|John Herschel knew that Iron is light sensitive, he made astronomical observations and wrote down the coordinates of what he had observed. In order to make copies he employed scribes, who rendered them useless, they knew letters and numbers but thought the decimal points were just blobs.
Add a couple of catalysts to Iron and that will speed up the its reaction to light. He used translucent paper to make his notes once he had the formula worked out and taught his servants, cheaper than scribes, how to make the copies, the first Blueprints.
One sees blue Cyanotypes with the highlights gone down the drains, the process was never intended to make tonal range prints.
However when I tone a print in the blue the highlights return and the shadows are darker resulting in an almost full tonal range. The toned prints are then resistant to alkali's as well as to acids and can be considered permanent.
"The woodland just off the trail"