TECHNOLOGY FOCUS - CERAMICS AND GLASS - Preferred Process: The partial partial pressure of the water vapor is 10-100 (preferably at least 15) bars; the temperature is more than 400 degrees C; and the ionic conductivity is at least 0.001 S/cm. (I) is impermeable to oxygen and hydrogen gas and has a degree of densification of more than 88% (preferably at least 98%).    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Materials: (I) is an oxide with oxygen atom vacancies, specifically created by deviations from stoichiometry and/or by doping. (I) is especially a perovskite deficient in oxygen atoms, preferably of formula AB1-zDzO(3-z)/2 (I') or AxByO3 (II').D = doping element, specifically a lanthanide or alkali(ne earth) metal, especially trivalent ytterbium (Yb3+);z = degree of doping;x, y = numbers other than 1;A = mono-, tri- or preferably divalent metal, such as divalent calcium (Ca2+), strontium (Sr2+) or barium (Ba2+);B = tri- or preferably tetravalent metal, such as tetravalent zirconium (Zr2+), cerium (Ce4+) or titanium (Ti4+).The perovskite is particularly ytterbium-doped strontium zirconate of formula SrZr0.9Yb0.1O2.95 (Ia) or ytterbium-doped barium zirconate of formula BaZr0.9Yb0.1O2.95 (Ib). Alternatively (I) may be a material with a fluorite, pyrochlor or apatite crystal structure. EXAMPLE - Tests were carried out on 88% densified pellets of ytterbium-doped strontium zirconate of formula SrZr0.9Yb0.1O2.95 (Ia). Nyquist plots were obtained for the pellets, which were placed in a furnace at 400 degrees C in dry form or under a continuous sweep with moist argon under a pressure of less than 1 atmosphere for various periods of up to 16 hours. The resistance was found to decrease (i.e. the conductivity increased) as the time of the sweep increased. When further Nyquist plots were obtained by the same procedure except that the pellets had previously been saturated with water by pre-pressurization with 15 bars of water in an autoclave at 200 degrees C (so that water was lost from (rather than taken up by) the pellets during the sweep), the conductivity decreased as the time of the sweep increased.