Cut the soap base into cubes that are around 1 inch in size.
Place into a microwave safe measuring jug.
Heat in the microwave for 30 seconds, remove and stir, then heat for additional 30 second increments, until fully melted, stirring well between each heating session.
Add mica powder, if using, and stir well until color is fully combined.
Add the infused oil or essential oil, and stir.
Add calendula petals, reserving a bit for the top of the soap. Stir to combine.
Pour soap base into soap mold.
Sprinkle remaining calendula petals on the top, pushing them into the soap.