Hydration of glass surfaces followed by intense drying causes the formation of surface cracks and loss of transparence. Relative humidity between 40 and 55 should be safe for preservation, occasional excursions from 35 to 65 being acceptable. Rh in case may be controlled by use of equilibrated silica gel. Leakage and quantity of gel. Avoiding heating of glass objects by lighting. Monitoring rh in showcase. Hygromeric half-time and the double barrier case.