In a small container, mix together the liquid glycerin, castor oil, and fragrance oil. Stir until all the wet ingredients are completely combined.
In a separate container, add the dry ingredients (baking soda, cream of tartar, corn starch, natrasorb bath and SLSA) one by one through a fine mesh sifter and into a large bowl. The sifter ensures the mixture is completely smooth and clump free. I recommend adding the SLSA very last. SLSA is extremely fine and powdery, and can become airborne very easily. You may want to wear a mask during this step to avoid breathing in any SLSA. Once all the ingredients have been added, slowly mix together the dry ingredients.
Pour the wet ingredients into the dry, and slowly work the wet into the dry. A spatula or your hands is the best way to work in the wet ingredients.
Once the dough is thoroughly mixed, divide it in half. Add your colors to each half and use your gloved hands or spatula to thoroughly mix in.
Prepare your tabletop. Place a large piece of freezer or parchment paper on your work space. Then, lightly cover the paper with a dusting of baking soda. You don't need a lot, just enough to keep the mixture from sticking to the paper. Then, take one half of the "dough" and form a long log shape. Use your hands to flatten and spread the mixture.
Place the second half of the dough on the top of the first half, and spread it out until it almost completely covers the first half. It’s okay if there is a little of the first layer still sticking out! TIP: If you'd like the layers to be super smooth, cover the top with another piece of freezer or parchment paper, and use a rolling pin to gently roll out and smooth the layers.
Now it’s time to roll! Starting on the side closest to you, begin to roll the dough up. With every roll, gently peel the paper back, then continue rolling.
Once the dough is completely rolled up, use your hands to gently press the layers together and compact the roll. You can also transfer the roll to a clean piece of parchment paper, and roll again as you shape it.
Use a sharp knife to carefully cut the roll into bars. Be very careful when handling the bars, as they will be very soft and very delicate.
Allow the bars to lay out for at least 3 days to harden. Depending on your climate, they may need over a week to fully harden. Once you’re ready to use, crumble the bars and place them into the tub while the water is running to create the fluffiest bubbles.
