Abstract: A process to synthetize zircon as an ultra thin coating and comprising: (A) mixing an alcoholic sol resulting from hydrolysis of a silicon alkoxide with an alcoholic solution of a chelated zirconium compound and in the presence of and with a doping agent, until a homogenous mixed sol is obtained; (B) forming a matured sol containing colloidal particles and/or the polymeric entities from the mixed sol; (C) forming a coating of the matured sol on a microporous substrate; (D) drying slowly the sol coating into a gel coating free of flaws; and (E) firing the gel coating at a temperature of 900.degree. to 1500.degree. C. to form an ultra thin zircon layer. Use of the zircon layer obtained above as an ultrafiltration membrane or as a protective coating.