“I had decided to do what I shall call a Ghost Swirl; a technique where a deliberate design is ’conjured’ into the soap without the use of any added colourant. It’s not essential whether the design is swirled with a skewer or a hanger, poured or spun, in the pot or in the mould. The essential thing is that contrasting shades are achieved purely by manipulation of water content and heat.”
I’ve talked about soap challenges before. I like challenges that inspire me to think a little harder, try new things, and push beyond what I do in my every-day soaping. For me it’s less about competing for sponsored prizes and much more about embarking on a personal creative journey, winning insight and experience.
In a Facebook group I run we regularly run challenges and so I know it’s tricky to set the theme for a good soap challenge. If the theme is too loosely defined and the directions too vague, people don’t know what to do with it. If the focus is too narrow (as in “Make an at-least-five-but-no-more-than-thirteen-colour piped soap emulating your paternal grandmother’s second best quilt”) it easily stifles imagination and creativity. Some kind of middle ground usually works best: define a clear framework, but leave plenty of space for interpretation and innovation within that framework.
This week I’m participating in a plain soap challenge. The instructions are clear; no colourant, no fragrance, just plain soap. Milk soaps are allowed.
That’s my kind of challenge, the kind I like; make whatever you like from simple ingredients and let the soap rather than the additives do the talking.
And I love plain soap. A good, well-made plain soap is sexy. It’s not hiding behind any multicoloured veils, it’s unapologetically naked in all its beauty – and begging to be used, not put on the mantle piece. No soap is better suited for showing off shape, texture and detail from a beautiful mould, and stamps seldom look better than they do on plain soap.
Now, this being a challenge I wanted to challenge myself and do something I hadn’t done before. I’ve done a lot when it comes to plain soap, plain soap and I are great buddies and hang out regularly, so this had to be plain soap with a twist.
As I was thinking about what I had done before it occurred to me that a year ago, while I was experimenting with water discount as a design tool (you can read about those experiments here), I had tried titanium dioxide (in the glycerine river experiments that you can read about here and here), mineral pigment and plant pigment. But I had never tried it without any colourant at all. In this present challenge I was allowed milk, but for my purposes milk would just be another colourant and I now had my heart set on trying my water discount technique without any colourant at all.
I had decided to do what I shall call a Ghost Swirl; a technique where a deliberate design is ’conjured’ into the soap without the use of any added colourant. It’s not essential whether the design is swirled with a skewer or a hanger, poured or spun, in the pot or in the mould. The essential thing is that contrasting shades are achieved purely by manipulation of water content and heat.
For this project I wanted to make a design with very fine lines and details to show off the contrasting shades. I chose a modified mantra swirl (which is what I used to call this type of swirls when I first started doing them long before I had ever heard of a Taiwan swirl 🙂 ).
To make a swirl with fine detail I needed a nicely fluid soap batter and so I began my project by choosing an oil formula that would not accelerate trace – even when mixed with a highly concentrated lye solution. I settled for a 1kg batch of avocado oil, castor oil, cocoa butter, coconut oil, olive oil and rice bran oil (in alphabetical order). Once I had decided on the oil formula I calculated the lye and divided it in three equal parts for three lye solutions. Two of the solutions I made to the strength of 44% NaOH and 56% water and one to the strength of 30% NaOH and 70% water. Then, I let all three solutions cool down to room temp.
Big difference in water content there. All three solutions in the picture contain the same amount of NaOH, but you can see from the volume that the solution in the middle has more water than the others. (The droplets on my plastic counter top cover are not lye solution, they are plain water from the outside of the containers that had been standing in cold water to cool down 🙂 )
I melted my hard oils and added my soft oils. I carefully stickblended everything in a big bowl and divided the oil mixture by weight into three equal parts.
While the oils were cooling down to room temp I lined my mould and inserted two dividers.
Once everything was room temp I added the lye solutions to the oils and blended each of the three batters to very light trace. The low water batters were poured in the side compartments of the mould and the high water soap was poured in the centre. I then pulled out the dividers and used a skewer to make a fine swirl. I was way too focused on my pouring and swirling to take any in-between pics, sorry about that 🙂
In this pic the soap is freshly poured. You can see how the high water soap from the centre section is lighter in colour than the low-water soap from the sides. This is typical in raw soap batter. I assume the water droplets suspended in the emulsion reflect more light than the oil does. The more water in the emulsion, the lighter the colour.
