine bubbles.

Beyond these characteristics salt is hygroscopic meaning that it attracts moisture from the air. In humid conditions salt bars and decorative salt crystals tend to get covered in droplets of water. Soapmakers call this ‘sweating’. Brine soap containing less salt than salt bars is less prone to sweating, But soap made with saturated brine can also sweat, especially if made with high water > saturated brine..

Salt scores high on the comedogenic scale. I.e soap with plenty of salt might not be your best choice if you are suffering from acne and break-outs. Also, on broken skin salt can cause quite a sting. But many users love salt bars and brine soaps.

Brine soaps revisited

In the early months of pandemic lockdown last year I had time on my hands and I decided to revisit brine soap after a pause of many years. I tested making a brine soap with a regular balanced formula with limited coconut to see how the salt would affect lather.

I usually make soap with very little extra water. Although the brine I used was saturated the amount of salt relative to oil weight was relatively small. Even so the soap was and continued to be very hard and it felt lovely with its cream-like lather.

But what I found most striking was the beautiful bright white colour. It looked as white as if I had added plenty of titanium dioxide – which I hadn’t. Since then I’ve used ‘salt brightening’ for various soaps.

So last week when I wanted to add bark extract to one portion of my soap and indigo to another, salt in the third portion was a natural choice for soap with natural colourants.

Translucent vs opaque

Trying to create crisp swirls in striking contrast to darker colours is hard with uncoloured soap. Even if you use oils that are very light in colour (typically coconut oil), uncoloured soap is still translucent (even more so if it goes through full gel phase). Being translucent it will not reflect light very effectively. This means that it will not stand out in fine swirls between dark coloured portions of soap. If you’ve ever tried swirling a predominantly dark soap with a small amount of uncoloured soap you know what I mean. The effect will always be duller than if the light colour in your design is completely opaque. That’s why pigments like titanium dioxide (by itself or in white mica) and zinc oxide are popular – even among those who are otherwise into natural colourants. They effectively make the soap opaque providing crisp contrast to darker colours.

All cosmetic titanium dioxide and zinc oxide is synthetic. Synthetic manufacturing makes it possible to eliminate impurities and to ensure standard particle size etc. Synthetic colourants may be more reliable than natural ones. But if you like to keep your products free from synthetics you might need an alternative to titanium dioxide and zinc oxide.

That’s where salt like the salt in last week’s swirled soap comes in. A very common question among new soapmakers is “How can I make my soap white naturally? When I posted about it many soapmakers had no idea that you could brighten soap with salt. And – come to think of it – I had never put brine soap and plain, uncoloured soap next to each other to see exactly how big the difference in brightness is.

Ta-da! Time for another soap experiment!

I wanted to test what the visual effect of brine vs. plain water would be in coldprocess soap. For the result to be relevant I needed to eliminate all other variables.

For my test batch I masterbatched a balanced oil mix with a 5% superfat. To my masterbatched oils I added some well-behaved and non-discolouring essential oils: rosemary, peppermint and some eucalyptus. I blended the oil and essential oil very well and then divided the mixture into three equal portions.

Then I made three lye solutions – each one in its own container. I started by weighing out equal amounts of water. To one of the containers I then added salt at 20% of the weight of the water in that container. I used coarse sea salt and mixed the solution until all the salt was dissolved, the liquid was clear, and no crystals could be seen at the bottom of the container.

At room temp at sea level you can dissolve salt in water to a concentration of about 26%. The salt concentration in my brine container was roughly 17% i.e. I could have added plenty more had I wanted to. But I didn’t want to use more salt than necessary; I was adding salt purely for visual effect.

Opportunistic water

Then I added equal amounts of lye to each of the three containers. As expected the salt solution (brine) turned milky white as the sodium hydroxide dissolved. Water loves salt and will readily get intimate with salt in a lovely, clear solution (think sea water). But as soon as you introduce the solution to sodium hydroxide salt gets mercilessly kicked out. Water then immediately hooks up with its new squeeze sodium hydroxide. And once water and sodium hydroxide are an item salt stands very little chance of coming between them anymore. Nature can be cruel..

Sodium hydroxide is readily soluble in salt solution, but it’s hard to dissolve salt in sodium hydroxide solution. When sodium hydroxide is added the salt in the brine precipitates in tiny crystals so small you can’t feel them and they can easily pass through a fine strainer.

Making waves

I added each lye solution to its oil mixture and blended each batter to very light trace. Using a couple of wavy dividers in a log mould I poured the brine soap in the middle and the plain soaps on each side. Then I removed the dividers. Because I used water sparingly (20% of oil weight throughout the log) I could ovenprocess the soap at 60C for a couple of hours without problems. Once the soap had cooled down completely I cut the log horizontally. This was the result:

I think that’s a pretty impressive contrast in ‘uncoloured’ soap. The only difference between the white wavy line in the centre and the other soap is the use of brine vs fresh water. In all three portions of soap the ratios of water to lye and water to oil were exactly the same. The oils and essential oils were masterbatched. And all portions went through saponification at the same time in the same mould.

This is a good illustration of how a relatively small amount of dissolved salt (low water with brine that was far from saturated) brightens coldprocess soap significantly. As the soap cures and dries the surrounding soap can be expected to lighten up too. But here the initial water was low, the soap did not go through full gel phase, and the fading will be insignificant.

Also, thinking of the log of soap as a whole, about two teaspoons of salt per 1000g oils is not going to reduce lather drastically and it’s not likely to cause a lot of sweating either.

As I cut the soap about 8h after pouring the brine portion was slightly firmer than the rest, but still very easy to cut smoothly with a fine wire.

What gives?

So how does brine brighten soap? The short answer is that it makes the soap opaque and because the soap is opaque it reflects more light than translucent soap. When you put the soap in front of a strong light you can see how the brine soap in the centre is uniformly opaque compared to the more translucent plain-water soap on the edges.

But what makes the brine soap opaque? I’m sure the answer has got to do with crystal structure. On the one hand the tiny precipitated salt crystals from the lye solution reflect light by themselves, but the salt is also likely to influence how the soap molecules arrange themselves in relation to the water/brine present in the soap. I assume that’s what causes ‘salt halos’ around larger grains of salt too.

The design I chose is quite simple, but given the crisp contrast here I think you could do interesting and more intricate brine designs. Whether brine can be used for ombre effects is for somebody else to test, but you can certainly use brine design to make dark colours lighter.

In any case I thought it was a cool experiment. Now we know just how much of a difference a relatively small amount of salt makes in the colour of ‘plain’ coldprocess soap..