The influence of high melting temperatures on the behaviour of polyvalent ions in silicate glasses

The influence of melting temperature, atmosphere and glass composition (the rmodynamical parameters) as well as that of holding time (kinetical paramet ers) on the behavior of the polyvalent ions iron, vanadium, chromium and ti tanium were investigated. As host glasses three different silicate glasses (33mol% Na2O - 67 SiO2, 15mol% Na2O - 85SiO(2) and 10mol% CaO - 10BaO 15Al( 2)O(3) - 65SiO(2)) were used. The glasses were melted in a temperature rang e of 1600 to 2000 degreesC at various (oxidizing, inert and reducing) melti ng conditions. The investigations show the possibility of thermal reduction melting at hig h temperatures under different atmospheres. Increasing melting temperature and/or decreasing oxygen partial pressure shift the redox ratios to reduced states. For polyvalent ions having a single redox transition the reached r edox ratios in the melt could be frozen. In the case of polyvalent ions hav ing more than one redox transition reactions during cooling may occur. Applying high melting temperatures lead to an acceleration of redox reactio ns of polyvalent ions in glass melts due to the increasing oxygen diffusion coefficients and decreasing viscosity of the melt. Constant redox ratios a re reached faster at high temperatures, too.