Weigh out all of the oils and add them to a slow cooker on low heat.
Weigh out and mix together the distilled water and glycerin.
Dissolve the Potassium hydroxide (KOH) in the distilled water and glycerin mixture in a well ventilated area. Add the KOH to the water and not the other way around. Use gloves and safety goggles when working with potassium hydroxide. the mixture will heat up and get hot. Avoid inhaling the fumes.
Add the potassium hydroxide solution to the oils in the slow cooker and begin to slowly mix them together.
Use an immersion blender to blend the oils and potassium hydroxide solution together. At first it will get opaque and thicken slightly, then it will look curdled. Keep blending!
Continue blending until it gets more opaque and begins to get too thick to blend with the immersion blender. I stopped blending as soon as I felt too much resistance so as not to burn out the blender, and my soap paste turned more solid almost immediately after I stopped.
Continue to heat the paste, stirring as best you can with a spatula or spoon every half an hour or so. You want to continue to cook the soap paste until it becomes translucent. A good way to check what your final soap should look like is to dissolve a small amount in some distilled water and see if it dissolves clear. If not, continue to process a little longer. You'll probably end up cooking the paste for a few hours.
Once processed, you can store the soap paste in glass jars in a cool, dark place until you are ready to dilute it to make your liquid Castile soap.
Depending upon how concentrated you want your soap, dissolve the soap paste in distilled water. I usually use somewhere between 2-4 parts water for every 1 part of soap paste. I dissolve small amounts at a time and leave the rest as a soap paste until I need to use it. The soap paste will keep for longer than diluted liquid soap.
You can now add in any essential oils that you want to use to add a fragrance to your soap, or you can keep in unscented. You can also add in a few drops of tocopherol (vitamin E) at this time.
Enjoy using your homemade liquid Castile soap.
This recipe makes 4-4.5 lbs. of soap paste which can be diluted to 10-20 lbs. of liquid soap, or 1-2 gallons, depending on your dilution rate.
Keep in mind that liquid soap is not as thick as commercial gels and surfactants. You'll find that this soap, like Dr. Bronner's, doesn't need to look thick to be a concentrated, effective cleanser.