COLORATION MECHANISM IN SILICATE GELS AND GEL-GLASSES CONTAINING IRON

The present investigation was carried out in order to study the incorp oration of iron oxide in several samples with a general composition xF e0-(100-x)SiO2 (x between 0.1 and 9.5) and 5R(n)O-xFeO-(95-x)SiO2 (R = Li, Na or Ca) prepared by the sol-gel method. Samples were gelated an d dried at 60 degrees C and thermally treated between 200 and 1000 deg rees C in oxidizing and reducing atmospheres. The color change and str uctural evolution were studied by VIS and NIR spectrophotometry, TEM a nd X-ray diffraction. When the temperature increases, the color change s from yellow to brown-reddishness, going through a bright ruby-red, a t the same time the absorption edge shifts to higher wavelengths and a t elevated temperatures to smaller wavelengths newly. A part of iron o xide is incorporated into the glass network and the remaining as collo idal particles, producing a double mechanism of coloration by optical absorption and dispersion. In the samples heat-treated in oxidizing co nditions all iron is present in the ferric state. When the heat-treatm ent is carried out in reducing conditions the presence of iron (II) wa s detected at temperatures of nearly 1000 degrees C.