Carboxymethyl cellulose (CMC) is a derivative of cellulose, in which part of the hydroxyl is linked to a carboxymethyl group (–CH2–COOH) as ether. Carboxymethyl celluloses are not soluble in water in an acidic form, but they dissolve well in basic solvents. They are used, e.g., to monitor filtration or to increase the viscosity of drilling fluids. Carboxymethyl cellulose is available in different viscosity grades and purity levels. Sodium carboxymethyl cellulose is able to form solid gels. The compound also strengthens the effect of emulsifiers and prevents undesirable substantive lumps. As it forms robust, smooth films, it is also used as a coating agent. Sodium carboxymethyl cellulose is the only cellulose derivative that can also form and stabilize foams.