My experience with thermal fax paper is that printed copy is best preserved. I discovered a couple of years ago, that manufacturers' faxed copies of data from the early 1990s have faded almost to unreadability.
Not black, but faded, as with disappearing ink Storage was in a file folder in a file cabinet in temperature and humidity-controlled environment not exposed to light for very long at all I've had blue-line copies similarly fade after about 40 years. Thermofax, anyone? --Chuck
