Luxembourg: Solar photo paper
Solar paper is only sensitive to blue or ultra-violet light. It may therefore be handled quite safely in rooms lit by light bulbs or muted daylight. Care should be taken in case of direct sunlight as this is composed to a larger extent of ultra-violet light.
This is how it is done:
Take a board as a base and place a sheet of solar paper on it with bluish side facing upwards.
Close the photos face down on the paper and cover the whole with a thin layer of kitchen cling film.
Expose the paper now in direct sunlight for 10-15 minutes, until the pale blue coloration on the exposed sections has almost turned completely white.
Then place everything in the shade or room, remove the film and rinse the paper at least 2 minutes in tap water. The picture will first disappear and then re-emerge as a negative image.
Once dried, the picture will have become light-fast. In time, the blue colour will darken considerably.