There are a number of conservation issues associated with the cleaning of stained glass and lead light glazing, not least of all that no chemical cleaner should be used. Before cleaning is undertaken it is of the utmost importance that a qualified person should identify if the painted detail of a stained glass window is in a condition that will allow cleaning to be undertaken. There are many windows that will have painted detail that is soft and in a condition that renders it water soluble as a result of poor firing of the painted detail at the point of execution. Following the visual inspection any cleaning work will need to be carried out by a qualified glazier/conservator so that conservation issues are taken into consideration.
The dirt/grime deposit that may be found on church windows is most commonly a combination of algae growth if condensation is an issue in the building, candle smuts, a deposit from the burning of incense, and the accumulation of general dust and cobwebs. These can, in most instances, be removed by wet cleaning with de-ionised water and cotton swabs, leaving considerable visual improvement.
View the process of re-leading an ancient stained glass panel from rubbing to completed panel here:
A stained glass window in the making >>