What’s Hot and What’s Not – A Study In Overheating

What’s Hot and What’s Not – A Study In Overheating

posted in: Auntie Clara's Blog, Blog Post | 55

Sensitive viewers be warned. This post is about soap overheating in various ways and features ugly soaps galore. For pretty pics I would suggest a meander through Auntie Clara’s photo gallery, a run through our Facebook photo stream, or a glance through our Instagram feed 🙂

In the world of handcrafted soap there’s recently been plenty of discussion about soaping temperature. Mixing cold process soap at hot temperatures may be a risky and unsafe method for beginner soapmakers but it may be an acceptable method with some merit for soapers with thorough experience and a good understanding of the soapmaking process. Too hot for one may not be too hot for another and seen from this perspective safe temperature is relative. But soapmaking is a complex art and there are other ways, too, in which temperature in soapmaking can be regarded as relative.

After my glycerine river and water discount experiments which you can read about here and here, I decided I wanted to learn a little more about how cpop or cold process oven processing (keeping cold process soap in a warm oven to speed up saponification and force gel) in combination with water discount affects soap. And so I’ve been experimenting with these things over the past year.

In the second glycerine river experiment I noticed texture distortion on the top of the soap. On the half of the log that was made with low water (left side in the pic), the top was beautifully smooth making a perfectly smooth background for my inverted stamp squiggles. On the ’river’ half, made with high water, the surface was initially smooth but puffed up in the oven and took on an uneven pattern – like cellulite. This was a little puzzling and so I decided to see if it was a random incident or if I could recreate the same effect in another experiment. This is the soap I made for the second glycerine river experiment:

Glycerine river soap by Auntie Clara's

 

Alien Brains and Silicone Rash

For my new experiment with heat, I chose my trusty oval silicone friand moulds. Saponification is an exothermic reaction, i e it generates heat. Soap is a good heat insulator and so the heat generated through saponification builds up inside the body of soap until it eventually finds its way out of the soap. The smaller the body of soap, the easier it is for the internally generated heat to escape. Likewise the distribution of volume in the body of soap (i e the amount of surface area per volume) will affect how quickly heat will be able to escape. 1kg of soap in a cube-shaped block mould will keep heat trapped inside for a long time, but 1kg of soap spread in a thin layer over a large area will cool down much faster.

My friand moulds are small (about 90g cured weight) which means that internally generated heat will escape quickly and the ambient heat in the oven will have a more direct impact on the temperature of the saponifying soap.

I used a simple formula with olive, coconut, palm and castor oils plus some non-accelerating essential oils. As with the second glycerine river experiment I mastebatched the oils and the additives and made two lye solutions, one with a 1:1.5 lye:water solution, and one with a 1:2.5 lye:water solution, both of which cooled down to room temp. I then mixed half the oils with the one lye solution and half with the other lye solution, which gave me one low-water soap and one high-water soap. I poured both soaps into their moulds at very light trace and waited for the viscosity to increase enough to mark the two soaps each with its own inverted stamp.

Then the soap in the silicone moulds went into a pre-heated 60C (140F) oven.

60C or 140F is lower than the 170F or 77C often suggested for cpop (cold process oven process), but my proofing oven keeps that temperature and I find it very useful for keeping soap nicely warm during saponification. Using my normal paper lined wooden log moulds and a low water formula I can keep soap at that temp for a few hours by which time the soap has passed its heat peak and doesn’t zap anymore. When using individual silicone moulds I usually avoid keeping the heat on for extended periods of time, but here I wanted to study the effect of doing just that.

After about 1,5h in the oven (heat turned on) the soap looked like this:

Auntie Clara's: high water soap going through gel phase

The high-water soap in front is at full gel and the low-water soap in the back has passed its heat peak and is in the process of cooling down while being kept at 60C by the ambient temp in the oven.

