As Wayne points out, you didn't just "accelerate" UV exposure, you also subjected it to direct sunlight millimetres from glass. That would have significantly impacted the heat levels on the media compared to a normal presentation hanging on a wall inside a home.
Surprisingly not. In the afternoon of the hottest day of the summer (44 degrees outdoor shade temperature), with no cloud cover and light streaming in through the window, the papers all measured between 37 and 39 degrees Celcius. This is because the near-white papers reflected most of the incoming solar radiation. In contrast, the black silicone sealant around the window measured at 88 degrees Celcius at the same time.
If I had printed the sheets, no doubt the temperature would have been a lot higher.
In any case, short periods of high temperatures are normal for photos and paintings displayed in home environments if they're displayed in any location that receives direct sunlight. Usually, a photo's location is chosen for aesthetic and interior design reasons, not print preservation concerns, and many of these locations receive direct sunlight at some time of the day at some point in the year. It's only museums, galleries and collections that deliberately locate works away from sunlight. And it doesn't take a lot of direct sunlight from an cloudless sky on a hot day to heat a printed image - often with dark inks, and usually stored inside an enclosed frame that essentially acts as a greenhouse and traps heat - to 50-60 degrees or hotter.
This is nothing like the environment in which most prints will exist, being either mounted to a board or behind glass or both, and sitting on a wall out of direct sunlight most of the time and certainly well away from an external window.
Blasting a print with 50klux xenon lamps for 12 or 24 hours a day also isn't anything like the environment in which most prints would be displayed. The point of accelerated testing isn't to simulate the environment the prints are going to be kept in - the only way to do that would be to hang sets of prints in various places for 50/100/200 years and see what happens. It's to simulate what will happen to paper when it's subjected to real-world fluctuations in humidity and temperature, as well as movement/flexion, after its been blasted by UV radiation to the equivalent of 50/100/200 years in normal display. In other words, what will happen to the paper when its been on display for a few decades and you then try to change the mat/frame, unframe it and send it by courier, move it to a new place or otherwise handle it in any other way, or what will happen to even a framed, unmoved print given a bit more time and normal humidity-related paper fluctuations.
You also had the media without an ink load, which may also impact (I don't know, but it's a pretty key difference so it shouldn't be ignored).
I was debating whether to run the test with or without ink. In the end, I went without ink because I didn't want temperature to become an issue (dark surfaces can become 30-40 degrees hotter than light-coloured surfaces in the sun) and because every print will have areas of light or no coverage (e.g. the print borders, at least a narrow strip of which won't be covered by the mat). If these light/no coverage areas fail, then the print as a whole has failed, whether the heavily-inked parts are OK or not.
As an Aussie, I can attest to the fact that putting your face against a North facing window in summer when it's hot is extremely uncomfortable - I dare say you could burn yourself (not just sunburn) with any prolonged exposure. We also have higher UV levels here than most other parts of the world, so the total impact on the media is quite extreme an unrealistic.
UV exposure here is definitely higher, but that's just like deciding to use 50klux lamps for permanence testing instead of 20klux lamps. Both are valid. Naturally, you can't directly compare results from a 50klux test against a 20klux test (due to the possibility of reciprocity failure), but, when you're testing multiple samples against each other, in the same environment, you can safely say whether one particular paper performs better or worse than another. In other words, the results are more qualitative rather than quantitative, but you can say the same for most accelerated testing regimes, since speeding up the UV illumination doesn't speed up gas fading, dark fading, humidity and biological effects which also deteriorate the print.
