Q. I went to see an art exhibit recently and noticed that many oil paintings on canvas had glass installed over the painting, like a watercolor. One canvas had an air pocket between the canvas and painting, but many had the glass against the canvas. Doesnt this keep the canvas from breathing and help them to develop rot?
A. Actually, a watercolor wouldnt be framed with the glass right next to the painting. The standard practice is to separate the painting from the glazing with a mat; if the work is floated (hung from the backing board so the edges of the paper can be seen) instead of matted, then a spacer is placed between the inside of the glass and the front of the picture plane. One widely available product that achieves this type of display is an item made by FrameTek called FrameSpace.
This arrangement keeps the surface of the picture from making contact with the glass to prevent physical damage to the picture and discourage the condensation of atmospheric water vaporwhich, as you astutely observed, could lead to the formation of mold or rot.
Although a current trend in framing is to bring art closer to the glass, I doubt the paintings you saw had the glazing actually touching the surface of the paintings. More likely they were slightly separated from one another by a spacer of some sort.
Artist/workshop instructor Jerry McClish is based in Bradenton, Florida.