EN
The article presents a number of factors destructive for stained glass windows: water, air pollution, mechanical and microbiological elements as well as inappropriate conservation in the past, symptoms of corrosion and changes in the chemical composition of the top layer. The second part of the article describes prophylactic operations whose intention is to protect (isolate) the windows from the above mentioned detrimental factors. Finally, the author deals with conservation: cleaning, mechanical stabilization by gluing cracks, duplications and strengthening as well as the replenishment of missing glass and paint, repairs of the lead strips and the covering of the stained glass with protective coatings. Already a superficial comparison of the state of preservation of windows which have remained in their original location (and not in museums) with earlier documentation shows rapid deterioration in recent decades. The symptoms of this process are particularly visible on medieval stained glass which is less resilient to corrosion due to its higher alkalis content and a relatively low presence of bioxide of silicon. The glass shows limited resistance to mechanical factors but none at all to chemical factors. The degradation of stained glass constitutes a corrosion during which certain components are diluted and partially change into non-transparent products of disintegration.