Gradually add the lye into the cold water, stirring until it dissolves well. Cover with a cloth and let it cool until about 110-120 degrees F, can take 1+ hours.
Heat the fat and other oils until liquid, then let cool to 100-110F (38-43C).
Slowly pour the lye mixture into the fats. Alternate between using an immersion blender in short 3-5 second bursts and stirring with a whisk. Check regularly for trace (see more in “TIPS” above). Do not over blend. If using clay, add during the last moments of mixing.
Once trace is achieved, pour mixture into your mold (use a silicone mold or line with parchment paper).
Tamp out any air bubbles by gently tapping or shaking the mold against the table
(Optional) Spray the top with alcohol if you want to prevent soda ash (purely for appearance to prevent any white film on the top).
Insulate the mold(s) with 1-2 bath towels, 1 covering the top and 1 wrapped around. Leave small individual molds for 24 hours, larger loaf pans sit for 48 hours.
Pop soaps out of silicon molds or turn upside down and tap firmly to release from mental molds.
Cut 1-inch thick bars with a sharp knife or soap loaf cutter.
Cure the bars for 6-8 weeks on a rack in a well ventilated cool, dark area. Each bar should be standing alone, not touching the others.
Test the soap after six weeks. If it leaves your hands feeling slippery or if your skin stings at all, it’s not ready yet.