This DIY Castile soap recipe is absolutely one of my favorits. It’s simultaneously one of the most inexpensive soaps you can make (making it a great beginner soap), but also the most gentle of cleansers at the same time.
Weigh out your essential oil, olive oil, distilled water and lye crystals in separate containers (use glass for essential oil) ahead of time. It makes things a lot easier.Apply your protective gear, including glove, safety glasses, long sleeves and an apron. The lye crystals and in turn the lye solutions and subsequent soap batter are all very caustic and can cause nasty chemical burns. Be safe and use your PPE.Then, go ahead and pour your lye crystals into the distilled water. Make sure to pour the crystals into the water to avoid splashing. As you pour, turn your head and try to lean away from it slightly so that if splashes do happen, you’ll avoid it more easily. Stir carefully with a stainless steel spoon.This will begin the chemical reaction and your lye solution should rapidly increase in temperature. Place to one side, ensuring the room is well ventilated. Be sure to not breathe any of the fumes in, as they are quite toxic.
Pour your olive oil into your chosen glass or PET plastic mixing bowl that you use to whisk up your soap batter, then pop it into the microwave to heat it up. You want it to be within 90-140F.Now we need to balance the temperature of the oil and the lye solution. Using a laser thermometer, check the temperature of your lye solution. If it falls between 90-140F, you’re good to pour. If it’s too hot, wait a little while and then pour. If, during the time you’ve waited for your lye to cool, your oil has cooled too much just pop it back into the microwave to get it back up to between 90-140F.Once you’re happy that both your lye and your oil are between this temperature range, pour your lye into the oil. Make sure to do it this way around, as you want to avoid splashing and losing any of the oil to the rim of the bowl.
Using a stick blender, blend the soap batter until you reach a light trace. You will notice immediately that the color of the oil has changed to a creamy, yellowish hue. This is called emulsion and is the precursor to trace.If you’ve made any other type of soap before, you may notice that this soap takes a lot longer to trace than others. This is fine and totally normal, just keep blending until you can pick up some of the mixture with a spoon, ladle it back onto the surface of the batter and it doesn’t immediately emerse. It should just sit on top. If you want a better description of trace, take a look at step 5 of our basic soap recipe.
Now that we’re at trace, it’s time to add our essential oil. Go ahead and pour the whole 35.5g into the batter and give it another blend. Be sure to work quickly, as all soaps have a tendency to seize when adding essential oils after trace.Once you’re happy that the essential oil has fully dispursed, move on to step 6. If you’re making a baby-safe, essential oil-free version, ignore step 5 and skip immediately to step 6. Although you may wish to blend a little longer than normal so it’s a little thicker than a light trace.
Working quickly but carefully so not to let the batter harden in the bowl, pour or spoon your batter into your soap mold. Once you’re happy you’ve got all of your mixtures in the mold, cover the top of the mold with a cut to size piece of cardboard and wrap with an old towel that you use only for this purpose (never use it to dry skin again, even if washed in a laundry machine).
Leave to harden in the mold for 24-48 hours. Then, you can remove from the mold and cut into bars.
Depending on the thickness of the cut, you should be able to get between 6-10 bars.