Display omitted •A rare wood ash glass tessera from Malbork Madonna was studied in detail.•It was found to be original mediaeval, from the area north of Alps, coloured by Co.•Opacification was achieved by Ca-phosphate grains, a Mediterranean technology.•Two phases detected in tessera for the first time: leucite and pseudowollastonite.•Pseudowollastonite indicates production temperature above ca. 1125 °C. While the natron and plant ash glass tesserae may be found on places of importance across the former Roman and Byzantine empires, wood ash glass tesserae are scarce. This is the first time a wood ash glass tessera is studied in detail. It was part of a magnificent 8-metres tall statue of Madonna in Malbork, Poland, created at the end of the 14th century and destroyed at the end of World War Two. It was found to be coloured by cobalt with possible impact of copper, and opacified by Ca-phosphate. Processes previously described in sodium-rich glasses were observed also in the studied potassium-rich wood ash glass tessera, such as diffusion of the respective alkali metal into the Ca-phosphate grains. The elemental composition of the tessera indicates that it is original – mediaeval, from the area north of Alps. Two phases were identified for the first time, to authors’ best knowledge, in any glass tessera – leucite (tetragonal KAlSi2O6) and pseudowollastonite (monoclinic CaSiO3). As pseudowollastonite is a high-temperature phase, it may serve as an indicator of production temperature, which was further supported by the study of polymerisation index of model glasses. This study contributes to the knowledge of old technologies and showed that the know-how for opacification was imported from the Mediterranean, while the raw materials employed for the base glass preparation were from the area north of Alps.