so, I was talking to a friend about electrochromic paper, paper that changes color in response to electricity, after seeing some old fax machine and audio recorder patents that mentioned it.

They use potassium ferrocyanide mixed with potassium cyanide (yikes…) which turns paper black when electricity flows through it. I tried it with just potassium ferrocyanide and it kinda worked, but sucked.
But that got me thinking about prussian blue, the pigment created in blueprints/cyanotype. It can exist in two different oxidation states, one of them is colorless and the other is blue. Could you electrochemically reduce the blue pigment into the colorless version?

You can! As long as the paper is slightly damp you can rest it on top of a conductive surface (I used a piece of SMD stencil) connected to around 25-30V and run the ground lead over the paper which will leave a white line in the pigment! It doesn't seem to be reversible though, or at least I can't get any current to flow in the opposite direction…

imagine combining this with a patterned PCB with contacts and a piece of ITO glass to make some kinda cyanotype e-paper display?! that would require getting the re-oxidation to work though

wheee, it's reversible when you use two stainless steel electrodes! cyanotype e-paper when?