Today in the studio I filled out a condition report for the second artwork that Nina is letting me handle. It's a small plein air painting by Charles Vezin, a moderately famous impressionist based in Pennsylvania and New York. There were no major damages - just some surface grime and a few cracks from how it was installed in its frame.
After that, Nina let me help retouch a beautiful gold gilded frame. It was too far beyond total repair, but the owner said that she likes how aged it looks, so Nina just stabilized it so that it'll stay relatively intact. My job was to add watercolor to tone down the areas where the gild was chipped away, revealing a bright white ground.
Conservators have a very strict code of ethics regulated by the American Institute for Conservation of Historic and Artistic Works (AIC) that asserts that repairs must hold up for a long period of time, but be as minimal as possible. Because of this, conservators must use gentle but effective and easily removable materials and tools whenever possible. This might include animal glues, very low heat tools (about blood temperature!), and in this case, watercolor.
It states that compensation for loss (areas where the original paint etc has chipped away) "should be reversible and should not falsely modify the known aesthetic, conceptual, and physical characteristics of the cultural property, especially by removing or obscuring original material." So basically, whenever you fill in holes where the original paint has chipped away, you have to do it in a way that doesn't completely mask the fact that it is a loss.
A conservator's job is to keep a piece of art from continuing to fall apart and to mitigate the appearance of any glaring damages so that the piece can be read without interruption as it was intended to when it was first made. However, they should not go so far as to mask the work itself.