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

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:

 

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:

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:

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.

 

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.

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.

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.

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.

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.

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:

 

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.

 

Caustic disaster inside the high water 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.