I was recently asked to reframe an original piece of art which had spent the last couple of years inside a cheap, store-bought frame. When I removed the mat, this is what I found underneath:
At first I thought the difference in colour between the matted area and the rest of the paper was caused by the mat itself as non-archival mat boards contain acids and other pollutants which can react with the paper. However, when I turned the paper over, I realised it was in fact the un-matted area which had been affected and that the damage had been caused by UV light.
Regular 2mm glass, often sold by glaziers as "picture framing glass" offers no protection from the UV rays present in sunlight and should not be used for original works, limited edition prints or anything likely to increase in value (be it financial or sentimental) over time. UV light can be particularly damaging to photographs, breaking down the chemical bonds between the molecules which form the image, causing it to fade over time. Cheap glass also has a green tint to it, which can affect the colours of the artwork behind it whereas conservation-grade glass often has colourants added to counteract this.
Under the guidelines handed down by the Picture Framers Guild of Australia, conservation-grade frames need to be fitted with glass or perspex which is at least 95% UV resistant. I have worked extensively with perspex in the past, but as a general rule I no longer use it. Although it's less heavy than glass, it scratches easily and has a much higher rate of expansion due to changes in temperature of humidity. In fact, the sort temperature fluctuations which are routine in Australia (not to mention the moisture we experience in the Dandenong Ranges) are enough to cause perspex to expand beyond the 2mm tolerance framers leave inside the frame to compensate, resulting in cracked mitres like this:
The standard glass I currently offer my clients is Tru Vue Conservation Clear, which blocks 99% of UV light from reaching the artwork. Non-reflective options are also available.