Manganese crystalline phases developed in high lead glazes during firing

Abstract

A High Temperature Synchrotron Radiation X-Ray Powder Diffraction experiment was performed to determine the manganese compounds formed during the heating and cooling of 70 wt% PbO - 30 wt% SiO2 mixture or the equivalent glass plus 10 wt% of MnO. The effect of adding calcite, dolomite and kaolinite were also studied. All mixtures were fired between 690°C and 1020°C in oxidizing conditions and analysed by Scanning Electron Microscopy. A sequence of manganese phases are formed during firing: bixbyite (Mn 2 O 3 ), barysilite ((Pb , Mn)Si 2 O 7 ), kentrolite (Pb 2 Mn 2 Si 2 O 9 ) and braunite (Mn 7 SiO 12 ). Kentrolite and braunite crystallise with different crystal habits during the heating and the cooling. If dolomite is present diopside ((Ca,Mg,Mn) 2 Si 2 O 6 ) is formed. If calcite is present, ganomalite (Pb 3 (CaMn) 2 Si 3 O 11 ), margarosanite (Pb(Ca,Mn) 2 Si 3 O 9 ) and wollastonite ((Ca,Mn)SiO 3 ) are also formed. Wollastonite can incorporate enough manganese to transform into bustamite ((Mn,Ca) 3 Si 3 O 9 ) at high temperatures. This leaves less manganese available for the crystallisation of kentrolite and braunite.