Cold-process soap involves lye (sodium hydroxide) which is caustic. Wear safety goggles, rubber gloves, and long sleeves. Work in a well-ventilated area and keep vinegar nearby.
Measure 142g sodium hydroxide (lye) and 340ml distilled water separately. Slowly pour the lye INTO the water (never the reverse) stirring constantly. The solution will heat to 80°C — normal. Let cool to 40°C.
Gently melt 500g coconut oil and combine with 300g olive oil and 200g castor oil in a large stainless pot. Cool to 40°C.
Slowly pour the lye solution into the oils, stirring. Use a stick blender in short bursts until the mixture reaches trace — a thick, pudding-like consistency. Add essential oils or colourants now.
Pour into a lined soap mould. Insulate with towels and leave for 24–48 hours. Unmould and cut into bars. Cure on a rack for 4–6 weeks before use.