<p>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.</p><p>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.<br>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?</p><p>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…</p>