Cobalt rich phases formed in processing natural erythrite at high temperatures

This study investigates whether natural erythrite was used before 1520 as a source of arsenic-free cobalt products for the production of blue glass and glazes. Historical records on glass and ceramics suggest a shift in cobalt sources or processing techniques around this time, with earlier European cobalt blue glass and glazes being arsenic-free and later ones containing significant arsenic levels. The aim is to determine the specific transformations of erythrite during firing, how it interacts with other compounds, and whether it forms phases that do not retain arsenic. To explore this, we conducted thermal treatments of erythrite alone and in combination with historically relevant minerals to assess its transformations and potential for arsenic removal. Materials characterization has been performed by XRPD, HT-SR-XRPD, XRF, and SEM-EDS.

The roasting of erythrite shows that the mineral decomposition begins at 227 °C, forming an amorphous phase with maximum arsenic loss by 635 °C. At 560 °C, cobalt arsenates (monoclinic and tetragonal Co₃(AsO₄)₂) crystallize, trapping arsenic within the structure. A transient cobalt arsenate appears at 627 °C but disappears at 755 °C, while at 775 °C, a cobalt-rich, arsenic-poor phase (Co₇.₈As₃O₁₆) forms. Therefore, arsenic remains locked within stable cobalt arsenates, indicating that simple roasting of erythrite, as could have been achieved in 14th–16th century ovens, is insufficient to eliminate arsenic below detectable limits in medium-resolution techniques such as PIXE and XRF.

Further experiments following historical recipes using erythrite mixed with borax and calcite (as described by Isfahani) revealed the formation of an amorphous blue glass containing cobalt, arsenic, and boron, along with crystalline phases such as NaCa₂Co₂(AsO₄)₃, Ca₃(AsO₄)₂, as well as cobalt rich phases such as Co-borate (Co₂Fe₀.₇Ni₀.₃BO₅), with SiO₂ and Al₂O₃ spinels ((Co,Ni,Fe,Al)₃O₄.