• Cold-process soapmaking is chemistry, and this recipe uses lye. Lye is a caustic substance that is completely neutralized in the soapmaking process, but it can be harmful if not handled correctly. Please read this soap making safety guidance before proceeding.

Make the Soap Paste

• Prepare your workstation with your tools and equipment. Put on rubber gloves, eye protection, and an apron. Carefully pre-measure the ingredients. The oils in the slow cooker, the water in a heat-proof jug, and the lye (KOH) in another container.
• Turn the slow cooker on to high heat and melt the coconut oil. When it's liquid, continue to the next step but keep the heat on.
• Dissolve the lye crystals in water. In an airy place, outdoors is best, pour the lye crystals into the water (not the other way around) and stir well. Don't be alarmed if it fizzes and crackles as this is normal for KOH. If you've made soap using sodium hydroxide, please note that the reaction between KOH is a little more active. Stir until the lye is completely dissolved.
• Pour the lye solution into the melted oils. Now it's time to blend. Put the immersion blender in the slow cooker and, turned off, use it as a spoon to gently stir the ingredients together. Bring it to the middle of the slow cooker next, press it right against the bottom and turn it on for a few seconds. Don't move it around while it's on. Now turn it off and use it to stir again. Repeat this until you see the batter thicken slightly. Put the lid on the slow cooker and allow it to sit for five minutes before coming back.

  The soap batter may look a little separated or chunky and different from other types of soap you've made before. Don't be too concerned.
  
• Repeat this process of stirring and blending and allowing to sit for a few minutes. After fifteen to thirty minutes it will thicken up to very thick 'Trace'. Keep stick blending until the soap becomes thick like really dry mashed potatoes and it becomes difficult to stick blend any longer.
  
• Time to cook. All this time the slow cooker has been on high heat and that's where you're going to leave it for at least the next three to six hours. Put the lid on, and let it cook for that time, stirring every thirty minutes. You can set a timer if that helps. During that time, the texture of the soap will change dramatically -- from the runny custard to mashed potatoes, to puffy taffy, to glue, to something that looks like a puffy mess with chunks of greenish or golden amber.
  
• Finally, after hours of cooking and stirring, the soap will all look amber paste. Some soap makers describe it as looking like vaseline. Once your soap looks like that, it's probably fully cooked. This soap paste is the first major step in creating your liquid hand soap.

  Note: if you cook and cook and don't seem to get anywhere. Unplug the slow cooker, cover it with a towel and let it sit overnight. Have a look the next morning and see what it looks like. Sometimes just letting it sit in residual heat overnight does the trick. If this doesn't work, keep heating it the next morning.
  

Testing the soap

• You now need to know if the soap is actually complete and if it's lye-heavy or not. Let's begin by seeing if there's unsaponified oil in the soap.

  Gently stir a teaspoon of soap paste into half a cup of scalding hot distilled water. Let it sit and dissolve, giving it another stir if it needs help breaking up. Let it cool completely then have a look. If there's oil on the surface, or if the liquid is milky and opaque then you still have unsaponified oils in the paste. Continue cooking it until it's much clearer. Just to be clear, milky means you can't see through it at all. If your liquid is translucent then you're good to go.
  
• Test the soap for excess lye by checking its pH. Dilute one part soap paste into ninety-nine parts scalding hot distilled water and cool to room temperature. Take the pH using strips (Litmus test papers) and check to see if the soap is between 9-10. Allow the paper to dry completely for the most accurate result.

  Liquid soap is supposed to be alkaline, but if it's above this amount then your soap is lye-heavy. Adjust down to the proper pH by adding diluted citric acid but don't go below 9 or it will destabilize.
  

Diluting the soap paste

• Once you've tested the paste, you can now dilute part or all of it. If you dilute the full amount, then you'll have more soap than you'll probably be able to use in a month. In that case, you will need to add a suitable broad-spectrum preservative.

  Alternatively, keep the soap paste stored in a jar and dilute part of it at a time. The soap paste does not need a preservative and has a shelf-life of up to two years. The shelf-life will be the closest best-by date of the ingredients you use (check your bottles). You can also use the soap paste on its own without diluting it.
  
• 100g of soap paste will give you approximately 200ml of liquid soap. Use more or less depending on how much soap you need. Measure the amount back into the slow cooker.
• Add the distilled water and glycerine to dilute the soap paste.

  Multiply the weight of the soap paste you're using by 0.8 -- this is how much distilled water you add to the slow cooker.

  Multiply the weight of the soap paste you're using by 0.2 -- this is how much vegetable glycerine you add to the slow cooker.
• Turn the slow cooker on to high and warm the contents through. Gently stir, turn the heat to keep warm, and leave for an hour. Come back after that time, stir again, squish any blobs gently, and turn the heat off. Cover the slow cooker with a towel and leave it to sit for several hours, if not overnight. The soap paste will go soft and mushy in the same way that a bar of soap will do if you leave it sitting in water. If you come back and it's not fully soft, you can add a little more heat and more gentle stirring. It's not an exact science, this part, and patience is essential.
  
• When it looks fairly liquid, cool it completely, and strain the soap through a sieve into another bowl. This will catch any chunks of soap paste. Add 10-20 drops of essential oil (per 200ml) if you wish, the preservative if you're using one, and bottle it up in squeezy bottles or pump bottles. It's ready to use immediately.