Prepare your workstation with your tools and equipment, and have your mold at the ready. Put on rubber gloves, eye protection, and an apron. Carefully pre-measure the ingredients. The olive oil can go in a small stainless steel pan, the water into a heat-proof jug, and the lye in another container. If you're using it, pre-measure the essential oil into its own small dish or ramekin.
Although listed as 'optional', sodium lactate is useful in hardening all soap recipes, especially softer soap like this castile soap recipe. It's available as a powder or in liquid form and if you're using the liquid form, you'll need one teaspoon of it. If you're using the powder form, use only half a teaspoon and dilute it in one Tablespoon of the water amount you've measured for the lye. Do this before you begin, and mix the powder and water into its own small dish.
Make the Lye Solution
Make your lye solution in a well-ventilated area, preferably outdoors, by an open window, or under a kitchen extractor that ventilates to the outdoors. With your goggles and gloves on, pour the lye onto the water and stir it in. Keep your face away from the steam that comes up and be prepared for the water to get very hot. Stir until the lye is completely dissolved and then set the jug in a basin of water to cool down. You want it to cool to 100°F / 38°C and can begin warming the olive oil as it cools.
Add the sodium lactate to the lye solution after it's cooled below 130°F / 54°C
Warm the olive oil
Warm the olive oil in its pan on low until it reaches about 100°F / 38°C. When both the oil and the lye solution are within about five degrees of one another, it's time to mix. Take your pan off the heat and proceed to the next step.
Stick blending to Trace
Pour the lye solution through a fine-mesh strainer and into the olive oil in the pan. Stir it together and then use an immersion blender (stick blender) to bring it to 'trace'. Trace is when the soap begins thickening up to a warm and thin custard-like consistency.
Using the stick blender to emulsify small batches of soap can be a little more tricky than larger batches. I recommend that you first dip your immersion blender into the pan and with it turned off, stir the mixture. Next, bring it to the center of the pan, and with both your hands, hold it on the bottom of the pan and blitz it for just a couple seconds. Turn it off and stir the soap batter, using the blender as a spoon. Repeat until the mixture thickens up to 'Trace'.
You've hit 'Trace' when you can drizzle some of the soap batter onto the surface of your soap and it leaves a trail. I prefer working at a very light trace since it settles nicely into molds. A thicker traced soap can literally be spooned up and plopped into containers. If you're a beginner, aim for somewhere in the middle.
Just to clarify, in small batches of soap I advise that the immersion blender is not moving while you have it turned on. Hold it down against the bottom of the pan and turn on to pulse. Only stir when the device is off, as stirring while it is on will create air bubbles and kick up caustic soap batter.I've placed a video at the bottom of this piece to show you the technique I use for using the stick blender. The recipe for the video is one I've shared showing how to use Cambrian Blue Clay to naturally color soap. I use short and controlled bursts of stick blending with gentle stirring.
Molding and Curing
When your soap is at trace, stir in the essential oil if you're using it (here's further information on other essential oils to use) and then pour the soap batter into molds. Leave uncovered and at room temperature or pop it in the fridge overnight if you wish. Leave the soap in the molds for at least 48 hours if not up to seven days. Handmade Castile soap can take time to firm up. If in doubt, just leave it a bit longer.
When the soap has hardened, pop it out of the molds and cure it. Space the bars out on wax paper / grease-proof paper someplace airy, dim, and at room temperature. This will allow the water that's left in the bars to gently evaporate out. There's detailed information on how to cure and store soap in this piece.
Technically, this castile soap recipe can be used after four weeks' cure time. However, pure olive oil soap can have weak lather and cleansing properties when used young so it's better to cure soap bars for several months and even up to an entire year before using it! It's a long time to wait but you'll be amazed at how much the soap improves over time.
Once made, your castile soap will have a shelf-life of up to two years. Check the oil bottle that you're using though -- the best-by date on the olive oil bottle is the best-by date of your soap. If your handmade soap is destined as gifts, check out these eco-friendly soap packaging ideas.