Toning black & white photos

Inspired by @getonthetrain's article on blueprints.

Way before digital photography (he said, sucking his pipe), there already were ways to change the colour of black & white prints.

I found out by accident. As a nerdy kid, I did of course have a chemistry set, my parents being unaware of the dangers of giving me one. One of the chemicals in the kit was potassium ferrocyanide, which could be used to detect iron ions: the solution would turn blue in the same reaction used to make the famous dye Prussian Blue.

One day, I spilled a drop of a potassium ferrocyanide solution on one of my photo prints, and to my amazement, the image under the drop disappeared! Because I knew about the reaction with iron ions, I took some iron(III)chloride I kept around for etching printed circuit boards, and applied it to the disappeared bit of the picture. The image reappeared, but in blue!

What I had stumbled upon was a process called metal replacement toning. Different chemicals and metal ions yield different colours; iron for blue, copper for red. The process can also be used to increase the longevity of prints by replacing the silver in the photo print with gold.

Below is a black & white photo I did this to on purpose. First, the image was bleached away with potassium ferrocyanide, then rinsed in water, after which iron(III)chloride was applied locally, then rinsed off.

After a while, the bleached bits came back in an orange-brown colour; I don't know what the chemistry behind that is.

I had forgotten all about this, until @getonthetrain posted his excellent article on blueprints, which are made using a different, but related process. See also @opheliafu's article on cyanotypes.

As an aside: these were the days well before the interwebs to look things up, so for quite a while, I thought I had discovered something new: turning photos blue! Eventually, it was my chemistry teacher who gently told me it was actually a well-known process. Shocks.