CMC gum is a water soluble long-chain molecule derived from cellulose. A small amount (typically 0.5% -2%) is included in brushing-type glaze recipes to extend brushing time by thwarting rapid water absorption into the clay. CMC creates a very strong application bond and it provides an excellent, tough surface to paint on.
It can be added to all types of glaze but be cautious about adding it to dipping glazes. CMC is a mild deflocculant which can cause settling. It also allows more glaze to drain off resulting in a thinner applied layer.
It can be mixed thoroughly and quickly in a blender. If hand mixed, add CMC to water and let it stand overnight before mixing. CMC is organic and will degrade without an added preservative, sometimes within weeks.
For more information on CMC gum, see the Digitalfire.com Reference Library of ceramic materials.