Despite its fragility, glass was a valued material in medieval medicine for its physical qualities of transparency, colorization, optics, and its chemical and olfactory neutrality. Colorless glass notably facilitated the observation of liquids such as milk or urine, the colors of which were analyzed in matula (night vases) whose use was widespread in the Middle Ages. In the Hôtel-Dieu museum at the Hospices de Beaune, visitors can see several glass suction cups (photo), the symbols of doctors in ancient iconography. These small circular containers with narrowed openings and rounded bottoms were edged with beads. They were applied side by side to the skin to attract the blood by revulsion after having rarefied the air.