In a large mixing bowl, thoroughly combine the baking soda, cream of tartar (or kaolin clay), SLSA. Stir in mica colorant if desired until completely combined.
In a small container, combine the oil, fragrance/essential oil, and Polysorbate 80 (if using).
Stirring constantly, slowly mix the liquids into the dry ingredients until they are completely combined.
Add citric acid to the mixture and stir until it is fully incorporated.
Test to see if the mixture is a moldable consistency - it should feel like slightly damp sand and hold together when squeezed in your hand.If the mixture is not quite wet enough to mold, spritz two to three times with a spray bottle of witch hazel and mix well. Repeat as necessary until the proper consistency is reached.
Fill both halves of the bath bomb mold with the bath bomb mixture, making sure to overfill both sides of the mold just a bit. Press both halves of the mold firmly together.
Carefully release the bath bomb from the mold (tap the mold lightly with a wooden spoon if needed) and allow it to dry completely for 24-48 hours depending on the temperature and humidity of your location.
The amount of fragrance oil and/or essential oil that will need to be added to your bath bomb mixture will vary widely depending on what you are using. Generally, fragrance oil can be added by the 1/2 teaspoon until the desired amount of fragrance is reached. Essential oils are often far more potent than fragrance oil, so we recommend adding 15-20 drops of essential oil to each recipe to start and then adding more in 5 drop increments until the desired level of fragrance is achieved.