The French stained-glass heritage represents 50% of the world’s total area of stained glass and 60% of Europe’s; it encompasses around 450 craft workshops. This glass-shaping technique is passed down from generation to generation, ensuring the continuity of the ancestral know-how.
Stained glass is composed of glass elements and metal parts. These metal elements are used to hold the glass pieces in place to form the design and to strengthen the structure. Traditionally, these elements are called rods (“baguettes”) and are shaped with a specific geometry in an “H” profile into which the glass pieces are inserted. These rods, and the solders at the intersection, are made of lead and tin alloy.
The use of lead in these alloys has been questioned for years. Indeed, the health issues that can occur due to handling the rods, along with the specific safety precautions required, have pushed regulation to a European level. The European Chemicals Agency (ECHA) carried out a public consultation to summarise the impact on workshops if this prohibition is formalised. If it can be demonstrated that the risk to users can be controlled, lead will continue to be used. Otherwise, a substitute material must be developed. Indeed, the consequence will be the inclusion of lead metal on the list of substances requiring authorization for use or handling, and applying for such authorisation will be time- and money-consuming, especially for small workshops. This potential evolution of regulations for this traditional material led Cerfav to study the development of an alternative material.
The goal of this work is to find a substitute alloy to replace lead in the event that it is banned. As Cerfav has both a laboratory and a stained-glass facility, these complementary capabilities allow for effective exchanges between the two environments. Moreover, the substitute, which will be lead-free, must also be shapeable using the traditional tools and know-how of stained-glass artists. However, as lead is one of the cheapest metals on the market, the alternative alloy will likely result in an additional cost.
It is from this perspective that the research was launched. At the beginning, the criteria to take into consideration for the new material are its malleability and the compatibility of the rod alloy with the solder one. Various compositions and geometries have been produced in a H rod shape design, as well as the complementary solder alloy. The cut and the solder test have been performed in Cerfav’s workshop with the expertise of its stained-glass teacher. After a first experimental plan, which studied parameters such as rod alloy, solder alloy couple, solder temperature, and aesthetic considerations, one of the tested compositions allowed for obtaining good malleability and compatibility with the solder alloy. To confirm these first results and evaluate with objectivity the alloy properties, a new project began in 2023 to study the mechanical and climatic resistivity of the different lead-free materials in comparison with the traditional lead material.
Three years after the start of the project, mechanical and climatic resistance tests have now shown encouraging results, leading to a new phase in the research. A test protocol and sample sets were distributed to stained-glass workshops so that artisans could share their feedback on the alternative material and identify the necessary adaptations to minimize the difference between the traditional lead rod and the lead-free alternative. The results are still under review, and additional analyses are scheduled.
Research into a new lead-free material for use in stained glass is still in its early stages: further research must be carried out over the next few years before identifying and validating a lead-free material capable of replacing the lead alloy used in stained glass.
Article by Dr Lucile Cornu, Cerfav, innovation@cerfav.fr
