A mixture, dispersed in water, of approximately 95 % inert substances capable of forming a vitreous glaze and approximately 5 % cellulose fibres is prepared, kept thoroughly agitated, and fed to dewatering equipment - such as the flat suction table of equipment of the type used for paper making - to form a layer; a flocculating polyelectrolyte is added to the mixture; the layer formed is treated by dewatering and drying and by surface treatment for retention and fixing.