Put on your safety gear, then weigh your ingredients.
Heat your oils and butter over low heat until it reaches 115°F.
In a well-ventilated area, add lye to your water in a heat-proof container. Stir until fully dissolved. Cool in an ice bath until it reaches 115°F.
When they're the lye water and oils are both 115°F, add them together in a large mixing bowl. Blend with an immersion blender until it reaches a light trace.
Add your essential oils and blend again.
Make a swirl by adding green mica to one side of the bowl and blue mica to the other. Mix them in place using the immersion blender. Use a chopstick to blend a couple of swirls in the bowl before pouring.
Pour in the soap mould. Let sit somewhere warm for 48 hours.
After 48 hours, remove the soap from the mould. Let it cure on a wire rack for six weeks before using.