One of the most frustrating things to see answered in soap forums is the question, “Why did my cold process soap crack?” Even well-known soapers tend to say that cold process soap cracks, because the soap got too hot. Heat is the cause, but it’s not necessarily a sign of too much heat. It can often be a sign of inconsistent heat. Usually you see this when the top of the soap is too cool to expand with the internal heat during gel phase, and it cracks. Many times, insulating your soap will actually prevent cracks from happening.
Clara Lindberg has written a wonderful blog on the topic of what’s hot when it comes to making soap (https://auntieclaras.com/2015/06/overheating-soap/). One of the many things she mentions is water content. I’ve never found any reason to use the standard 38% water as a percent of oils. I actually don’t love using water as a percent of oils. Instead I prefer liquid:lye ratio and lye concentration, but that’s a different topic for another time. 38% water as a percent of oils is way more water than I can rationalize ever needing in cold process soap. More water means a hotter soap. When you control the amount of liquid, or water, you use in your recipe, you’ll have a much easier time controlling the outcome of your soap.
I tend to keep my water content just under a 2:1 water:lye ratio. Then I use my oven to ensure an even and full gel phase, and I even oven process milk soaps. I preheat my oven to 170F, turn the oven off, turn the oven light on, and leave the soap in the oven, uncovered, overnight. When I began soaping this way, my soaps stopped cracking. I honestly thought it was a far-fetched idea to insulate my soap when so many people kept saying the cracking was from overheating, but lo and behold, adding or retaining some of the heat prevents cracking. Teri from Tree Marie Soapworks mentioned that, "Cold soap cracks. Warm soap expands," referencing Clara's blog, and I'm glad I took the advice of these skilled soap makers.
The soap cracks when the top cools quickly and can’t move with the warm soap which is expanding in the middle. The cooler top cracks against the internal heat. If you’re making 100% coconut oil soap, have a high water content, or include additives that make the soap hotter (sugars for example), then it's possible the soap is cracking and may be about to volcano. I've only seen anything close to a volcano once, and it was with a 100% coconut oil dish soap I was making. If your water content is not too high, then it's most likely fine, but exercise caution and keep an eye on your soap until you have built your confidence with this and know what to expect.
What happens when you use the standard water in a soap calculator and add heat through insulation or oven processing? Loads of things can happen, and most are less than desirable. If you want to force glycerin rivers for a design choice, then this might be the way to go. However, the results often may not be what you love. Refer back to Auntie Clara’s blog I linked earlier. I’m not reinventing the wheel by doing my own experiments and documenting them for this blog post when Clara has such a beautifully written blog already. Plus I’ve already seen many of the things mentioned in her blog, and I don’t really want to do them again just for the sake of documenting them.
To summarize, I think it is rarely necessary to go above a 2:1 liquid:lye ratio when making cold process soap. I wish someone had told me that sooner when I was a new soap maker. I can’t even figure out why we tell new soap makers to use the full water amount (Soap Calc standard 38% water as percent of oils) until they get used to making soap. The full water amount is what causes so many problems anyway. To keep your soaps from cracking, remove the wild card of excess water, and insulate your soap. Keep a close eye on it until you know how your recipe will behave, and have a safety plan in place in the event something does go awry. For example, maybe place your mold in a large soap-safe container to catch potential volcanoes. Honestly, I think it’s good advice to have a safety plan in place regardless of your water content while you’re learning how to make soap or when trying something new.
Soap making involves a great deal of trial and error and making adjustments until you find your sweet spot with how you are happy and comfortable making soap. Your main focus is to following safe soap making practices, and then enjoy the learning process.