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Why does melt and pour soap ‘sweat’

October 19, 2018

Good question!

It’s been really wet around the Sydney region of Australia lately and as such many people have noticed a strange thing happening with their melt and pour.

I decided to tackle this in a video which you can view below.

If you are more of a word-lover you can read all about it below too. I just want to clarify that making videos is a bit more off-the-cuff than writing so I might say a couple of things in a not 100% perfect way.  A melt and pour soap recipe isn’t majority humectant but the humectants are the part of the formula that gives the majority of the problems.

Anyway, let’s carry on and sorry about that first-frame-face I’ve not perfected the art of looking cool on camera yet…

And the transcript notes:

Typical melt and pour INCI list:

Aqua Glycerin Sodium Stearate
  Sorbitol Sodium Laurate
  Propylene glycol  


  • Heavy in humectants.

Humectants are water binders.

They also like to share water evenly between the formula and the environment.

When the environment is very wet (high relative humidity) they may sweat as there is more water outside the soap than it can handle. It attracts the water but the humectant can’t accept the water because it’s already swollen full like a soggy sponge.

There are lots of different humectants, common ones being.

  • Glycerin most common in cosmetic MW 92 g/mol
  • Very popular in M&P Soap. MW 182.17g/mol
  • Propylene Glycol. Often cheapest and best at binding water 76 g/mol
  • Honey – mix of fructose and glucose mq 180 g/mol

Smaller MW = higher water binding capacity. That is because in every gram of product there are more molecules of humectant each with their teeny tiny hands out waiting to grab the water!

The higher the MW = lower water binding capacity so sorbitol holds less water per g than glycerine.

If you are the formulator of a melt-and-pour base you can create a more humidity tolerant product by playing  around with different humectants to get the best skin feel, price and humidity resistance while not affecting the cleansing power of the bar or its appearance.

If we graph the relative humidity of different percentages of glycerine solution we see, somewhat weirdly, that the lower % of glycerine solutions have a higher relative humidity. A 10% solution graphed at around 98% RH whereas a 50% glycerine solution was at 84%.  Most of the time the relative humidity of the air is between 40-80%.  Humidity less than the soap will not lead to beading, it is only when humidity is higher, so if we formulate with that in mind we should keep our glycerine (or glycerine equivalents) at 50% of more of the formula.  Practically that could be quite hard hense the reason that more powerful humectants such as propylene glycol are used,  so the percentage of humectant can be low but the formula relative humidity can remain high.

Glycerin action

Manufacturing / melting proceduce influence.

When making melt-and-pour if you over-heat your mixture you evaporate off some water, this concentrates what’s left thus making your humectants a higher percentage weight of your formula than before.  This can make your soap MORE likely to bead so don’t over heat!

The weather.

The air can hold different amounts of water at different temperatures but that’s more of a bother to us than the soap!

As the temperature goes up so does the airs capacity to hold moisture.  A 20C day  at 80% relative humidity has 13.8 g water per m3 on average whereas at 30C, 80% humidity air contains 24.3 g per m3 water even though the relative humidity is the same.  This temperature difference and heavy air can make us feel more yucky as the temperature rises but it doesn’t necessarily bother the soap as the soap is only interested in % relative humidity.   When the relative humidity inside the soap is less than the air humidity outside of the soap the soap will attract moisture to it and you see this as beads. These beads don’t necessarily damage the soap but do make it feel slimy and wet and look unattractive.

For a product seller or formulator, it’s the relative humidity that’s important as that’s what triggers the soap to bead, not the temperature.  The only other thing that a higher temperature can do is (perhaps) make the soap more prone to mould due to the higher amount of water around. But that’s another problem.

Most melt-and-pour soaps will attract moisture at some point and while unattractive, this is quite normal.  If you don’t have the time or lab space to do any experimenting you can either seek out a humidity tolerant ready-made melt and pour base or you can shrink-wrap your finished soaps.   Another last thing you could try if you are in a high humidity area all the time is popping the ready-made soaps in a dehydrator after manufacture as this will help take out any excess moisture that has been introduced to the soaps by the environment.  The soap should then have a little excess capacity to deal with a bit of outside moisture, at least for a little bit.

So there we have it!

Amanda x

One Comment leave one →
  1. Deepa Chauhan permalink
    January 20, 2020 8:59 pm

    Very informative Thanks 😊

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