Foxing is actually mold growth and the stains that result from that growth. Photographs especially should never touch the glass, because the emulsion can adhere to the glass and is not removable without destroying the image. Most paintings on canvas or linen are not glazed, so this is not an issue.
In picture framing, matting is common on 'paper art' such as prints and watercolors. It used to be that matting material was made of wood pulp,
which is extremely acidic. These mats caused "acid burn" on artwork where the acid mixed with air and light. A brownline, sharp on the mat side and fading out toward the art's edge characterizes the burn. There will not be much burn from the back mat because no air was in contact with the art.
More modern matting material is made of 100% cotton fiber, or is a meythol-cellulous fiber buffered with agents that neutralize the acid in the fibers. An even newer board has been developed to absorb acid and actually correct an unbalanced environment. The art conservation world has embraced this product as superior to 100% cotton or 'rag' board. At this point, though,100% cotton is seen as the more stable board for picture framing.
Fine art should never be mounted, but should instead be secured to the mat with small tabs or hinges of linen or paper. Because hinges are 'reversible' and neutralin acid, they are the preferred way to position and hold a piece of art in a mat or frame. There are several types of hinges that can be used to secure a work in a frame depending on the weight and size of a particular piece.
Photo corners are another way of securing art in a frame without attaching anything to the art. These are little paper pockets made of folded paper and secured to the substrate (mat) to hold the art in place. This method is good or art that is being shipped because it will not allow the art to swing or move out of place.
