An account of restoration work (1985-1988) done on medieval relics of stone objects and ceramic pavements in the crypt under the Cathedral of Gniezno is given. There are relics of old foundations, walls, stonework, sculpture, tombs, and ceramic pavements made of terracotta floor plates. The relics were subjected to scientific examinations with different techniques, including microscopy, chemical analyses, and physical tests. The analyses showed that the oldest mortars were made of clay and low-quality gypsum. Different kinds of hard stone and mortars were consolidated by a recrystallization method using barium hydroxide and catalyst to recement disintegrated structures. The pavements were lifted and transferred on movable supports. The treatments are considered successful.