In a large mixing bowl, thoroughly combine the baking soda, cream of tartar, and SLSA.
Divide the dry mixture evenly into two medium bowls, and stir the red mica colorant into one of the bowls. Leave the second bowl uncolored.
In a small container, combine 1 tablespoon of fractionated coconut oil, 1/2 tablespoon of Polysorbate 80, and fragrance or essential oil (see notes below). Stirring constantly, slowly mix the liquids into the red dry ingredients until they are completely combined.
In another small container, combine the remaining fractionated coconut oil, Polysorbate 80, and fragrance/essential oil. Stirring constantly, combine the liquid ingredients with the dry white ingredients.
Add 3/4 cup of citric acid to each of the bowls, and stir until it is fully incorporated into the mixture.
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 alternating stripes of red and white mixtures. Make sure to overfill both sides of the mold 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 can 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. For any essential oil or fragrance oil used, add half of the desired amount to the first small container of liquid ingredients and the other half to the second container.