To refresh memory, lets take a look at the typical heat and phase progression of low and high water soaps in general: Everything else equal high water soaps take longer to saponify and therefore to generate heat than low water soaps. In low water soaps saponification is relatively quick and more heat is generated in a short space of time resulting in a quick peak of higher temperature. On the other hand, high water soaps enter gel phase at a lower temp than low water soaps, stay in the gel phase longer, and take longer to cool down. For anybody interested in reading up on this I recommend Kevin Dunn’s book Scientific Soapmaking; The Chemistry of the Cold Process (2010).

But for me the interesting thing in the picture above is the puffing going on in that gelling high water soap – and the total lack of it in the smooth low-water soap. Once the soap had cooled down and was out of the oven, the high water soap looked like this:

Musical 'Alien Brain' soap by Auntie Clara's

Although I quite like my G-clef,  I’d say the oval shape in combination with the fleshy colour and the undulating ’veins’ make for a rather unfortunate design – bordering on the rude 😉

So, what is the explanation to this change in surface texture? I believe it’s a fairly simple case of water expansion. When water heats up it expands. This is what makes steam engines work and what causes volcanoes in soapmaking. It’s also what makes hot process soap shrink in the mould as it cools down.

Aided by the superimposed heat in the oven, the water in the gelling high water soap expanded and caused the surface to puff up resulting in this texture sometimes referred to as ’alien brains’. In the low water soap there was simply much less water to expand and so the surface remained more or less intact. The top of the high water soap ’overheated’, but the low water soap didn’t. Yet they spent the same amount of time in a 60C oven.

Another interesting outcome of this experiment was that even though the top of the low water soap came out looking pretty and smooth, both the high water and the low water soaps had severe cases of ‘silicone rash’ on the sides.

Silicone rash is the uneven, ’pock mark’ texture that occurs on soap making contact with hot silicone. Typically this happens when oven processing soap in silicone moulds. Here you can see clearly how ’blisters’ have formed on the inside of the silicone mould and how the soap has a fine bubble texture in each blister.

Silicone rash by Auntie Clara's

 

So, even if a steep water discount can help prevent glycerine rivers and ‘alien brains’ in oven processed soap, it doesn’t appear to be an effective means to prevent silicone rash. I thought this was interesting and so I decided to have another go at comparing high and low water soaps oven processed in individual silicone moulds.

 

Speed Up with Pine Tar

This time I made a pine tar soap with plenty of tallow and as before I added a low water lye solution to half my oil-tar mix and a high water lye solution to the other half. Pine tar accelerates trace considerably so I poured at thicker trace this time than the previous time. By the time the soap hit the 60C oven a few minutes later, it had set up properly and was very firm. The soap spent about 1,5h in the oven, heat turned on, and was left to cool down in the closed oven.

The outcome was interesting again. All the bars, low and high water alike, had super smooth sides and there were no signs of alien brains anywhere. The formula was different, but a big difference to the previous experiment was also the higher viscosity of the soap as it was introduced to the oven heat.

Pine Tar Soap by Auntie Clara's

So, is silicone rash in oven processed soap prevented more by high viscosity than by low water content? (obviously, the most effective way to prevent it is to not use silicone moulds)  Yet another experiment was called for.

 

Thick and Thin

This time I made one batch of low-water soap. I blended it (oils and lye at room temp) to emulsion and poured half the soap. Then I blended the rest to thick, but still pourable trace. The soap spent about 30 minutes in a 60C oven, after which I turned off the heat.

Silicone rash on soap poured at light trace

The bars on the left were poured at emulsion and the bars on the right were poured at thick trace. In this low-water soap that spent a relatively short time in the oven you don’t get much of the ’mini bubble’ texture inside the blisters, but other than that the silicone rash presentation is typical: conical craters with a shallow incline next to the silicone wall and a steeper incline towards the vortex. In contrast the series of tiny holes in the top right bar look quite different: the edges of the holes are clearly defined and at a steep angle to the silicone wall. These holes were not the result of soap blistering on hot silicone but happened as air was trapped in the soap as it was being poured into the mould.

