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 200 g of coconut oil, 200 g of olive oil and 100 g of castor oil into a slow cooker of 4.5 litres or more, 500 g of distilled water into a heat-proof jug and 115.5 g of potassium hydroxide flakes into a dry container, then close the flake tub at once: it pulls water from the air.

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. Potassium hydroxide fizzes loudly and gets very hot as it dissolves; let the solution 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 paste

   Pour the lye solution into the oils and stick blend in bursts of a minute or two, stirring between them. With as much water as oil, the batter stays thin and milky for a long time, then thickens, separates and comes back, sometimes more than once; keep going until it holds together as a thick, opaque paste that no longer separates when you stop, which can take 30 to 60 minutes.

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 liquid that has separated and stirring the paste down if it swells, then stir every 30 to 45 minutes.

6. Cook until translucent

   Over 3 to 4 hours the paste turns pale gold and translucent from the edges inwards; this much water makes it softer and slower than a stiff paste. 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 1115.5 g that went in, because steam left the pot, and the dilution needs the real figure.

9. Work out the water

   The paste holds 604 g of soap: the 500 g of oils plus the 104 g of pure potassium hydroxide in 115.5 g of 90 % flakes. At 25 % soap, a good strength for a pump bottle, that makes 2416 g of liquid soap however much steam the cook drove off, so the water to add is 2416 g minus your paste's weight, about 1.17 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. Scent it

    Stir half a teaspoon of the lavender and lemongrass, mixed, into 85 g of the warm soap and wait half an hour. If the sample stays clear and keeps its thickness, stir the 10 g of lavender and 5 g of lemongrass oil gently into the whole batch.

12. 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 pump bottles.

13. Keep it clean

    Diluted, this is three-quarters water, 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.