At the onset of WWII, acres of stained glass windows were removed from churches to avoid destruction. Most of them have been replaced, except in cases where the churches themselves were too badly damaged. The ensuing documentation gave rise in 1952 to the Corpus Vitrearum Medii Aevi, for which France planned 60 volumes, only two of which have been published so far due to lack of funding. Many windows were destroyed in the 17th and 18th centuries (Revolution). The 19th century started the work of restoration, but many of the originals were lost or sold illegally. At present, the Office of Inspection of Historical Monuments is responsible for restoration. Aside from breakage (in wars, children's games, pigeon shoots, supersonic booms, etc.), the glass itself is subject to deterioration, becomes opaque, and dissolves (mostly from air pollution). Ongoing research in various government laboratories addresses these problems.