Paper is made from fibers that stick together because of the starch inherent in the fibers. Dust can easily imbed itself in between those fibers. When that happens, the starch breaks down and the paper eventually breaks apart. Glass acts as a dust barrier protecting the surface of the paper from the dust that’s in the air. That’s why we need to put glass over paper borne art.
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The purpose of a frame is to focus your eye. To focus the eye of the viewer, so that your peripheral vision isn't distracting, doesn't distract you from the content.