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Rustic slow cooker hot process bar

Three thick beige hot process soap bars with rough, spoon-textured tops beside a jug of avocado oil and a bowl of yogurt on a blue backdrop

A thick, rustic bar cooked in a slow cooker, so the lye is spent before it reaches the mould. Avocado oil stirred in after the cook adds to the superfat, yogurt keeps the hot soap soft enough to press into the mould, and lavender and patchouli scent it. Palm-free.

Skill
Intermediate
Method
Hot process
Batch
1000 g of oils about 1511 g of batter
Hands-on
1 h 30 min
Cure
6 weeks
Gel phase
In the pot it gels as it cooks
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What you need

For 1000 g of oils, which fills a 1.5 litre loaf mould. Weigh everything, liquids included.

Olive Oil · 40.0 % 400 g 14.11 oz
Coconut Oil · 25.0 % 250 g 8.82 oz
Shea Butter · 15.0 % 150 g 5.29 oz
Cocoa Butter · 10.0 % 100 g 3.53 oz
Castor Oil · 5.0 % 50 g 1.76 oz
Avocado Oil · 5.0 %, stirred in after the cook 50 g 1.76 oz
Sodium hydroxide (NaOH) · the lye, 100 % pure 134.0 g 4.73 oz
Distilled water · for a 31 % lye solution 302.9 g 10.68 oz
Plain whole-milk yogurt · warmed, then stirred in after the cook 31 g 1.08 oz
Sodium lactate (60 % solution) · 2 tsp, stirred into the lye solution once it has cooled a little 13.1 g 0.46 oz
Lavender essential oil · 2.0 % of the oils, after the cook 20 g 0.71 oz
Patchouli essential oil · 1.0 % of the oils, after the cook 10 g 0.35 oz
Superfat
4 %
Temperatures
Oils and lye solution below 77 °C (170 °F), cooked on low, scent in below 82 °C (180 °F)

Resize or change it in the calculator

Method

  1. Get ready

    Put on splash goggles, gloves and long sleeves and keep them on until the soap is in the mould: hot, uncooked batter is caustic and sticks to skin. Weigh 400 g olive oil, 250 g coconut oil, 150 g shea butter, 100 g cocoa butter and 50 g castor oil into the slow cooker, the 50 g of avocado oil into a small jug for after the cook, 302.9 g distilled water into the lye jug and 134.0 g sodium hydroxide into a dry container.

  2. Make the lye solution

    At an open window or under the extractor fan, add the sodium hydroxide to the water, never the water to the lye, and stir with your face turned away until it runs clear. Let it cool for a few minutes, then stir in the 2 tsp of sodium lactate.

  3. Melt the oils for the cook

    Melt the coconut oil, shea butter and cocoa butter in the slow cooker on low, then stir in the olive and castor oils. Keep the avocado oil aside.

  4. Blend to trace

    With the oils and the lye solution both below about 77 °C (170 °F), pour the lye into the oils and stick blend to a firm trace. A well-blended batter is less likely to separate as it heats.

  5. Cover and cook

    Lay cling film over the pot, put the lid on and cook on low, checking every 15 to 20 minutes without lifting the lid more than you need to. Never leave it: hot batter can froth up and climb out of the pot.

  6. Fold the edges in

    As the soap at the edges turns glossy and translucent, fold it into the centre and stir down any foam. If it swells past twice its starting volume, stir it down and turn the heat lower.

  7. Cook until translucent

    Keep going until the whole batch is glossy and translucent, like petroleum jelly, with no opaque streaks when you fold it over, usually about an hour. When in doubt, give it another 15 minutes.

  8. Add the avocado oil and yogurt

    Turn off the heat. Warm the avocado oil, the yogurt and your spatula slightly, then stir the oil and yogurt through the soap; cold additions make hot soap seize around them.

  9. Scent it below 82 °C

    Let the soap cool to below about 82 °C (180 °F), then stir in the lavender and patchouli oils quickly, before it stiffens.

