Lumen print, fennel
Jill Enfield has a good overview of the process , including some really interesting examples.
Basically, it's just putting photo printing paper out in the sun. These prints should not be developed (they will just turn black), but can be fixed. The fixing will wash out a lot of the color, but make the print stable. Gold toning seems to work well.
Lumen print, garden flower
Different paper will produce different results, and you can use old/expired or even fogged paper or film.
The prints here I made were all done on Ilford Warmtone paper. Note that it's just ordinary black and white photo paper, but it gets really vibrant colors when left in the sun. These prints were gold toned, and no fixer was used.
Lumen prints in progress
Online I've found people recommend 20-60 minutes for fiber paper. Gina recommended much longer times, 5-6 hours. The color did seem to become more vibrant as time went on.
I did get impatient and my prints were only exposed for 3 hours. I'd like to experiment more on time.
Lumen print in progress
Moisture (from the plant) seems to play a big part in the color shifts.
Some of the detail from these I find pretty interesting, there's almost a 3D feeling. I'm not sure how much is a result of the toning vs. the print.
I'm not that crazy about the prints I made, but it was fun and easy. I'd definitely like to try them again and see if I can get better results.
Give it a shot if you haven't already! All you need is some old photo paper!