Glass is a material widely used since the ancient periods in the field of cultural heritage. Its composition has been varied during the history according to the production technology and to the availability of raw materials and it strongly influences the chemical stability. The leaching processes of two types of potash-lime-silica glasses, extensively used during medieval time in Northern Europe, were studied. They show a low chemical resistance due to the low content of network formers such as SiO and to the high 2 amount of network modifiers such as K and Na.