  10. Pack the mould

    Spoon the soap into the mould, pressing it into the corners, and bang the mould on the worktop to knock out air pockets. Texture the top with the back of a spoon.

  11. Unmould and cut

    Once it has cooled and firmed, often within a few hours and certainly by the next day, unmould it and cut it into bars.

  12. Cure for six weeks

    The bars are safe to use once set, but cure them on a rack with air all round for about six weeks, turning them now and then: hot process soap made with this much water takes at least as long to dry as cold process.

Tips

  • The cook runs at a 4 % superfat on the other oils, and the avocado oil goes in on top once the lye is spent, so it stays in the bar as extra superfat: about 8.4 % in all. Even at the worst case of the published saponification values the cook keeps about 1 %, with the avocado as the margin on top. Forget to hold it back and it simply cooks with the rest; the lye is the same, so the bar still comes out at about 8.4 %.
  • The calculator link ticks the avocado oil as an after-the-cook oil, leaves it out of the lye and asks for the same 134.0 g of sodium hydroxide at a 4 % superfat.
  • Judge the cook by eye, never with your tongue. Fold the soap over and look for opaque, creamy streaks coming up from the bottom; oil pooling on top means it has separated, so stir it back together and keep cooking.

Why make it

This is the bar for someone who wants to use their soap sooner and likes a rustic look. The slow cooker spends the lye before the soap goes into the mould, so hot process soap is essentially ready to use once it sets, and the lavender and patchouli, stirred in after the cook, are not dulled by lye (David Fisher). The formula is a balanced everyday one, cleansing 17 and conditioning 56 on our scale, firm from shea and cocoa butter and free of palm oil, so it suits most skin. Its thick, spooned texture is part of the character, not a fault. The honest caveat is that safe is not the same as finished: this much water still has to leave the bar, and it will be harder and last longer after six weeks on the rack.

Sources

  • David Fisher, The Complete Photo Guide to Soap Making · Creative Publishing International, 2018

What tradition, makers' experience and research say about soaps like this, each labelled for what it is. It is not medical advice, and no soap treats a skin condition: if you have one, ask a doctor or pharmacist.

What the numbers say

  • Lauric acid 12.3 %
  • Myristic acid 4.7 %
  • Palmitic acid 11.7 %
  • Stearic acid 11.8 %
  • Ricinoleic acid 4.4 %
  • Oleic acid 43.3 %
  • Linoleic acid 7.5 %
  • Other 5.2 %

Saturated : unsaturated 44 : 56

Hardness 41 29–54
Cleansing 17 12–22
Conditioning 56 44–69
Bubbly lather 21 14–46
Creamy lather 28 16–48

Computed from the fatty acids of the fats above. The bands are what the craft has settled on, not chemistry — a bar outside one is not wrong.

From the guides

All guides
Intermediate 13 min read

Hot process soap, step by step

Cook soap batter in a slow cooker until the lye is spent, then stir in the superfat oil, fragrance and colour. A full recipe with the calculator's numbers.
Intermediate 11 min read

Superfat and lye discount: why recipes leave some oil unsaponified

What superfat and lye discount mean, how the calculator applies them, where makers set them for each soap, and why oil added at trace isn't the oil left over.
Beginner 8 min read

Curing and storing handmade soap

What curing does to cold process soap and what it doesn't, how long it takes, how to set bars up to cure, how to tell when they're done, and how to store them.

Where these proportions come from

Our own palm-free formulation: olive oil and a quarter coconut oil, with shea and cocoa butter for a firm bar and a little castor oil for lather, cooked at a 4 % superfat, with 5 % avocado oil stirred in after the cook as extra superfat, about 8.4 % in all. The method is the slow cooker hot process of David Fisher's Complete Photo Guide to Soap Making, as our hot process guide gives it: water at 2.5 times the lye, sodium lactate in the lye solution and yogurt after the cook to keep the soap spreadable.

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