Prepare your workstation with your tools and equipment. Put on rubber gloves, eye protection, and an apron. Carefully pre-measure the ingredients. The solid oils into the pan, the liquid oils into a jug, the water into another heat-proof jug, and the lye in another container.
Stir the indigo into the dry lye crystals. Indigo doesn't disperse very well in water and you'll notice that after the next step. Mixing the indigo with the lye at this point will help with clumping though.
Next, dissolve the lye (Sodium hydroxide) crystals in water. In an airy place, outdoors is best, pour the lye crystals into the water and stir well. There will be a lot of heat and steam so be careful. Try not to breathe it in. Leave outside in a safe place, or in a shallow basin of water to cool.
Melt the solid oils in a stainless steel pan on very low heat. When melted, remove from the heat and set on a potholder. Pour in the liquid oils. If you have the olive and castor oils in the same container, stir them together first before pouring into the pan. Castor oil is pretty sticky and it's easier to pour when mixed with a lighter oil.
Measure the temperatures of the indigo lye-water and the oils. You should aim to cool them to about 110-120°F / 43-49°C.
Add the indigo lye solution to the pan of oils. To avoid creating air bubbles, pour the lye solution against the side of your stick blender or a spatula. This slow and steady pouring method is what soapmakers do to create professional-looking bars with fewer imperfections.
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 of seconds. Turn it off and stir the soap batter, using the blender as a spoon. Repeat until the mixture thickens up to 'Trace'. This is when the batter leaves a distinguishable trail on the surface. The consistency will be like thin custard.
Working quickly, pour the soap into the mold. Give it a tap to settle it. I'm using the six-cavity silicone mold listed in the equipment list for this recipe. Feel free to use another of your choice though.
Ensure your soap gels to get a truly vivid blue soap. With smallish silicone cavity molds it's a little trickier to gel soap than in loaf or larger molds. What I do is place the mold on a thick wooden cutting board (it's advisable to protect the wood with a layer of grease-proof paper). Then I put it in the oven at 170°F / 77°C and keep it at that temperature for at least half an hour, if not a full hour. I judge the time by having a peek every now and again and checking to see that the soap batter turns a dark color throughout. After that, I'll turn the oven off and leave the soap inside overnight.
The next day, take the soap out of the oven and set someplace to rest for another day. Once 48 hours have passed, pop the soap out of the mold.
Cure the bars for 28 days. Curing means leaving the bars spaced out on a protected surface out of direct sunlight and in an airy place. This allows the extra water content to fully evaporate out. Here are full instructions on how to cure soap.