Who would have thought that this would be so easy?
I have folders of old faxes, which were printed on thermal fax paper over ten years ago. I do need to keep some of these records, unfortunately.
There's a couple of ways to resurrect faded old faxes.
First, you can scan an old fax in a scanning machine. I have a five-year-old Canon Pixma that does scans. There's a setting where I can set the scanning settings myself. By doing so, I can control the contrast, brightness, and most importantly -- the "levels" of the resulting scan.
I show the settings that I used to resurrected some faded fax sheets a few days ago. It simply involves moving the left slide for the levels (the black level) almost all the way to the right (white). By doing this, you are bringing out any possible dark text on the white sheet of paper so that it appears even darker.
You can see the resulting scan of the faded fax here. It's amazing -- all the text is legible.
A second way to resurrect faded faxes is to simply photograph them with a digital camera. I simply took a photo of the faded faxes and opened it in Photoshop. I then went to "Levels" and crushed the blacks again. Voila! The faxes are readable again. This is amazing -- digital cameras are capable of seeing far more than the human eye. It was impossible to read the most faded part of these faxes, and simply by taking a digital photograph and adjusting the black level of the image -- I can now read the faxes.