CMC Gum

Gum CMC is not a plant gum. It is cellulose that has been treated to dissolve in water, as a sodium salt. In the bucket it thickens the water, which slows settling. On the pot it dries to a film that binds the glaze layer to the bisque. A glaze containing CMC applies thicker, brushes smoothly and takes a second coat without lifting the first. Dipping glazes commonly use less than one part in a hundred, and brushing glazes a few times that. Once it is mixed into water it has no dependable shelf life. Bacteria break the chains down and the thickening does not come back. Some batches spoil in days and others last months, so it is usually mixed in small amounts. The dry powder keeps far longer.