PREPARATION OF COLORED SILICA GLASSES MADE BY SINTERING OF PARTICULATE GELS

Large pieces of silica glass can be easily made by sintering particula te gels prepared from fumed silica powders. These compacts can be dope d by soaking with alcoholic solutions of salts of a large variety of c olouring ions of the transition and rare earth elements. As the sinter ing of the compacts in a zone furnace with a silica glass tube inside is not restricted with respect to the atmospheres, higher states of va lence of the dopants with additional colours can be achieved under oxi dizing conditions in contrast to the conventional melting of silica gl ass in crucibles of molybdenum or tungsten which have to be operated u nder reducing conditions. In reducing sintering atmospheres red copper and gold ruby glasses can be prepared. Therefore, it is favourable to prepare compacts with smaller particles which show also smaller pores . The mechanism of the formation of the colloidal metal particles is n ot yet clear and needs further investigations.