Since 1756, the National Manufacture of Sèvres Porcelain has been designing and producing world-renowned ornamental porcelain, made of a ceramic material elaborated at high temperature, most often covered with a transparent glaze (called “enamel” in this work) and topped with a layer of a glaze or containing pigment crystallites, which contributes to the color and brilliance of the object [1]. Centuries of improvement led to lead borosilicate compositions for both the enamel and the glaze, in various proportions of PbO, and they now require a compliance of their formulas in order to follow the evolution of chemical safety standards, including the REACH classification [2]. The present study is therefore part of the search for new lead-free enamel and glazes formulations that have similar properties and meet the same quality and aesthetic requirements as the current lead ones. 
 
In this work, we have developed a new glaze with a composition close to those currently used but in which lead oxide (PbO) has been totally replaced by bismuth oxide (Bi2O3) , and various transparent enamels with and without bismuth.[3,4] Bismuth oxide is a good substitute for lead oxide, due to its similar physical-chemical properties and low toxicity. The goal is to develop “greener” glaze and enamel compositions while trying to maintain the same physico-chemical characteristics  : a high refractive index guaranteeing brightness, a high hardness and chemical stability, a coefficient of thermal expansion close to that of the substrate, a low crystallization and phase-separation tendency during glaze cooling in the furnace, as well as melting temperature and  viscosity  which will enable them to be used in the same temperature range as current lead compositions. In this study, to comply with the know-how of the Sèvres Manufacture, the working temperature should be around 840°C for the glaze and around 1120°C for the enamel.
 
The different enamels and glazes prepared for this study have been characterized in terms of density and behavior as a function of temperature (viscosity, glass transition temperature  and crystallization tendencies). Associations of leaded and unleaded glazes with leaded and unleaded enamels on a porcelain support were carried out, to study and understand on the one hand the interactions between the porcelain with the different enamels, and on the other hand the interactions between the transparent enamel and the colored glaze that overlay it. These interactions were characterized with SEM-EDX, as well as with XRD.
 
These studies have shown the necessity of having a lead-free enamel under the glaze to avoid the diffusion of lead in the glaze. They have highlighted the different diffusions of ions taking place within the glasses particularly the aluminum ions.
 
[1] A. D’Albis Traité de la Porcelaine de Sèvres, 2003
[2] ECHA, « ANNEX 1 in support of the Committee for Risk Assessment (RAC) for evaluation of limit values for lead and its compounds at the workplace », 2020.
[3] G. Gao et al., Opt. Mater., vol. 32, no 1, 2009
[4] C.-Y. Wang et al., J. Non-Cryst. Solids, vol. 363, 2013