There is a huge controversy surrounding art restration, especially in Italy where so many of the world's most precious works are located. If a restorer isn't careful, they may cause irreversible damage by not knowing the compounds of the piece or scraping a little too hard.When it was anounced that the Sistine Chapel would be restored, people all around the world voiced concern for the most famous church on earth. So why did it need restoration?
Such things as sunlight, humidity, and regualar old Air can add to the damage done to a painting. plus, those figures were just plain dirty. heh, get it? DIRTY!
Light(with some help from oxygen)'s Effect on oil Paints and Varnishes
All light waves carry radiation. When this radiation comes in contact with an object, the high energy that's associated with it excites electrons and can displace them. When this happens bonds between atoms can break, especially in organic compounds. All materials used in the Sistine Chapel are organic compounds. The breaking of bonds can cause colors to fade. Also, the heat can cause physical changes, expanding or contracting of materials which may result in cracking or "crazing" of some surfaces.
Paints are made with long carbon-chain molecules called polymers. Radiation from strong or repeated daylight will excite the atoms in these polymers making them unstable allowing a variety of reactions to take place. One of these is a free radical reaction. Free radicals are short lived atoms or groups of atoms caused by fission(the splitting of an atomic nucleus into almost equal parts) of a covalent bond. These are very reactive because they have open valence electrons. Auto-oxidation may also occur which could could involve an oxygen gain, hydrogen loss or the loss of electrons. The oxidation rate increases with time.
These reactions may dry the oils in the paints or fade them to the point where sketches underneath are evident.
These same effects on the traditional Medieval varnishes made of tree resin will cause cross-linking reactions. Several will most likely occur, including causing the compounds to become more polar, broken down into smaller compounds and mabybe bond with the polymers in the paint.
So...pretty wacky huh?