Few things are more frustrating than pouring a beautiful batch of soap… only to come back and find cracks running across the top. Soap cracking is a common issue—especially in cold process soap—and the good news is that it’s usually easy to fix once you understand why it happens.
Let’s break down the most common reasons soap cracks, what’s happening inside the soap, and how to prevent it in future batches.
The Most Common Reason Soap Cracks: Overheating
In most cases, soap cracks because it gets too hot during saponification. When soap heats up too quickly, the inside expands faster than the surface can handle. The result? Cracks, splits, or even volcano-like eruptions.
Overheating often happens when:
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The soap is heavily insulated
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The room temperature is high
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Sugars or milk are used
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The recipe contains fast-heating oils
Cracks usually appear within the first few hours after pouring.
High Sugar Content
Sugars increase heat during saponification. While sugar can boost lather, too much can cause your soap to overheat and crack.
Common sugar sources include:
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Honey
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Milk (goat milk, cow milk, coconut milk)
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Beer or wine
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Sugar or sorbitol
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Fruit purees
When using sugar-heavy ingredients, soap at lower temperatures and avoid insulation.
Soaping at Too High a Temperature
If your oils and lye solution are too hot when combined, the soap can race through gel phase and overheat.
Soaping above 120°F increases the risk of:
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Cracking
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Partial gel
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Soap volcanoes
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Rough, uneven tops
Most soap makers have better results soaping between 90–110°F.
Heavy Insulation or Forced Gel Phase
Wrapping your mold in towels or placing it in a warm oven can push soap beyond its heat limit—especially with milk, honey, or sugar in the recipe.
If your soap begins to crack while still soft, it’s a sign that it’s overheating. Removing insulation or moving the soap to a cooler area can sometimes stop the cracking from getting worse.
Fast-Moving Recipes
Certain oils speed up trace and generate more heat:
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Coconut oil (high percentages)
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Palm oil
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Tallow
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Butters (shea, cocoa)
Fast-moving recipes can trap heat quickly, especially in thick pours or loaf molds.
Fragrance Oils and Essential Oils
Some fragrance oils accelerate trace or increase heat, which can contribute to cracking. Spicy and bakery-type fragrances are common culprits.
Essential oils like clove, cinnamon, and nutmeg can also increase heat and should be used carefully.
Low Water (Water Discounting)
Using less water causes soap to thicken faster and trap heat. While water discounts create harder bars sooner, they also increase the risk of overheating.
If you’re seeing cracks, try increasing your water amount slightly in your next batch.
How to Fix Cracked Soap
If your soap cracks while still soft:
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Gently press the crack together with a gloved hand
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Spoon or smooth the top with a spatula
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Spritz lightly with isopropyl alcohol
If the soap has already hardened, it’s usually a cosmetic issue only. The soap is still safe and usable.
How to Prevent Soap Cracks in the Future
To avoid cracking:
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Soap at lower temperatures
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Avoid insulating milk or honey soaps
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Reduce sugar additives
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Use individual cavity molds for heat-heavy recipes
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Watch your soap during the first 2–4 hours
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Consider refrigerating or freezing milk soaps
Is Cracked Soap Still Safe to Use?
Yes. Cracks are almost always cosmetic and do not affect the safety or cleansing ability of the soap. As long as your recipe is properly formulated and cured, cracked soap works just as well as smooth bars.
Final Thoughts
Soap cracking is your soap’s way of telling you it got too hot, too fast. Once you understand the role temperature, sugars, and insulation play, preventing cracks becomes easy. With a few small adjustments, you’ll be pouring smooth, crack-free soap every time.