Put on gloves and a mask. Mix the SLSA, baking soda, corn starch, and cream of tartar together slowly in a bowl so that you don't get the SLSA fine powder up in the air. Set aside.
In a separate bowl, mix the glycerin and oil together. Add the wet ingredients to the dry ingredients and stir with a wire whisk or use your hands to thoroughly combine (just make sure you are wearing gloves). Mix until the mixture is smooth without any clumps. If the mixture is too dry and crumbly, add more glycerin.
Add the essential oils or fragrance oil that you would like to use. Be sure to follow safe essential oil dilution guidelines. If you'd like to make multiple colors in your bubble bars, then divide your dough into separate bowls, one bowl per color you want to use. Add the coloring and mix completely so there aren't any color specks and the color is uniform.
Place a piece of parchment paper or wax paper onto your work surface. Pat your first color of dough down onto the parchment paper so that it is flat and rectangular. Place another piece of wax paper over the top, then gently roll over the top with a rolling pin to make the dough nice and smooth and somewhat flat.
Place the second half of the dough over the top of the rolled out piece, then pat over the top so the second color is spread out on top. Place a piece of parchment paper over the top and use a rolling pin to smooth out the second color directly over the first color until it covers (or mostly covers) the first color and is somewhat flat and smooth.
Remove the parchment paper and use your fingers to press in any cracks or creases and make a rectangular shape. Starting on a short end close to you, roll the end over itself like you would when making cinnamon rolls. Use the parchment paper to gently roll the dough, then use your fingers to press it together as you go. Use your fingers to smooth out any cracks. Keep pulling the parchment paper up to roll the mixture nice and tight until it is completely rolled. Gently squeeze everything together so you don't have any cracks.
Use a sharp knife to slice each bar. You'll get about 10 good-sized bars, depending on how you rolled your dough and how thick you cut your bars.
Place your bubble bars on a parchment paper-covered baking sheet to dry. Drying time will vary based on multiple factors, but don't touch the bubble bars for at least 3 days, since they will crumble if not completely dry. If you want to test them after a few days, then gently touch an end piece to see if it is rock hard. The bubble bars are done when they are completely hard. Store your bubble bars in an airtight container for up to a year.
To use a bubble bar, start your bath water running with warm water. Crumble a bubble bar in your hand drop the crumbles right under the running bath water. You'll immediately notice the bubbles start forming and the water changing color. Enjoy your nice relaxing bubble bath!