In soaping forums I see many people looking for help controlling trace or trying to figure out why a batch went wrong. Well-meaning people often suggest that they need to soap while their oils and lye are within 10°F (5°C) of each other. Let me talk about why this answer bothers me.
I frequently make soap with my lye solution around 75°F and oils around 95°F. I am able to create intricate swirls like this. However, if I soap with my lye solution at 150° F and oils at 160°F, I can expect for my soap to move along much more quickly even though they are within 10°F of each other. Then there’s the heat transfer method where you use the hot lye solution to melt your hard oils and butters. Those are just three counterexamples (when you really only need one) for the suggestion to soap with oil and lye between 5-10 degrees of each other. Why do people still push this concept?
I do see reputable soapers mention that the oils and lye should be within a specific range of each other AND mention that they should keep them at a certain temperature. Soap Queen recommends soaping between 120°F and 130°F (https://www.soapqueen.com/bath-and-body-tutorials/tips-and-tricks/back-to-basics-how-temperature-affects-cold-process-soap/). This blog states that books and soapers recommend oils and lye be within 10°F of one another, but I didn’t see an explanation of why. I have not seen evidence that they need to be within a certain range of each other. Modern Soap Making suggests keeping your temperatures below 110°F to help control trace and keep your soap batter behaving nicely. They even mention that your oil and lye do not need to be within 10°F of each other. Here’s a link to their really great blog on the topic: https://www.modernsoapmaking.com/blog/controlling-trace-in-cold-process-soapmaking
Why in the world do so many people suggest the oils and lye be within 5°-10° of each other? Why are we not explaining more frequently a certain range for soaping overall instead of how close in temperature the oil and lye need to be? I know it would be confusing and frustrating to a new soap maker to have their oils and lye within a certain range of each other only to experience acceleration, because they were 145°F and 140°F, which is quite warm. I don’t see where it can be a safety concern, but it does seem like an outdated concept, and it’s frustrating to see misinformation on the topic. Your lye and oils do NOT need to be within a 5°-10° range of each other, but it would be best to stay at a cooler temperature if you want to keep your batter fluid. I tend to soap with room temperature lye, around 75°F, and oils around 90°F-100°F. As you can see, I am able to make some good soap like this. We will all have our own sweet spot as to what temperature works best for us and our recipes, but it probably isn’t going to be based on how close in temperature the oils and lye are.
As always, safety first, and then have fun.