Based on this it looks as if the viscosity of the soap is a more important consideration than water discount when it comes to preventing silicone rash. Whether the thermal conductivity of the soap batter changes as saponification progresses and oil and lye is turned into soap and glyceroI I don’t know. If it does, that could be an explanation to why the firmer soap seems less affected by the contact with hot silicone. Firmer soap also tends to be warmer and the smaller differential in temperature between the soap inside and the hot silicone could be another explanation. By letting the soap rest for a while after pouring and before introducing it to a hot oven you let it get firmer and that may help prevent blistering.

 

Soap, Sweet Soap..

High water soap seemed to be more prone than low water soap to overheating while being exposed to oven heat, but so far all these experiments had been made with basic formulas without additives particularly prone to heating up in soap (pine tar accelerates trace even at very low concentration, but at low concentration it’s not particularly prone to causing cracks or volcanoes in soap). Now I wanted to really put this to the test by adding sugar to the equation.

Sugar (as in cane sugar, beet sugar, honey, milk and beer) burns easily and is prone to heating up in soap. The general recommendation is to keep temperatures low and not to insulate (and definitely not oven process) soap made with e g milk because of the risk that the soap will overheat causing cracks on the top, separation and hazardous volcanoes at worst. It’s a good recommendation and for the beginner who makes soap with maximum water I would suggest following this recommendation. The following experiment will show you why.

The formula was again a blend of olive, coconut, palm and castor. To my 1kg of oils I added 20g of fat free powdered milk and 20g of honey. That’s plenty of sugar for one batch of soap. The oil mixture and lye solutions were brought to room temp. I added a 1:1.5 lye:water solution to the one half of the oil mix and a 1:2.5 lye:water solution to the other half. Each half of the batch was poured at thin trace. Then each half of the batch was given its own inverted stamp squiggle and the soap was put in a pre-heated 60C oven.

Milk & Honey soap freshly poured

Milk & Honey soap freshly poured

After 30 minutes in 60C the high water soap with the G-clef has begun sweating. The low water soap in front is darker than the high water soap and is looking OK.

High water milk & honey soap sweating

After 45 minutes the low water soap still doesn’t show any signs of overheating. The high water soap seems to be in full gel – and oil is beginning to pool on top.

Overheated high water milk & honey soap

This did not look good so I turned off the oven and let the soap cool down.

 

The next morning my oven processed batch of milk & honey soap looked like this. Low water soap on the left, high water soap on the right:

_DSC6760

Overheated milk & honey soap

 

I’d say it’s a pretty remarkable difference, given that water content is the only thing that distinguishes these two soaps from one another.  They were made from the same master batched oils and additives and they shared their time in the oven, yet the high water soap overheated spectacularly whereas the low water soap is perfect. Exactly how the high water soap separated at 60C when the low water soap didn’t I can’t explain. Water expansion coinciding with heat development in a particular phase is likely to be the explanation, but I’ll leave it to the chemists to explain what happened on a molecular level.

Interestingly the sides of the low water soap are perfectly smooth without any signs of silicone rash even though the soap was poured at very light trace and spent 45 min in a 60C oven. The explanation could be that I had to wait about 20 minutes for the high water soap to thicken up enough to make a decent inverted stamp on it (a bit of a waste in hindsight, but I had no way of knowing beforehand how successful my attempt at destroying this soap would turn out to be.. 🙂 ). In that period of time the low water soap may have firmed up – and heated up – enough to withstand blistering in the oven.

 

Overheated milk & honey soapCaustic disaster inside the high water soap..

Overheated milk & honey soap..but a rather fetching stalagmite structure in this perfectly formed heat cave.

Please note that these soaps were part of a small batch in small moulds where internally generated heat escapes easily. To keep a high water, high sugar soap like this in a big mould in a heated oven for an extended period of time could result in volcanoes and serious damage to the oven. Also, I used an oven temp of 60C in all the above experiments. Higher temps than that may well cause overheating in low water soap too.

 

Conclusion

In oven processing cold process soap, correct oven temperature is relative. What the right oven temperature is and for how long you can keep the soap in this temperature without overheating, depends to a great extent on the composition of your soap. Also, the size of the oven, the size and shape of moulds and the amount of soap kept in the oven at a given time will affect temperature and heat retention.

