Cobalt enamels through time: arsenic's influence and removal in historical production techniques

The presence of elements associated with cobalt in historical enamels provides relevant information on the minerals used to produce them and their origin.
Starting from around 1520 we witnessed the systematic appearance of arsenic in blue enamels produced in the Mediterranean. Until this date and starting from the 15th century, an arsenic-free cobalt pigment was used instead.
The chemical composition of the two glazes is however compatible, showing in both cases the presence of Ni, Fe, Zn associated with Co.
Both pigments may have been produced using the same cobalt and arsenic minerals: erythrite, smaltite and skutterudite coming from the Erzgebirge region (Gratuze et al., 1996; Soulier et al., 1996). Then, the absence of arsenic in one of the two glazes could be attributable to different production processes like the roasting of the ores, which was used to produce saffron, or to the utilize of different fluxes employed to make the glaze, two cobalt by-products that began to be produced in the Erzgebirge mining district starting from the 16th century (Meltzer, 1716).

Recent studies on skutterudite thermal behaviour (Molera et al., 2021) revealed the difficulty of completely removing arsenic from the mineral, however it cannot be excluded that this element can be more easily removed from other minerals with Co/As ratio greater than 1:3.

This study aimed to replicate historical saffron and enamel recipes on erythrite and clinosafflorite to evaluate the loss of arsenic in the final products.
Several roasting experiments, reaching temperatures up to 1020°C, were performed on mineralogical samples from Bou Azzer (Morocco), even using different fluxes and studying the products obtained through different techniques (XRD, XRF, SEM-EDS). Some diffractions were performed during heating using synchrotron radiation.

With these treatments arsenic was not completely eliminated, but cobalt phases with reduced arsenic content were often obtained. The presence of calcium, sodium, and lead promotes the formation of different arsenates inside the glaze.
Heating clinosafflorite powders up to 1020°C resulted in the formation of cobalt-rich phases and As-Co-Fe-Ca phases. A mixture of CaO and borax with erythrite heated up to 900°C allowed to obtain Co-Fe-Ni oxides and Ca-Co-Na-Ni arsenates. Clinosafflorite roasted with quartz and ash promoted the formation of arsenic-free cobalt phases. The reaction and blue colouring of a quartz grain and the formation of different phases of Co-Na-Ca silicates, Co-Fe-Ni phases and different types of arsenates were obtained by heating clinisafflorite with borax at 1020°C.

REFERENCES
Gratuze, B., Soulier, I., Blet, M., Vallauri, L. (1996): De l’origine du cobalt: du verre à la céramique. Revue d’archèometrìe, 20.
Meltzer, C. (1716): Historia Schneebergensis Renovata. Das ist: Erneuerte Stadt- u. Berg-Chronica Der im Ober-Ertz-Gebürge des belobten Meißens gelegenen Wohl-löbl. Freyen Berg-Stadt Schneeberg.
Molera, J., Climent-Font, A., Garcia, G., Pradell, T., Vallcorba, O., Zucchiatti, A. (2021): Experimental study of historical processing of cobalt arsenide ore for colouring glazes (15 16th century Europe). Journal of Archeological science: reports 36.
Soulier, I., Gratuze, B., Barrandon, J.N. (1996): The origin of cobalt blue pigments in French glass from the bronze age to the eighteen century. Revue d’Archéométrie, 20.