First, add the Epsom salts, citric acid, baking soda and cornflour / starch to a large bowl.
Using a whisk, stir the dry ingredients, mixing them for a few minutes until the are completely combined.
Add the mica powder or turmeric powder if using and stir until combined.
In a jar, add the carrier oil, water and fragrance oil. Secure the lid and shake well.
Very slowly, one drip at a time, mix the wet mixture into the dry mixture, whisking as you go. Whisk until all the wet mixture has been added and the texture of the new mixture is all the same.
Add the mixture to bath bomb moulds (in the link above) overfilling each on the two halves. Press them together and give a quick twist to secure.
Add the bath bombs to set in the moulds for about 6 hours. Then, remove and let them finish drying on the counter.
If they don't easily release from these moulds, tap the bottom of them gently with a spoon.