As far as water content goes, low water soap can take higher oven temperature and the same oven temperature for longer than high water soap can. It’s pretty intuitive that soap without heat sensitive additives can take more oven heat than soap with heat sensitive additives, but it’s not quite as intuitive that soap with heat sensitive sugar can take more oven heat if made with less water. Yet, that seems to be the case. If it is the case, it would be interesting to know if it holds true beyond oven processing: e g is high water cp soap more prone in general to making volcanoes than low water soap, everything else equal?

For now I’ll stick to what I said earlier: when working with full water and additives prone to heating up, keep temperature low. When deciding on what heat strategy and thermal management scheme to choose for your cold process oven process project, consider the composition of the soap as well as the volume and size, shape and material of the mould. What’s too hot for one soap may not be hot for another one. And, everything else equal, shorter heat exposure and letting soap set up before oven processing may help cut down on silicone rash. Finally, cutting down on water may seem like a daunting prospect, but when working with additives that heat up in soap, a sizeable water discount might in fact be a prudent precaution. Discounting water may cause the soap to set up faster, but with less water heating up and expanding, it may also prevent cracks and volcanoes from happening.

Overheated milk & honey soap

55 Responses

  1. Once again great research!

  2. Very interesting synopsis higher water content allows all types of travel and mischief in soap! Love the heat tunnels much like monster teeth or cave dwelling formations always cool to look at in photos! So this begs the question about those notorious floral fragrances that move quickly. Would they benefit with a low water soap and slow moving oils? Always a pleasure Clara thank you so much.

    • The answer to your question will depend on what your aim is. If your aim is to avoid signs of overheating (volcanoes, cracks ets) a steep water discount might be the right way to go. If your aim is to keep the soap fluid for as long as possible to be able to add colour and do fancy, fluid swirls, a steep water discount is not going to help you. Like so often in soapmaking, perfection is about making exactly the right compromise 🙂

  3. Cathy Gordon

    Extremely interesting. Had to go back and read your other articles again, also. Thank you for publishing your results. I will definitely take this info into consideration in OP.

  4. Thanks for the fascinating experiments and great explanations!

  5. ….And wonderful photos too!!

  6. Hilda Bahner

    Fantastic article , thanks !

  7. Clara,

    Gosh, I really enjoyed your article. Well written and well thought out. I have a question and a few thoughts.., I also keep Kevin Dunn’s book close at hand. I understood that all soaps go through gel phase, and you’ve proven some come out the other side very different. If that’s so, would the process of keeping the soap at a lower temp before pouring and then adding heat, therefore allowing it to begin to thicken, slow the molecular change or cause the molecule to connect differently? I know you said you’d leave that part to the scientist (which I am not) however, I wondered if you saw that and if you thought, maybe, water interferes with that process or is it just the expansion of the water. And, I believe, you’ve proven that water percentages (-/+) changes how those molecules connect. (I’m referencing page 309 in Scientific Soapmaking). I hope this makes sense, I truly would love to understand this… However, thank you for your insightful article and for doing the experiment.

    • I would love to understand it too! 🙂 Not all soaps go through gel phase. In order for a soap to enter gel phase it needs to reach a certain temperature. What that temperature is depends on the composition of the soap; the oil formula, the water content and possibly also additives like salt, sugar, clay, colourants and fragrances. So, just because you oven process a soap with 60C oven heat on for an extended period of time, it doesn’t necessarily follow that the soap goes through full gel phase if the soap is made with a low water lye solution. Figure 22-1 on page 307 illustrates this nicely. I’m sure the water content influences the molecular structure and stability of the soap, but I don’t know how.

  8. Thanks Clara for that information, something to think about when doing coop, I must be doing something right as I have never had any of that happen, however I warm my oven pop in the soap and turn it off because I’m a little scared of overheating.

