Measure out the olive oil and add it to the slow cooker on low heat.
Make the lye solution
Weigh out the glycerine, water, and potassium hydroxide (KOH).
Mix the water and glycerin.
Carefully add the KOH to the water and glycerin mixture. (Not the other way around!) Do this step in a well ventilated area.
Stir the KOH into the water until it dissolves. It will be cloudy at first, but then it will clear up.
Make the soap paste
Slowly add in the the KOH mixture to the warm olive oil, and gently stir them together in the slow cooker.
Once they are mixed together well, use an immersion blender to blend the ingredients together. In a few minutes the mixture will begin to thicken and look like mayonnaise. It will later look like a creamy pudding.
Shortly afterward, the mixture will begin to look grainy like mashed potatoes or applesauce. Continue to blend.
As you continue to blend, it will start to get creamy again. Once you reach this point, you can stop blending with the hand held blender and can leave the soap paste cooking covered in the slow cooker.
Cook the soap paste
As you cook the soap, it will continue to thicken and will become more translucent. To achieve a clear liquid soap we'll need to cook the soap paste for 3-4 hours. During that time, check on it and stir it up every half an hour or so.
Check for doneness
To check for "doneness," we will look to see if the soap paste dissolves clear. To do this, take a small amount of the soap paste and dissolve it in distilled water. If the resulting soap is clear, the soap is finished cooking. If it's cloudy, continue to cook the soap paste for another half an hour before checking on it again.
Dilute the soap
To make a liquid Castile soap, dissolve some of the paste in distilled water. I ususally use a ratio of one part soap paste to 2-3 parts distilled water. It can be left overnight to dissolve on its own, stirring ocassionally, as needed. You can also use heat to speed up the process.
Video
Notes
A slow cooker is perfect for this sort of job because it will gently warm without burning, and keeping things at a steady temperature. If you don't have a slow cooker, you can bake the paste in an oven or cook it in a double boiler.
Take breaks while blending so as not to burn out the motor of your immersion blender.
If at any point the mixture becomes too thick to blend with the immersion blender, switch to mixing with a wooden spoon.
If you've cooked for more than 3-4 hours and it still isn't clear, there may be other issues. Tap water can give a cloudy soap. For other possibilities, check my post on troubleshooting liquid soaps.
This recipe makes 4.5-5 lbs. soap paste which can be diluted to 10-20 lbs. soap or more. That ends up being 1-2 gallons, or more, depending upon the desired concentration.
Keep in mind that liquid soap is not as thick as commercial gels and surfactants. Liquid soap is of a thinner consistency and doesn't need to be thick to be concentrated and work well.
Undissolved soap paste can be stored in glass or plastic containers in a cool dry place for more than a year. (I've had some for several years without problems.)