Before the days of the digital imaging, which has put a camera in everyone’s pocket, photography was a chemical process.
Light sensitive silver salts fixed in a get on cellulose acetate film were exposed to light and on development these created a negative image. These in turn could be used to make a positive image on light sensitive paper, also containing silver salts.
One of the early pioneers of photography, William Herschell, also developed a photographic process that uses iron salts, the cyanotype process. It fell out of favour partly because the monochrome images were in shades of blue but mainly because the paper was less sensitive and demanded longer exposure times.
However, this lower sensitivity means it can be safely handled in a school laboratory, as long as the light is not too bright, and it is also much cheaper than silver photography. As such, it is an ideal introduction to one of the major uses of a photochemical reaction.
Incidentally, the blue tones of the cyanotype print are where we get the word blueprint from today.
A more detailed version of this process with other recipes can be found in the Workshope section under Cyanotypes.