  9. Thanks for sharing Clara! Very Interesting Info!

  10. Thank you so much for your time and effort in posting your findings. Very informative and thorough! Your contributions (including this posting and your study on water discount) are appreciated in our soaping community. Love your warning to sensitive viewers LOL

  11. Thank you very much for sharing your studies! These articles have been enlightening and most helpful. 🙂

  12. Fantastic article! Thank you

  13. Kerry Lamb

    Great info as always. I was wondering what brand of silicon moulds you use? I purchased 10 x red silicon ones some time ago but even after half a dozen batches the red was still leaching pink on to the soap. Usually I use column moulds but I am a curious child 🙂

    • Mine are from Silikomart and with these I’ve never had any problems at all with colour bleeding from the mould. I have another red mould from another supplier, and if I leave the soap to stand in that mould for more than 12h it will discolour from the mould. Not all silicone is the same. These are very good.

  14. Great post!

    What about the pH? I made a soap cpop with 40% lye solution and it came out with high ph and it wouldnt drop, its been more Than 3 weeks now and I stil have ph=10.

    • Provided that you have at least the same weight of water as NaOH in your lye solution (less water and your NaOH will not dissolve properly) the amount of water should not affect the pH. In the saponification reaction water is merely the catalyst needed to ‘introduce’ the NaOH to the oils. You can look at the water as a friend introducing two strangers. Whether the friend is tall or short doesn’t really matter; as long as she’s present (a 50/50 lye/water solution is enough for that) introductions can be done. If your soap contains unsaponified lye it’s likely to be due to incorrect calculation of lye, incorrect measuring, insufficient blending of lye and water or lye solution and oils – rather than to the water discount.

  15. Debbie Rich

    Thank you so much Clara! May I ask, the tiny brown specks……. Is this the honey? I made a high water soap and HP’d in the oven. The soap had honey and milk in it. I used the heat normally recommended. Well, it sure was not long before I had those brown spots all over. I tried and tried to get it out. Definitely separation. I finally gave up, trying to save it. I doubt there is enough lye. Smells odd, lol. I did find a German man, who did a tutorial on Salting OuT. Saved for another day down the road.

    But am curious if this is just the honey….the milk….or both?

    I did decide that if I ever did try that method again, much less water!!! 🙂 less heat, definitely!!! Just never want to make the brown spot mistake again. 🙁

    So far, my best batch, a basic palm freeMilk Soap…..no other additives but Kayolin,TD, Oxides……..Split Method……Is Perfect!!! My picky son loves it! 🙂 I CPOP,d. I do wish I had picked a less “manly scent”. For me 🙂 ha!! 🙂

    Thank you again Clara …….I have studied very hard. And before I set out on my soap journey. But I have learned much more from you…..much more detailed. And very appreciative of your kindness.

    • Honey crystals can easily give brown specks in soap. The best way to avoid brown honey specs is to make sure that the honey is well diluted in water before mixing with the oils so that you don’t have any crystals left.

  16. Interesting experiments Clara! Thank you for posting. I’ve been soaping for 2 years and I’ve never CPOP’d; today I will follow some of the guidelines in this article to make my first CPOP soap. Not my strong point, but I’ll try to take pictures and keep copious notes.

    • I hope your first experiment went well. Cpop works well for me because it’s a way of controlling saponification temperature here in my house where room temp varies a lot depending on the season. I hope it can work well for you, too.

  17. Clara This is another great article packed with very valuable info. Thank you so much for reporting. I was wandering why some of my soaps had curly tops, now I know. It is going to be hard to find the correct proportion of water and heat to gel soaps without overheating too much. especially as it depends on the oil mix and the additives. One really has to make very precise notes of each soaping process! Many thanks! I am looking forward to your next post !

