Mica and Glitter
In addition to adding color to soap, you can also add special features, such as mica and glitter. Mica is, quite simply, beautiful. It is sparkly, shimmery, and even pearly, and comes in a dazzling array of colors. It is put to best effect when used in a transparent or translucent application. You can use it to color opaque soaps, but the shimmer shows up best when it has light hitting it from more than one side.
Add micas in much the same way as you would add mineral pigments. Stir the amount of color you want to use into a little of the soap, then incorporate that back into the main mass. Some micas lose their color completely in the high alkalinity of lye soap, especially cold process. Your mica supplier will have this information for you.
When using mica in casting soap, add it a little at a time, as you can easily add more. If you find you have more than you want, dilute the over-colored soap with more plain. Unless you've gone really overboard with the color, this will help you get back to the depth of color you want.
Glitter is very beautiful in transparent soaps. Use a small amount for a delicate twinkle. Use a lot for an outrageous sparkle. Make sure that if you give soap with glitter to someone that they know not to use it as a facial soap as it is easy to get soap in your eyes when you wash your face. It would be very easy to get a tiny piece of glitter in your eye that would then scratch your cornea as you blink.
Glitter is generally made from polyester and will melt like plastic if exposed to high heat. You need to take this into consideration when adding it to hot soap mixtures. Add glitter to casting soap that is about 130°F. If you add it as an accent in cold process, add it at trace as you would a spice or herb. In hot process, add it at the very end, when the soap is coolest.
Since pieces of glitter are bigger than mica or mineral pigment, sprinkling it right into the soap works well. Be sure not to throw it in all at once, or it will clump. If you get clumps, just break them up on the side of the pan with your spoon.