Manganese crystalline phases developed in high lead glazes during firing. Issue 9 (August 2022) - Molera, J.
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 (Mn2 O3 ), barysilite ((Pb, Mn)Si2 O7 ), kentrolite (Pb2 Mn2 Si2 O9 ) and braunite (Mn7 SiO12 ). Kentrolite and braunite crystallise with different crystal habits during the heating and the cooling. If dolomite is present diopside ((Ca, Mg, Mn)2 Si2 O6 ) is formed. If calcite is present, ganomalite (Pb3 (CaMn)2 Si3 O11 ), margarosanite (Pb(Ca, Mn)2 Si3 O9 ) and wollastonite ((Ca, Mn)SiO3 ) are also formed. Wollastonite can incorporate enough manganese to transform into bustamite ((Mn, Ca)3 Si3 O9 ) at high temperatures. This leaves less manganese available for the crystallisation of kentrolite and braunite. Highlights: Sequence of manganese phases formed in high lead glazes during firing. High temperature Powder X-ray diffraction experiment. SEM images of the manganese silicates formed during heating and cooling.