This study led by researchers from the MECAMAT Research Group at the Universitat de Vic – Universitat Central de Catalunya, in collaboration with the Universitat de Barcelona, ALBA Synchrotron Light Source, and Universitat Politècnica de Catalunya – BarcelonaTech, has unveiled new insights into the formation of iron-manganese crystalline phases in high lead glazes during firing. The findings have been published in the Journal of the European Ceramic Society.

Key Findings:

• Historical Context: Lead glazes, colored and decorated with iron and manganese oxides, have been used since ancient times. The study identifies new crystalline compounds such as haematite, andradite, melanotekite, kentrolite, braunite, jacobsite, and magnetoplumbite in lead-glazed ceramics from the 11th to 17th centuries.
• Experimental Approach: Using X-ray powder diffraction at the ALBA synchrotron, the researchers studied various mixtures up to 928°C, examining the influence of Al2O3, CaO, and MgO. Laboratory recreations of glazed ceramics were analyzed at temperatures from 850°C to 1020°C using optical and electronic microscopy.
• Crystalline Phase Formation: The study established the sequence of crystalline phase formation in different systems. Key findings include:
  • Haematite (Fe2O3) recrystallizes as hexagonal platelets in PbO-SiO2 mixtures and as cubic-shaped crystals in kaolinitic mixtures, stable up to 1020°C.
  • Barysilite (Pb8(Fe,Mn)3+(Si2O7)3) forms above 600°C in calcitic mixtures with Mn and disappears below 826°C.
  • Melanotekite (Pb2Fe3+2O2(Si2O7)) forms around 650°C and disappears below 930°C.
  • Kentrolite (Pb2Mn3+2Si2O9) and Fe-kentrolite (Pb2(Mn,Fe)3+2Si2O9) form in mixtures with Mn and Fe, melting above 950°C.
  • Pyroxene forms above 800°C in all dolomitic mixtures, stable up to 1020°C.
  • Andradite (Ca3Fe3+2(SiO4)3) forms above 850°C in calcitic mixtures, stable up to 1020°C.
  • Braunite (Mn²⁺Mn₆³⁺(SiO₄)O₈) forms above 950°C in mixtures with Mn,
  • Jacobsite (Mn²⁺Fe³⁺₂O₄) forms above 928ºC in mixtures with Fe, Mn. Al, Ca and Mg
  • Magnetoplumbite (Pb(Fe3+,Mn3+)12O19) forms above 980ºC in mixtures with Fe and Mn

Significance: This study enhances our understanding of ancient production techniques and the final appearance of lead glazes. By elucidating the sequence of formation and stability of different crystalline phases, the research provides valuable insights into the historical use of iron and manganese oxides in ceramics.

For more information, please refer to the full article in the Journal of the European Ceramic Society.

 

Iron-Manganese crystalline phases developed in high lead glazes during firing

 

To download the paper free until 29 march 2025

https://authors.elsevier.com/c/1kZs63PCJlXNN0

 

Fig. 2. (left) Sequence of crystalline phases in the R7-Fe, R7-FeMn and R7-Mn mixtures according to the synchrotron HT-PXRD experiment up to 928ºC (solid bars) and crystalline phases identified by SEM and XRD (dashed bars) of samples fired at 950ºC, 980ºC and 1020ºC. (right) SEM-BSD images of the crystalline phases at 928ºC, 950ºC and 1020ºC.