Add the baking soda, epsom salts, corn starch, and citric acid to a medium glass or metal bowl. Use a whisk to stir well.
Add the almond oil, water, and essential oil to a small jar with a lid, secure the lid, then shake well to combine the liquids together.
Slowly pour the liquid into the dry mixture, while whisking the entire time. See tip below, as you may not want to use all of the liquid.Whisk well until you have a nice texture that will form.
Add calendula petals, stir to combine.
Add the orange mica if using and mix to combine.
Add mixture to one half of a bath bomb mold, overfilling it, then do the same with another half.
Press the two halves together, twist lightly to remove any excess mixture, and set in a cool, dry place to set for at least 4 hours or overnight. If your climate is wet or cold, allow additional time. A great way to test is, when the bomb is hard to touch the bath bombs are ready.
To release, lightly twist to remove the top, then lightly tap the bottom of the other.
Store in an airtight container until ready for use.
Tip - in humid climates, you may need to decrease the amount of liquid you use, so slowly pour some in while whisking, then ¼ of the way through, check the texture to see if you can form it in your hand. You want it to be able to form, but not be too wet.