Why Did My Soap Crack? Understanding the Heat of Soap Making

Have you ever checked on a batch of soap only to ponder “Why Did My Soap Crack”? If so, you’re not alone. This is a common issue in cold process soap making. Let’s dive into why this happens and how you can prevent it.

Cold Process Soap: Cracking from the Heat Within

When a loaf of cold processed soap cracks it’s always due to overheating. Cold process soap can overheat when the soap goes through what is called “gel phase” too quickly or at to high of a temperature.

When cold process soap overheats, it can crack on the top as the middle expands and pushes outwards. This crack allows the heat to escape through the surface of your soap. Often, the heat issues stop at the crack as enough heat was able to escape allowing the soap to cool. However, if the soap continues to heat up, your soap can bubble out of the mold like lava from a real life volcano.

Here are the factors that can cause your soap to crack:

• High temperatures: If your soap mixture or the room temperature is too high, your soap may overheat and cause the top to crack, or even the soap to volcano in the mold.
• Insulating too much: Wrapping your soap mold in towels or blankets, forcing a gel with CPOP or heating pads, can cause the soap mass to retain too much heat.
• Recipe factors: Using ingredients that naturally heat up, like honey or milk, can contribute to overheating.

Solutions to Prevent Cracking and in-the-mold Volcanoes

Let’s take a deeper look into each one of the overheating and crack causing issue above and figure out how to prevent your soap from overheating!

Keep Soaping Temperatures Low to Prevent Overheating.

If you’re wondering why your soap cracked, it’s probably the temperature. This includes the temperature of your oils, lye solution, the room temperature, and the temperature of the surface your mold is sitting on!

• For cold process soap, I typically soapy at “room temperature”. This really just means that
  • my lye solution container is cool to the touch,
  • I don’t heat my liquid oils,
  • and hard fats are warmed just enough to be fluid.
• I keep my soaping room at about 76-78F. Any higher (for me) and some of my recipes will quickly heat up, crack, and/or volcano out of the mold. We’ll talk more about additives and formulations below.
• Keep the mold and soaping surface cold for sensitive formulations that are prone to overheating.
  • I have stainless steel tables for my work surface which conducts heat (and cold). If it’s an extra sensitive recipe, I’ll put icepacks on the counter tops to cool them down, and thus cool down the soap.
  • Ice packs can also be used to keep things cool on a “typical” kitchen countertop where the molds sit flat on the surface. Molds with an elevated bottom allow more airflow, and airflow helps keep things cool. You can see how I set up icepacks in the video on this article about the gel phase in cold process soap.
  • Which brings me to fans! If I’m making a slab of 100% coconut oil soap (which gets really hot really fast), a fan is a must to disperse the heat generated by the slab of soap.
  • And yes, if you have a refrigerator or freezer that your mold fits into, and where your soap wont be mistaken for food, and won’t come into contact with food (think overheating or spilled soap), then you can also place your mold in fridge or freezer. I’m not a huge fan of this option as I don’t like moving molds filled with soap if it can be avoided.

Recipe Factors that Can Lead to Cracked and Overheating Soap

Using ingredients that naturally heat up, like honey or milk, can contribute to overheating. And, soaps high in saturated fatty acids (fats solid at room temperature) are more prone to overheating. You could adjust your recipe, and sometimes that’s the only solution, but I would first recommend trying the temperature control tips above. If that doesn’t work, then adjust your recipe.

One example is the Whiskey Soap we made. When we added the whiskey to the batter before adding the lye solution, things got hot FAST, and we had soap on a stick. Adding the whiskey after trace helped, but it was still going to be difficult to pour a large batch of soap before it thickened up too much. So, we had to reduce the amount of whiskey in the formulation. With less whiskey, it soaped like a champ!

Here’s a list of common soapmaking ingredients that can cause your soap to overheat, along with a few additional notes about why they can cause your soap to crack from overheating:

• Sugar and anything with sugar including:
  • purees
  • milks
  • alcohol
  • aloe vera
  • oats
  • etc.
    Here’s more information about adding food (including sugar) to your soap.
• Saturated fatty acids (solid fats)
  • Coconut oil heats up very quickly!
• High water content (anything above 1.8:1 water:lye for recipes high in saturated fatty acids always overheat for me)
• Fragrances and Essential Oils (not all of them)
  • Cinnamon, Clove, and other spicy essential oils speed things up. This can be a good thing if you want to accelerate trace, but keep things cool once in the mold.
• Some botanicals
  • A large slab of indigo soap will need some airflow for sure.
  • Some resins will also accelerate trace and can cause overheating. (see my chaparral soaps in the gallery below).

And before I wrap this up, here are a few examples from me and other Soapy Friends of some overheated cracked soap!

Remember, soap making is both an art and a science. Sometimes, despite our best efforts, soap will do its own thing. Cracks and volcanoes don’t mean failure; they’re just part of the soap making adventure. Your soap is still clean, safe, and ready to lather up!