Weigh all your ingredients: coconut oil, olive oil, tallow, castor oil, distilled water, lye and optional essential oils if using. Next, place salt in a small bowl and activated charcoal in a separate small bowl.
Add coconut oil, olive oil, tallow and castor oil to the slow cooker. When they are almost fully melted move on to the next step.
Make sure that kids and pets do not have access to the lye water you are about to make, then put on your protective gear and add your water to a medium-sized heat-resistant glass or ceramic bowl. Choose a well-ventilated area to mix in the lye. I prefer to mix outside, but some soapers like to mix in their kitchen sink with several windows and a fan or air purifier running. If you're new to working with lye, see the FAQ section for safety information.Make sure you have a long-handled spoon or spatula ready. I use a this silicone spatula/scraper.While wearing your protective gear and taking care not to breathe the vapors, slowly add the lye to the water while mixing gently. Order is important here, so make sure it is the lye you’re pouring into the water.Keep stirring until the lye seems mostly dissolved. The mixture will be cloudy at first, but will turn clear in 5-10 minutes. When it's clear, add the salt and stir again until the salt is fully dissolved, then proceed to the next step.
Add the lye water to the slow cooker (being careful not to splash) and stir a few times with your spatula or long-handled spoon.
Using the stick blender begin mixing toward “trace.” I've found that if I mix while holding the immersion blender straight down instead of at an angle it doesn't splatter outside of the container, which keeps my protective equipment clean and reduces cleanup. You’ll know trace is achieved when the mixture has the texture and thickness of a light pudding.
Add the activated charcoal and mix gently with a long-handled spoon or silicone spatula. I've found that activated charcoal seems to accelerate the saponification process (conversion of fats into soap), so at this point I set a timer for 10 minutes to remind me to check on the soap.
After the soap has been cooking for about 10 minutes, I check on it and give it a stir. During the cooking process the oils should rise up the sides like a wave and then fold back into the mixture, but sometimes if they're not stirred often enough they can overflow out of the slow cooker. After the first 10 minute check, I stir the soap every 15 minutes until a total of 55 minutes has passed.
When the soap is ready, it should look like soft, fluid mashed potatoes. Here's how to test your soap for readiness according to Dr. Kevin Dunn, author of 'Scientific Soapmaking.""1) Take a gloved finger and wet it.2) Rub the wet finger over the surface of your soap to gather up a coating of diluted soap/lather (does not have to be frothing).3) Very carefully touch the lather on your finger to only the very tip of your tongue - i.e., behave as if your finger might be covered with something gross or dangerous, instead of covered with something delicious like chocolate. Don't go licking the bar of soap itself, and don't go rubbing your lathered finger all over your tongue. Just a quick touch or 2 of your lathered finger to the very tip of your tongue is all it takes to be able to detect excess alkali/unreacted lye in your soap, which will make itself known by a zapping or stinging sensation on the tip of your tongue.4) Repeat the above on each side of your soap.5) Rinse your mouth out with water (or lemon juice if you desire) and spit."If there's no zapping or stinging sensation, it's ready. If you don't want to do this test, the only other reliable testing method I know of is to purchase a waterproof pH tester kit and make sure the soap ph is between 8-10. .Note: It's really important to make sure the soap is finished cooking and therefore all the lye is converted – otherwise it can burn skin.
If you’re adding essential oils, wait until the mixture cools to 140F - using your thermometer to check - then add them. Otherwise the heat will evaporate the essential oils instead of allowing them to be infused into the soap.
Spoon mixture into your mold and firmly tap it on your work surface the floor a few times to get rid of any air bubbles. (It shouldn't splash, but just in case make sure you're still wearing your protective gear.
Fill your sink with hot, soapy water and wash all tools well in hot soapy water while wearing your protective equipment. Rinse very well after washing.
Let the soap set for 24-48 hours, then cut into bars. The mold I use will make ten 1-inch bars, but I like mine a little thicker so I cut the loaf into eight bars. In an area with good airflow, place the soap bars on a rack/tray with about an inch of space between them.Allow them to dry out and harden for a week or two. Charcoal makes bars a little soft at first, so although you can technically use them after a few days these just get better with time.