I have heard that you should never put glass over paintings on canvas, but I see it done this way in museums. What's the truth?
It used to be believed that glass was unnecessary (or even detrimental to the artwork because it didn't allow the artwork to "breathe"), and the function of glazing was handled anyway through the use of varnish. Nowadays, the myth about paintings needing to respire is starting to fade, and framers are realizing that the use of glazing provides protection from dust, insects, physical contact, and even ultraviolet light (with UV glazing). For this reason, many museums now glaze their paintings, although most museums use UV acrylic (or even the newer anti-reflective acrylic) instead of glass, since acrylic poses less risk of damage to the artwork should the glass shatter.
In any case, IF a painting is glazed, then it is imperative that a spacing mechanism is used to prevent direct contact of the artwork and the glass or acrylic. Often, the choice over glazing a painting comes down to finding a balance between aesthetics and preservation. While there are legitimate preservation benefits to glazing an oil painting, many people prefer the traditional look of an unglazed painting. It really depends on how much importance you give to the unglazed look, versus the protection afforded by glass or acrylic.
