One of my favourite parts of soapmaking is choosing fragrance. I love imagining how a particular scent might translate into a bar of soap, deciding which colours to use, and planning the swirls and pattern. But it has to be said, some fragrances are an absolute NIGHTMARE to work with.
They might smell gorgeous in the bottle, but add them to a perfectly well-behaved batch of soap and suddenly all your carefully laid plans go out of the window. And it’s all down to chemistry.
First, a quick recap of how soap is made
When I make cold process soap, I combine oils and butters with a carefully calculated solution of sodium hydroxide (lye) and water. The ingredients begin reacting in a process called saponification, eventually producing soap and glycerine. As I mix everything together, the liquid gradually thickens.
Soapmakers call this thickening trace, and it’s an important stage because the consistency of the mixture affects what we can do with it.
A thin, fluid mixture is perfect for creating intricate swirls and patterns.
A thicker mixture is better for some textured designs and decorative tops.
Ideally, I want to control how quickly the mixture thickens so that I’ve got enough time to create whatever design I’ve planned. Unfortunately, some fragrances really don’t want to be controlled.
What is acceleration?
In soapmaking, acceleration is when the soap mixture thickens much more quickly than expected.
Imagine I’ve just mixed a batch of soap to a lovely, pourable consistency. I’ve got my colours ready, my mould waiting and a beautiful swirl planned. Then I add the fragrance. Within moments, the mixture starts thickening.
And thickening.
And THICKENING.
Suddenly, instead of having a lovely fluid mixture to pour into the mould, I’ve got something resembling very thick custard. Or even mashed potato.
Neither of which is particularly helpful when you’re trying to create delicate swirls. In extreme cases, like in the photo above, the mixture can become almost solid in the mixing jug. Soapmakers call this seizing (aka ‘Soap on a Stick’). At that point, the priority is no longer creating a beautiful design. It’s getting the mixture into the mould at all*
And yes, I’ve had my fair share of batches where my artistic ambitions have been reduced to getting the soap into the mould before it sets solid. Fragrance testing is essential. Although it has to be said that I’ve had fragrances perform reasonably well during testing and then surprise me by being tricky when I made a full batch. Usually because I don’t pay as much attention to my oil temperatures as I probably should. Hotter oils can mean faster tracing and acceleration, so I really shouldn’t be surprised. Mea culpa!
Why do floral fragrances cause acceleration?
Floral fragrances have a reputation among soapmakers for being particularly troublesome. Lilac, lily of the valley, jasmine and certain rose-type fragrances are among those that can cause rapid thickening.
But why?
Well, a fragrance isn’t usually a single ingredient. It’s a blend of different aromatic compounds, each contributing something to the final scent. Some of these compounds can affect the behaviour of the soap mixture when they’re introduced into its strongly alkaline environment, affecting how quickly it thickens or how stable the mixture remains.
For some floral fragrances, whether they be essential oils or fragrance oils, the result is a very noticeable acceleration. However, not every floral fragrance behaves this way. For example, lavender, in my experience at least, does not noticeably accelerate trace. And two fragrances that both smell like roses might contain very different combinations of aroma compounds. One might behave beautifully, giving me plenty of time to work, while the other might have me frantically reaching for the mould.
This is why the actual composition of a fragrance matters far more than simply whether it’s described as floral.
What about spicy fragrances?
Ah yes, the spicy ones. Cinnamon, clove and some other warm, spicy fragrances can be particularly challenging in cold process soap. One interesting compound is eugenol, which is a major aromatic component of clove essential oil and is also found in cinnamon leaf oil.
Eugenol belongs to a group of compounds known as phenols, which can behave differently in the highly alkaline conditions of freshly made soap. Eugenol-rich essential oils and fragrance blends are well known among soapmakers for their tendency to accelerate trace, sometimes dramatically.
Other aromatic compounds found in spicy fragrances can contribute to difficult behaviour too.
This is particularly noticeable when I’m making my autumn and Christmas specials, whose fragrances often combine spicy notes with sweet or floral ones. A fragrance described as something wonderfully comforting, perhaps involving cinnamon, cloves, vanilla and a few other delicious things, can be a lot less comforting when you’re actually trying to make soap with it. It’s enough to take the festive cheer right out of you, ha!
There is also a separate safety consideration with some spicy essential oils. Ingredients such as clove and cinnamon can have strict usage limits because of their potential to irritate or sensitise the skin.
So working with them isn’t just about whether they’ll ruin my swirls. They also need to be used at appropriate, assessed concentrations in the finished product.
So essential oils can cause issues too?
Absolutely. It’s a common misconception that essential oils are somehow gentler or easier to work with because they come from plants. But the compounds that cause acceleration don’t particularly care whether they originated in a flower, a leaf, a piece of bark or a laboratory.
An essential oil rich in a troublesome aromatic compound can behave just as badly as a fragrance oil containing that same compound. Equally, some essential oils and fragrance oils behave beautifully.
Once again, it’s the chemistry that matters, not whether the ingredient is described as natural or synthetic.
And then there’s vanilla…
Now, vanilla is a slightly different problem. While some fragrances make the soap thicken unexpectedly, vanilla-containing fragrances are notorious for making soap change colour. And when I say change colour, I don’t mean a subtle adjustment.
I’m talking about beautifully pale soap turning beige, caramel brown or even dark chocolate brown. The culprit is usually a compound called vanillin.
Vanillin is one of the main aromatic compounds responsible for vanilla’s familiar sweet scent. It occurs naturally in vanilla beans, but it can also be manufactured and is widely used in fragrance oils. It’s found not only in vanilla fragrances but also in many scents described as creamy, sweet, gourmand or bakery-inspired. Cakes, biscuits, custard, marshmallows and all sorts of other delicious-smelling things like that.
