Enamels in stained glass windows: Preparation, chemical composition, microstructure and causes of deterioration

Stained glass windows incorporating dark blue and purple enamel paint layers are in some cases subject to severe degradation while others from the same period survived the ravages of time. A series of dark blue, green–blue and purple enamel glass paints from the same region (Northwestern Europe) and from the same period (16–early 20th centuries) has been studied by means of a combination of microscopic X-ray fluorescence analysis, electron probe micro analysis and transmission electron microscopy with the aim of better understanding the causes of the degradation. The chemical composition of the enamels diverges from the average chemical composition of window glass. Some of the compositions appear to be unstable, for example those with a high concentration of K2O and a low content of CaO and PbO. In other cases, the deterioration of the paint layers was caused by the less than optimal vitrification of the enamel during the firing process. Recipes and chemical compositions indicate that glassmakers of the 16–17th century had full control over the color of the enamel glass paints they made. They mainly used three types of coloring agents, based on Co (dark blue), Mn (purple) and Cu (light-blue or green–blue) as coloring elements. Blue–purple enamel paints were obtained by mixing two different coloring agents. The coloring agent for red–purple enamel, introduced during the 19th century, was colloidal gold embedded in grains of lead glass.