Once the soap had set up slightly (the soap was still glossy, but didn’t move when I moved the mould) I transferred the mould to my pre-heated 60C ( 140F) oven.
These are three phone pics ( these are pics for my personal records, please excuse the bad quality) of the soap in the oven. You can follow the gradual change in colour.
Here the soap has just been transferred to the oven; still wet and the high water soap clearly lighter than the low water soap.
Here the soap has been in the oven for 45 minutes. The low water soap is light in colour and the high water soap is darker and moving into gel.
After one hour. The high water soap is now in full gel. Note how the centres of the ’leaf’ shapes in the high water soap are lighter than the surrounding high water areas going through gel phase. Some soda ash has developed – no doubt encouraged by my frequent opening of the oven 🙂
At this point I turned off the heat, left the oven light on and went to bed.
Next morning I unmoulded the soap and cut it – first vertically in blocks and then each block horizontally.
This is what I got:
That’s pretty spectacular considering that there was nothing but masterbatched base oils, NaOH and water in this soap.
So how does this work? Why is the high water soap darker in colour than the low water soap?
Simply put the answer is that the difference in water content influences the pace at which the soap saponifies and how the soap reacts to ambient heat. The difference in reaction to ambient heat results in different gelling behaviour which in turn results in a shade difference in the final soap.
Those who have read Kevin Dunn’s book Scientific Soapmaking carefully, will know that (everything else equal) a low water soap goes through saponification faster and enters full gel phase at a higher temperature than a high water soap does. In this case the 60C temperature kept in my oven was not high enough to force the uncovered low water soap to enter full gel phase. The high water soap, however, did go through full gel at that temperature. But, even though the low water soap did not go through full gel phase it probably was fully saponified before the high water soap was.
So, by manipulating the water content I was able to manipulate the gelling behaviour and by manipulating the gelling behaviour I was able to manipulate the relative depth of colour in the final soap. That’s pretty cool.
I need to point out a couple of things here. The ambient temperature is crucial. 60C seems to be a sweet spot where high water soap gels and low water soap doesn’t. Raising the temperature with say 10C might well change things because when the temp is raised high enough the low water soap will eventually enter full gel too. Whether it will end up looking exactly like the gelled high water soap I don’t know. Exactly at what ambient temp this low water formula (without additives in a batch and mould this size) enters full gel phase I also don’t know because soaping with a big water discount and being more keen on quick saponification than actual gel phase I usually just oven process at 60C. Different oil formulas and various additives like milk (sugar), salt, clay or fragrance may also cause changes in gelling behaviour.
The other thing is the relative difference in strength between the lye solutions. The smaller the difference in water concentration the closer the gelling behaviour of the soaps will be. So, if you want to make sure that the soaps show different gelling behaviour you want your low water soap to be properly low in water. You should, however, never attempt a lye solution stronger than 50%.
What I find particularly fascinating about this Ghost Swirl is that the visual effect is not just two-tone (from gelled and ungelled soap as can be expected); it’s multi-tone. On the one hand you get the difference in shade between the low water and high water soap, gelled and un-gelled soap, but you also get that very interesting dark contour line everywhere were low water soap borders on high water soap.
Exactly why that line looks as dark as it does (it’s well darker than the gelled soap in general) I don’t know. My guess is that the temperature fluctuations on each side of that border line (by the time the high water soap is reaching its heat peak, the low water soap is already well in the process of cooling down) cause continuous pressure and tension changes which result in a crystal structure different from either the main portion of the low water soap or the main portion of the high water soap. Different crystal structures will reflect light differently and therefore look different.
The fact that you can make well-defined designs and swirls inside soap without any colourant is pretty cool, but once again the most interesting thing here may well be the way the top looked when I unmoulded the soap the next morning.
As you can see there’s a good coating of ash on the surface. And not a single speck of that ash is on the low water soap. That’s interesting.
Let’s recap: both the high and the low water soaps have an identical oil formula and superfat, both soaps were poured at light trace and both spent equal time uncovered in the same ambient temp in the oven. The high water soap, now covered in ash, went through full gel phase and the low water soap, now free of ash, didn’t. I don’t think full gel phase as such increases the tendency for ash to develop, but it doesn’t seem to prevent it either. The significant difference here is the water content and the ash development is exactly in line with what I’ve observed before: everything else equal ash seems more likely to happen on high water soap.