Sun is usual the reason for fading. Some Flouresent tubes emit strong UV light that causes fading.
Displaying photos near heat sources can discolour the surface which makes it look like they're fading. Black and white photos faded mainly because the washing process was too short for all the chemicals to wash out.
Sepia toning making the bw photo into a brownish colour prolonging the lifetime of a photo emensly to qualify as archival quality.
In the beginning of colour photo printing, from the late 60s, only some colours faded more than others to turn photographs red or yellow after 20 years. Early hand coloured photos faded but the paint it was coloured with stayed, to give the impression of being an original painting.
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