Foxing is the term used to describe when old paper turns brown and/or begins to show brown spots due to its age. This is a fairly common phenomenon on older books and pieces of ephemera. There are a few theories as to why this happens, but the most common belief is that it is a fungal growth or that it might be caused by the oxidation of minerals found in the paper.
Although this is unfortunate, and some collectors don’t like seeing foxing on their pieces, it’s mostly not a big deal. It generally doesn’t affect the quality of the paper, just the coloring.
Here are some examples from our collection: