Coconut liquid hand soap

Method

1. Get ready

   Put on sealed goggles, nitrile gloves and long sleeves, and keep them on until the paste is cooked. Weigh 250 g of coconut oil, 175 g of olive oil and 75 g of castor oil into a slow cooker of 4.5 litres or more, 356.4 g of distilled water into a heat-proof jug and 118.8 g of potassium hydroxide flakes into a dry container, then close the flake tub at once.

2. Make the lye solution

   At an open window, add the flakes to the water a little at a time, stirring, never the water to the flakes. It fizzes loudly and runs hotter than sodium hydroxide; let it cool below 82 °C (180 °F), and keep it away from aluminium, tin and zinc, which it attacks, giving off hydrogen.

3. Warm the oils

   Melt the coconut oil in the slow cooker on low, stir in the olive and castor oils and bring them to 65 to 71 °C (150 to 160 °F).

4. Blend to a stiff paste

   Pour the lye solution into the oils and stick blend in bursts of a minute or two, stirring between them. Keep going through the false traces, when the batter thickens and then separates or thins again, until it sets into a stiff, opaque paste that stretches like taffy and the blender can barely move.

5. Cover and cook

   Put the lid on and leave the cooker on low. Check after 15 to 20 minutes and again 15 to 20 minutes later, stirring back any separated liquid and stirring the paste down if it swells, then stir every 30 to 45 minutes.

6. Cook until translucent

   Over 2 to 3 hours the paste turns pale gold and translucent from the edges inwards. It is ready to test when no opaque, creamy patches come up as you fold it over.

7. Test a spoonful in boiling water

   Stir 28 g (1 oz) of paste into 59 ml (2 fl oz) of boiling water until it dissolves, and let it cool. Clear or faintly hazy means the paste is done; milky, or with an oily film on top, means another hour of cooking and a second test.

8. Weigh the paste

   Turn off the cooker, let the paste cool and weigh all of it. It will weigh less than the 975.2 g that went in, because steam left the pot, and the dilution needs the real figure.

9. Work out the water

   The paste holds 606.9 g of soap: the 500 g of oils plus the 106.9 g of pure potassium hydroxide in 118.8 g of 90 % flakes. At 25 % soap, a good strength for a pump bottle, that makes 2428 g of liquid soap however much steam the cook drove off, so the water to add is 2428 g minus your paste's weight, about 1.49 g for every gram of paste if it lost none; the liquid soap dilution calculator does the same sum.

10. Dilute it overnight

    Break the paste into a large pot or jar and pour on all but the last 150 g of that water, boiling hot. Stir gently without whipping up foam, cover it and leave it overnight; the next day, at room temperature, add the rest only if it is still thicker than you want.

11. Judge the thickness with water, not salt

    This soap is mostly coconut and castor oil, with too little oleic acid for salt to thicken it, so it pours thinner than a shop soap. If you want it thicker, dilute your next jar of paste with less water; diluted soap cannot be thickened back.

12. Dilute further for a foaming pump

    For a foaming dispenser, mix one part of the finished soap with one and a half parts of distilled water, which brings it to about 10 % soap.

13. Settle and bottle

    Leave the soap covered for a week or two so any haze sinks, then pour the clear soap off the sediment into washed and dried bottles.

14. Keep it clean

    Diluted, this is three-quarters water or more, and it can grow bacteria and mould once they get in: never top up a half-empty bottle, and keep the rest as paste in a closed jar, diluting a little at a time. To keep diluted soap for long, or to sell it, it needs a preservative that works at pH 9 to 10, and soap for sale needs a challenge test as well.