This work elucidates the mechanism responsible for the lustre effect of scheelite-based glazes for single-firing wall tiles. The surface decoration is obtained with a thin film composed of a Si–Ca–Zn–Al–K–B frit and 10 wt% WO3 on zircon-engobed substrates for single-firing wall tiles (maximum temperature of 1130 °C for 50 min). The observed lustre effect is sub-adamantine and pearlescent. It is sub-adamantine because scheelite nanocrystals at the surface, with a relatively high refractive index (n = 1.93), cause considerable reflection of light. The lustre is also weakly pearlescent because the nano-crystals oriented with the (004) plane parallel to the surface give interference with the underlying glassy layer (n ≈ 1.5), where randomly dispersed scheelite crystals occur. This model apparently applies to the glazes decorated with ceria, although the latter exhibits iridescence due to the high refractive index of ceria (n = 2.05) which yields stronger interference effect with the underlying glassy substrate.
Mechanism of lustre formation in scheelite-based glazes / Gualtieri, Alessandro; Canovi, Lorenzo; Viani, Alberto; Bertocchi, Paolo; Corradini, Cecilia; Gualtieri, Eva Magdalena; Gazzadi, gian carlo; Zapparoli, Mauro; Berthier, Serge. - In: JOURNAL OF THE EUROPEAN CERAMIC SOCIETY. - ISSN 0955-2219. - STAMPA. - 33:(2013), pp. 2055-2064. [10.1016/j.jeurceramsoc.2013.03.018]