CMC stands for carboxymethyl cellulose. It's a powder that makes the sugar paste thicker/firmer (the formed parts hold the shape better) and speeds-up the drying time. Other names for CMC are tylose powder and tylo powder.

CMC is NOT the same thing as gum tragacanth. CMC is a synthetic substitute for gum tragacanth. CMC works quicker than gum tragacanth, so you don't have to wait many hours for the paste to get thicker/firmer.
To better understand how CMC works, you can try this simple EXERCISE:
  • Roll a few same size (f.ex. 2 cm) balls of the paste you usually use. As I mentioned earlier, CMC is usually added to fondant or to a mix of fondant and gum paste.
  • Add different amounts of CMC to them. Add just a tiny amount to the first one, increase the amount with each ball and add quite a lot to the last one.
  • During kneading observe how the thickness/firmness changes. Stretch the paste and see how it behaves. Line the pieces of paste from the smallest amount of CMC to the largest amount of CMC. Keep that order through the whole exercise so that you know which paste is which.
  • Knead the pieces again. Roll balls. Which of them hold the shape well? Which are smooth?
  • Flatten them (press them from the top). Do you need to use the same pressure for all of them?
  • Roll out the pieces of paste quite thin (f.ex.1-2 mm). Release the paste from the board. Which paste stretches the most? Try to cut out some shapes (using cutters). Do you notice any difference?
  • Roll thick sausages. Leave them to dry for a few minutes (f.ex. 5-10 min). Use a clean, dry scalpel to cut them in half. Clean the blade after each cut. Did some sausages get flattened during cutting? Did you get clean cuts?
  • Leave the pieces to dry overnight. How firm are they on the next day?