Abstract: When water is removed from a body of wet intumescent material by heating it for a period of time during which water is allowed to evaporate, microbubbles tend to form in the resulting body during the later stages of drying and/or during aging of the body.In order to reduce this tendency, during at least a part of the heating period there is introduced into the atmosphere in contact with the intumescent material one or more gases which is less soluble in the intumescent material than is oxygen, and/or one or more gases which is the vapor phase of a solvent for the body. The most preferred solvent vapor is steam, and preferred less soluble gases include SF.sub.6, CF.sub.4, N.sub.2 and cyclohexane.