The blue printing method is also called the iron cyanide (iron-Prussian blue) printing method.
It can produce a white image and shaded areas with blue midtones. Other colors can also be produced, including blue images on a white background, and photographs can be made on photographic paper or cloth washed with starch slurry, or other materials using negative or transparent positives.
Using this principle, try to reproduce a roll of film ten years ago, using the keychain as the carrier to regenerate it.