Why does vanillin turn soap brown?
When vanillin is exposed to the strongly alkaline conditions of soapmaking and oxygen in the air, it can undergo a chemical reaction called oxidation. This produces coloured substances that cause the soap to darken. The amount of vanillin in the fragrance affects just how dramatic the change will be.
A fragrance with a relatively small amount might turn the soap a pale beige. One containing a high proportion can produce a much deeper brown. And the particularly frustrating thing is that the colour change doesn’t necessarily happen immediately. I might make a little test batch, pour it into the mould and be delighted with how it looks.
The following day, when I unmould it, it might still look lovely. Then, over the next few days or weeks, the colour begins changing. Sometimes the outside of the bar darkens first because it’s exposed to the air, while the inside remains lighter. Eventually, the darker colour might spread through the bar.
Imagine carefully creating a beautiful pastel design, only to discover a fortnight later that most of it has turned brown. It first happened to me very early on during my soap-making journey, before I’d heard of the vanilla/vanillin issue. I adored the fragrance and made a pretty pink and lilac soap, only for it to very quickly turn various shades of unappealing brown over the next few days. Gutted, to put it mildly.
Does brown soap mean something has gone wrong?
It might feel like it if you’re not expecting it, but vanillin discolouration is a cosmetic change. It doesn’t mean that the soap has gone bad or that the fragrance is unsafe. A properly formulated and assessed bar can still be perfectly lovely to use. It just might not look anything like the soap you originally planned.
And, of course, brown isn’t necessarily a bad colour. If I’m making something that smells like chocolate, coffee, caramel or a delicious pudding, a rich brown can work beautifully.
The challenge comes when the fragrance smells like vanilla cupcakes and you’ve planned a delicate pink and white design. Chemistry doesn’t care about your colour scheme…
Any other ways fragrances can cause trouble?
Um, nope! Acceleration and discolouration are just the beginning…
Ricing: Sometimes a fragrance causes little lumps to develop in the soap mixture, giving it a texture rather like cottage cheese. These can sometimes be blended out, but that risks thickening the mixture even further. This one is particularly fun, haha!
Separation: Occasionally, a fragrance can interfere with the soap mixture staying properly emulsified, causing oils or other liquid to separate out. This is more than a cosmetic problem, because a badly separated batch may not saponify evenly and will probably be unsafe to use as it stands.
Overheating: Some fragrances can contribute to the mixture becoming excessively hot, causing cracks, wrinkled surfaces or even a soap volcano, where the expanding mixture pushes its way out of the mould. Yes, soap volcanoes are a real thing!
Fading or changing scent: Some fragrances smell incredible when first added but fade dramatically during curing. Citrus essential oils are particularly notorious for this. Other fragrances can change character entirely, losing some notes while others become more noticeable.
And finally, vanillin isn’t the only ingredient that can cause unexpected colour changes. Other fragrance components can also affect the finished colour of a bar, sometimes quite unpredictably.
So when I’m testing a new fragrance, I’m not just checking whether I like the smell. I’m also interested in how it behaves during mixing, whether it causes any unexpected colour changes, and how well the fragrance survives the curing process. There’s not much point creating a gorgeous smelling bar of soap if the fragrance has vanished by the time it’s ready to sell!
Can I stop fragrances from misbehaving?
Sometimes I can manage their behaviour, but I can’t always prevent it. For fragrances that accelerate, I can adjust my working method. I might avoid excessive stick blending, work at a suitable temperature, keep the design simple or plan to get the mixture into the mould fairly quickly.
If I know a fragrance is particularly troublesome, there’s not much point planning an elaborate swirl that requires ten minutes of leisurely pouring.
With vanillin, I can sometimes design around the expected colour change. or example, I might deliberately use brown in the design, choose colours that will work alongside it, or leave part of the mixture unscented to create some contrast. Take Sugar Drops, for example, a soap I make occasionally. Its fragrance contains a high percentage of vanillin, so I deliberately add it only to the bottom two layers, the black and the brown, leaving the top two layers unscented so they don’t become discoloured.
There are products intended to help prevent vanilla discolouration, but their effectiveness in cold process soap can be limited. Personally I don’t bother with them.
It’s a good job I enjoy a challenge!
So, the important thing is paying attention to how a fragrance behaves in soap, rather than judging it purely on how lovely it smells in the bottle.
I think one of the things that surprises people about soapmaking is just how much chemistry is involved. From the oils and butters I choose, to the amount of water I use, the temperature of the mixture and the fragrances I add, every decision can affect the finished bar.
Even though I’ve been making soap for years, there are still occasions when a fragrance behaves unexpectedly. Fortunately, a batch that doesn’t turn out quite as planned will usually produce perfectly good soap. It might just look a little different from what I had in mind.
So next time you see one of my soaps with a beautifully intricate swirl, spare a thought for the chemistry involved in making it possible. And if you see a bar with a rather more rustic design? Well, perhaps that was exactly what I intended. Or perhaps the fragrance had other ideas.
I’ll leave it to you to decide. 😂
Vicki x
*If this happens to you, one possible rescue technique is to allow the soap to gel in the mixing bowl. As it heats up and becomes more translucent, it can sometimes soften into a Vaseline-like consistency, making it easier to transfer into the mould. This requires experience and careful temperature monitoring, and the finished batch must still be checked for safety. It’s an advanced technique, though, and I’d always recommend beginners start with fragrances known to behave well in cold process soap.
