How to Make Hot Process Soap (Slow Cooker Method)

What Is Hot Process Soap?

Hot process soap uses the same ingredients as cold process soap, but you cook the soap batter in a slow cooker until saponification is complete. That means the bars are safe to use much sooner. The trade-off is texture: hot process soap is thick when you mould it, so bars look rustic rather than smooth and swirled.

Hot Process vs Cold Process

• Hot process is usable after about a week; cold process needs 4–6 weeks
• Fragrance goes in after the cook, so less of it burns off
• Hot process is harder to swirl and gives a rustic finish
• Both use exactly the same lye calculation

Equipment

• A slow cooker you only use for soap
• Digital scale accurate to 1 g
• Stick blender
• Heat-safe jug for the lye solution
• Silicone spatula and a sturdy spoon
• Loaf mould
• Gloves, goggles and long sleeves

Step by Step

• Run your recipe through a soap calculator to get the lye and water weights
• Melt your oils in the slow cooker on low
• Make the lye solution: add lye to water, never water to lye
• Pour the lye solution into the oils and stick blend to trace
• Cover and cook on low, stirring every 15 minutes
• The soap will bubble, rise, then turn glossy and translucent, like mashed potato or petroleum jelly. This usually takes 45–90 minutes
• Turn off the heat, let it cool for a few minutes, then stir in fragrance and colour
• Press firmly into the mould and tap it on the counter to remove air pockets
• Unmould the next day, cut, and let the bars dry

Get a hot process soap recipe with exact amounts →

Tips for Smoother Hot Process Soap

• Use a little more water than for cold process to keep the soap fluid
• Add a spoonful of yoghurt or sugar dissolved in water after the cook for a smoother texture
• Work quickly when moulding; the soap stiffens as it cools
• Do not let the soap boil over. Stir it down if it climbs the sides of the cooker