The most characteristic feature of northern European medieval glass, used in the cathedral windows, is the high content of K2O and CaO, resulting from the use, after 1000 A.D, of beech and fern ashes as fluxing agents. Three glass varieties with medieval-like composition were prepared using recipes deduced from archaeometric studies with different amounts of potassium for fluxing (15 - 20 - 25 K2O weight %). Aging experiments were carried out on the reproduced glass samples by exposure to boiling concentrated sulfuric acid and to high temperature and high pressure bi-distilled water. The glass weight loss and the alteration layer thickness were measured and the aging solutions were analyzed through AAS to detect K+ ions release. For the w...