Why is the drawing blue? How are they made?

Blueprint (photochemical photosensitive material) The blueprint in photography is made by cyanogen salt printing, which is widely used in civil engineering, architecture and many other aspects. There are two kinds: blue line with white background and white line with blue background. Both methods are based on the following photochemical reactions: Fe3++ light → Fe2+, that is, the blueprint photosensitive paper made of photosensitizer with ferric salt (ammonium ferric oxalate) and blood red salt (K3Fe(CN)6) as the main components is overlapped with the description, printed in mercury lamp or sunlight, and exposed in a place without black lines. By photochemical reaction, or replacing red blood salt with yellow blood salt, unexposed Fe3+ turns blue, and a blue line graph with white background can be obtained. The photosensitive wavelength of the blueprint is about 238 ~ 435 nm. For every photon absorbed, only one atom's Fe3+ is reduced to Fe2+, so the sensitivity is extremely low.