The best feature of stained glass windows is that its appearance is constantly transposed by the different light it gets. Eventually dirt, soot and grime build up on the glass from pollution, smoke, and oxidation. Burning of incense or candles in churches eventually cause carbon deposits to build up on the glass.
Eventually these deposits over the years reduce the amount of light that goes through the windows and give them a dull muted appearance. Cleaning the windows will remove these films and deposits and restore its original appearance. There are different types of cleaners depending on the condition of the windows.
The first attempt at cleaning the windows should be with water alone. If cleaning with the water alone doesn't give you the desired effect try using a nonionic detergent.
Stained glass windows often become covered with a yellowed layer of shellac, varnish or stains which may requires alcohol or solvents to remove. If these methods do not work try a stronger solution, provided that the windows are not painted. Stained glass can be cleaned with acetone, ethanol, isopropyl alcohol or mineral spirits to remove these deposits. Be sure to remove all chemical residue with a nonionic detergent and thoroughly rinse off the residue with clean water.
Before attempting to clean any painted windows be sure that the paint was properly fired on. If the paint is secure it can be cleaned with soft sponges and cloth using one of the above methods. Remember to always try cleaning with water first before trying any harsher methods. If the paint was not properly fired or simply applied cold this can cause the paint to flake off during cleaning. If this is the case be sure to use extra care while cleaning them.
Never use any acidic or abrasive cleaners on stained glass windows they may harm the window. Household glass cleaners that contain ammonia should also be avoided, ammonia can often cause a negative reaction with the putty or came.