If you’ve ever made a shampoo bar, you’ll know the last step is a rather ambiguous “age the bars” for what can sound like a rather random length of time. I decided I needed to know how long shampoo bars actually need to age before they’re ready to use. I also wanted to know if that aging time varied between different types of formulations and different ways of shaping the bars. So, I whipped and pressed up an experiment to figure it out with a bunch of spreadsheets and math, and I’m sharing what I learned in this post (and the partner video, of course).

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Why do we age shampoo bars?

Shampoo bars are left to age for the same reason cold process soap is; so they can dry out. Aging is what takes the bar from a soft, pliable dough or crumbly damp mixture to a rock-hard bar that will last a long time once its in use. We want to let the bars age long enough that they’ll be hard and have a long life in a damp shower, but we also want them to be ready to use somewhat quickly.

Even if a shampoo bar formulation doesn’t contain straight water, there will be some water found in liquid ingredients like liquid surfactants and liquid hydrolyzed proteins. Cocamidopropyl Betaine, an amphoteric surfactant I often include in my formulations, contains about 65% water, so if a formulation includes 10% Cocamidopropyl Betaine that means it’ll also contain about 6.5% water.

The questions

  • What’s the optimal shampoo bar aging time? At what point have they lost the most water for the shortest possible drying time?
  • How does the formulation impact aging time?
  • How does the pressing/shaping impact aging time?

My hypothesis’

  • I think chunkier bars will dry out more than the smoother, more condensable bars.
  • I think the hand pressed bars will dry out more than the machine-pressed bars.

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The experiment

I began by making two batches of shampoo bars: my Chocolate Rhassoul Shampoo Bars and my White Chocolate Peppermint Shampoo Bars. To keep things as clear as possible I’ll call them the “Rhassoul” and “White Chocolate” formulations from here on out.

I choose these formulations because they’re quite different from one another in terms of consistency and water content. The Rhassoul bar mixture is quite chunky, using only stick-style surfactants, while the White Chocolate formula makes a much more dough-like mixture as it uses finely powdered surfactants. Water content-wise, the Rhassoul bars contain approximately 3.6% water, while the White Chocolate bars contain about 6.3% water. The White Chocolate bar mixture isn’t noticeably wetter; it simply requires more water to become a soft workable mixture because there’s more surface area due to the smaller particle size of the ingredients.

Once the mixtures were made, I created a selection of bars in a few different ways.

The Rhassoul mixture is so piece-y/chunky that it needed to be pressed on all sides to come together, so I made three cube bars using The Bath Bomb Press and one using the same mould, but hand pressing with all my might.

The White Chocolate mixture is much more interested in coming together (it’s very dough-like), so I made one pressed bar (using The Bath Bomb Press + their round shampoo bar mould), one hand-shaped bar, and one “smushed as much dough as I could into a plastic mould” bar.

I then left the bars somewhere with plenty of air circulation (a wire basket) and weighed them every 24 hours for three weeks to see how much weight they lost, and how quickly.

I used a Google sheet to convert the daily weight loss to percentages, and then graphed the changes over time so we could see which bars lost the most weight, and when each one hit its “most water lost for time passed” point.

The results

Which bars lost the most weight?

Despite the White Chocolate bars containing quite a bit more water than the Rhassoul bars, the Rhassoul lost more weight. After 10 days the average weight loss for the Rhassoul bars was 1.8% vs. 0.8% for the White Chocolate bars.

The bar that lost the most weight was easily the hand-pressed Rhassoul bar, losing 2.9% of its weight in 10 days, while the average for all the other bars after 10 days was 1.12%.

If we compare the bars pressed by The Bath Bomb Press, the Rhassoul bars lost on average 1.44% in 10 days, while the White Chocolate one lost 0.75%. With just one of the White Chocolate bars and three of the Rhassoul bars this isn’t a perfect comparison, but I’d say the difference of nearly double indicates the Rhassoul bars will always lose more water than the White Chocolate bars.

The machine pressed Rhassoul bars were clustered quite closely together, with the % of weight loss dropping as the original bar size dropped. That is, the smallest bar lost a larger percentage of its weight than the largest bar. This makes sense to me as the smaller bar would have a larger surface area in relation to mass than the larger bar (1.07 weight to surface area vs 1.11).

All three of the White Chocolate bars were clustered quite closely together, even though one bar was quite a bit smaller than the other two (so it lost a higher % of weight comparatively). The hand shaped bar lost the least weight, though by quite a narrow margin.

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What’s the optimal shampoo bar aging time?

I’d aim for a doubling of the first day weight loss. Basically: weigh the bar after 24 hours and figure out what % of its weight was lost. Wait for that number to double and you’ll have some pretty hard bars.

The White Chocolate bars lost an average of 0.25% of their weight in the first 24 hours, and then hit 0.51% on day 4. The Rhassoul bars lost an average of 0.54% of their weight in the first 24 hours, and hit 1.07% on day 5. After that point, the loss dropped off quite a lot, with most days the loss being below 0.1% overall weight.

So, if you live somewhere similarly dry, or you’ve got a dehumidifier, 4–5 days should be fine. If you live somewhere more humid, you can figure out what works for you by tracking the weight of your bars for a while; whenever the bar has doubled the water loss from the first day, I’d say those bars are ready for prime time!

How does the formulation impact aging time?

The formulation dramatically impacts how much water the bar will lose. The denser the bar, the less water it loses. The finer the mixture/dough, the more dense the bar will be. So, the finer/smoother your shampoo bar mixture is, the less water it’ll lose.

That said: the actual aging time didn’t vary that much. In the end, the White Chocolate Bars doubled their first-day water loss in about three days, while the Rhassoul bars took four.

How does the pressing/shaping impact aging time?

The doubling time didn’t change much; what changed was how much water was lost, and that ended up depending more on the formulation.

A bar made from finer powders can be pressed or hand-shaped without drying time or water loss changing very much. A chunkier bar that is hand pressed won’t end up being anywhere as densely packed as a machine-pressed bar, and as a result will lose a ton more water in the same amount of time. I wonder about the structural integrity of the hand-pressed bar compared to its machine-pressed counterparts. Will it be more crumbly throughout use as it clearly has more air in it, which means it has more space for water to work its way into the bar? 🤔