    • You are right; when it comes to overheating there are many variables to consider. Being aware of this is a first step on the path to being able to control it. I’m glad if my post helps you in the right direction 🙂

  18. […] been reading about the effect of water content on Aunty Clara’s blog, and I thought that with all the sugar in my soap, maybe low water content is the way to go, […]

  19. Brilliant post once again Clara!

  20. Dear Clara,

    A big fan of yours from Korea. Due to language problem, I don’t think I fully understood your post this time, but ejnoyed to read.
    May I ask you one question? I wanted to put rose petals and cranberry seeds on top of my cold process soap, but they all turned out to be black or brown colors due to heat from gelling. And I found out that it was recommandeded to put the batch right into frezer to prevent gelling, but not sure if this is going to be OK. Sorry to bother you, but would be highly appreiacted if you answer me. Here in Korea, people say that preventing gelling is not recommnaded as we beleive the quality of soap will not be good as the one with insulating. Thanks a lot!! Lea

    • You can definitely put any soap in the fridge or freezer to prevent gelling. However, the most efficient way to make sure that the soap does not enter gel phase is to do all of these things:
      1. Soap with room temperature oils and lye
      2. Do a steep water discount
      3. Don’t use ingredients or additives that cause acceleration
      4. Keep the soap cool, in the freezer or refrigerator
      Refrigerating and doing a water discount are not alternatives for avoiding gel; they work well in combination.

  21. Amazing work, your experimentations are pieces of art and you are helping so many soapers with your research. Thank you for that! You are an inspiration 🙂

  22. I love your designs on top. Is that just done at really thick trace ?

    • The designs on top are made at medium to thick trace. You’ll find a tutorial in my blog post about the inverted stamp technique.

  23. Hi clara

    i desperately want to produce cold process soap which does not go through the gel phase . . i see you mentioned things above to avoid this but can you tell me if my type of mould will affect this and if it is popossible to combine water discount and a low oven temperature whilst still avoiding the gel phase?
    thank you

    • Soap enters gel phase when it reaches a high enough temperature. What that temperature is depends on the water content in the soap. The higher the water content the lower the temperature needs to be for the soap to enter full gel phase. I.e. by reducing water you can prevent soap from entering gel phase at relatively low temperature. How heat builds up in soap depends on a number of things: the oils and additives used, the mixing temperatures, the amount of agitation before pouring into the mould, the material and thickness of the mould, the size and shape of the mould, the temperature of the mould, the way the soap and the mould is insulated, any external heat sources etc. So, in order to avoid gel you need to combine the What and the How. You need to reduce water so that the temperature threshold for gel phase becomes higher and then you need to manage the heat development so that the soap doesn’t reach that critical temperature. The smaller the moulds the less heat will build up in the moulds during saponification. Because heat travels upward wide, low moulds retain less heat than upright moulds. Single bar moulds made from thin plastic will typically be at one end of the spectrum and massive, upright block moulds will be at the other end of the spectrum. Avoiding gel phase while oven processing is possible if the soap contains minimal water, the temperature is kept low (I don’t go over 60C at any point) and the moulds have a relatively large surface area.

  24. I just made a batch of soap that came out looking just like your ‘caustic disaster’ picture: complete with void, little teeth and leaking fluid–is the oily fluid glycerin that has separated due to the addition of something? in my case i added, honey, oatmeal, cinnamon and cloves to a soap batter containing Crisco, olive oil and coconut oil. goat milk was my fluid for the lye solution. i did use a little more milk in the lye solution because i didnt want it to trace so quickly.
    i make my soap in a long box mold and wrap it in a towel. i have only had this happen once before, same basic recipe but with powdered sage, citronella, tea tree, lemongrass and eucalyptus essential oils.
    i cant sell it this way but is it safe to use if i let it rest for several weeks?
    what is the oily leakage?
    what are the crystal teeth growing in the void?

    • Cinnamon and clove essential oil both accelerate trace and milk and honey both contain sugar and heat up a lot. By adding more milk you added more liquid which made the soap more prone to overheating. Together all of these things made the perfect conditions for overheating. Next time you use both milk and honey you’re more likely to save the soap by using less liquid than by using more liquid.

  25. I have to say as a new soapmaker I greatly appreciate the experimentation you and others undertake only to then graciously share the result with the community. THANK YOU.

  26. Thank you! Exactly what I was looking for. I like my soaps to gel, but didn’t know how to do that when using individual molds.

  27. Your research is always detailed and insightful, and your work is beautiful! Thank you so much for sharing.

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