Most seasoned soapmakers are well familiar with the phenomenon commonly known as glycerine rivers, TD (titanium dioxide) rivers, TD crackle or glycerine separation. The terms are all descriptive of what the phenomenon looks like: translucent rivers or streaks in a body of opaque soap. I’m not a chemist so I won’t attempt to give any conclusive explanation of the chemistry that goes into the formation of glycerine rivers, but when soap goes through gel phase, particles of colour pigment added to the soap seem to gravitate and stick to the greasy, water-repellent tails of the soap molecules while leaving the water-soluble heads of the molecules without pigment. This happens sometimes when soap coloured with titanium dioxide and other colour pigments goes through gel phase – but not always.
Soap with glycerine rivers is not technically inferior to other soap. However, the translucent ‘rivers’ are more water-heavy and will shrink more than other parts of the soap as water evaporates during cure. This can be seen as a slight indentation of the translucent areas. Some soapmakers like the look and ’embrace the rivers’, others think the rivers are ugly and some, like me, would just like to be able to understand the glycerine rivers so that we can make them happen or not make them happen as we please.
Over the years I’ve tried to read up on the reasons for glycerine rivers and I’ve asked more experienced soapmakers about it, but the only decisive answer I got was “it’s because of heat” and “don’t gel if you don’t want rivers”. Well, I wasn’t satisfied with that because I let almost all my soaps go through gel phase and sometimes when using colour pigments I would have rivers – but sometimes not. So yes, colour pigment and heat are prerequisites for glycerine rivers, but I felt there had to be more to the true answer.
Going over my own soaping history I often had glycerine rivers when I first started soaping. That frequency has decreased and I rarely have them at all these days. Interestingly other soapers have said the same thing. So what have I changed? Yes, I have tweaked recipes and I like to go with lighter trace than I did in the beginning, but the one big difference is water discount. In the beginning I would always use a lye/water ratio of at least 1/2.3. Now I regularly use a lye/water ratio of 1/1.4. So, my feeling was that it was water-related. I’ve been talking about this, but yesterday I decided to finally put my hypothesis to the test – properly.
To test the significance of water-content on the formation of glycerine rivers I had to rule out as many other variables as possible. I felt the best way to do this was to test it in one single batch. That way I could make sure that the oils, colour, fragrance and temps including ambient temp and meteorological circumstances were all exactly the same.
To keep my experiment as simple as possible I chose a very basic formula for my test soap: 20% coconut oil, 40% palm oil and 40% olive oil with a 5% superfat. Since I wanted to avoid anything that would cause acceleration of saponification I chose straight lemongrass eo for fragrance (I also anticipated ugly soap for home use and hubby likes lemongrass ). For colour pigment I used 1 tsp of titanium dioxide mixed with a small amount of glycerin for my batch of 1000g of oils.
I added the colourant and fragrance to the oils and stickblended carefully before adding the 1/1.4 lye/water solution which was warm but not hot. I stickblended the batter to very light trace. I then divided the batter into two identical jugs, weighing them to make sure that I had divided the batch exactly in half. Then, while stirring with a whisk, I added enough warm water to the one half for the lye/water ratio to be 1/2.4 in that half.
I wanted to ensure that both halves got equal heat. Since heat travels upwards I felt that placing one half of the soap on top of the other might jeopardize equality and so, using a lengthwise divider I poured both halves of the batch side by side in the same log mould and placed it in a 60C oven for 4h to ensure that the soap would go through gel phase.
I cut it this morning and yes, you guessed it, I wouldn’t have been doing all this talking if the results hadn’t been pretty spectacular! The same batch with the same oils, same amount of colourant, same fragrance and same external heat produced plenty of spectacular glycerine rivers on the water-heavy side but no rivers at all on the water-discounted side. The only other variable here except water content is that I hand-mixed the high-water half a little more than the low-water half to incorporate the added water.
So, for all of those who say “I’d love to get rivers but I don’t seem to be able” this is pretty good news. Just up the water and your chances will grow. And for those who say “I’m bummed because I want to force gel but my soaps are full of rivers” it’s equally good news: by doing a steep water discount you may be able to avoid them. What this experiment does not tell us is where the line is drawn; at what exact water concentration (heat, pressure, formula, fragrance, colourant etc being equal) do glycerine rivers start showing up? But that’s a different experiment and I will leave it up to somebody else 🙂