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Troubleshooting: Volcano Soap & Soap Cracked on Top

soap science and info
blue soap starting to volcano

In the Modern Soapmaking Troubleshooting series, we explore various soap making problems and find out what went wrong plus share some tips and tricks to put the smack down on issues that pop up.

Today we’re talking about…

Volcano Soap & Soap Cracked on Top

What are volcano soaps and/or soaps cracked on top? Soaps that volcano out of the mold expand and rise out of the mold, and can completely volcano out of a mold and cover any surface near the mold. Cracked soap tops are a precursor to volcano soaps, usually. A forming crack in the top of the soap is a telltale sign to pay attention!

What causes the problem? Usually, overheating is the cause of volcano soaps or soaps that crack on the top. However, other factors can come into play, like overdrying the top of the soap.


Most soapmakers are familiar with the gel phase, when the interior of the soap changes colors and looks like jelly as it heats up during saponification. (Not all soaps gel!) If the outside of the soap begins to solidify, and the interior continues to heat up, the heat becomes trapped.

If it gets too hot, the top of the soap will crack and some of the heat will begin to escape. However, if it continues to heat up, it will begin to expand through the crack and volcano.

Overdrying can also cause soap cracked on top. For instance, if you spray too much 91% isopropyl alcohol on your soap tops, it can dry out the top of the soap. When the top of the soap dries out, any changes underneath the surface (such as gel phase) can cause cracks.

How do I prevent volcanoes or cracked soap tops? If you are having issues with your soap cracking on top or creating a volcano out of the mold, try to:

  • Reduce your overall soapmaking temperature to lower your chances of overheating. (Especially with heaters like floral/spicy fragrances, high amounts of sugar in your soap like milks, juices, etc.)
  • Reduce insulation of your soap in the mold by elevating it for more air flow or by using a less insulating mold.
  • Reduce the amount of 91% isopropyl alcohol that is sprayed on the soap to prevent ash.
  • Place a piece of plastic wrap on the top of the soap after pour, and remove it as the soap begins to gel. This will prevent the top of the soap from hardening as much, and lessen your chances of the crack showing up or making a permanent impact in the final appearance. (Do not do this with high temperatures or additives that cause heat.)

Is the soap usable? Soaps affected by overheating are usually still completely safe for usage as long as they do not separate in the process (not common!). They may not be as pretty as what you had in mind though, especially after a volcano.


Can the soap be fixed? If you are dealing with a crack:

  • Elevate the soap mold, remove any insulation, and turn on a fan directed towards the soap to increase air flow. Once a crack appears, it can be too late to attempt to use the fridge or freezer to drop the temperature as the inside of the soap is already heating. However, if you have a dedicated fridge or freezer for soap making, it is worth a shot (if you don't mind cleaning it up if it volcanos!)
  • Oftentimes, the soap will settle back down together and the crack will be barely recognizable in the final soap. However, sometimes it does not. If you are concerned, you can use a spatula (it will be extremely hot!) to press the soap back into place.

If you are dealing with a volcano:

  • Using gloved hands and a curved spatula (the soap will be extremely hot!), try to scoop the overflowing soap into a secondary mold. Once a soap starts to erupt out of the mold, it is often too late to save the aesthetics of the batch. At this point, it is more about creating usable soap and minimizing the mess.

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