Further to my last post, another synthesized blue lurks in my paint box – Prussian Blue. It is a complex dark blue pigment that was first synthesized around 1706 by the paint maker Diesbach (whose first name I couldn't find) in Berlin. Since its discovery, Prussian Blue has been used extensively in making paint, and is the traditional "blue" in blueprints. Strangely, It has been used as an antidote for certain kinds of heavy metal poisoning – perhaps a story for another day.
The synthetic Prussian Blue filled a gap left by the loss of knowledge of how to make Egyptian Blue. It is a stable and relatively light-fast blue that is cheaper than ultramarine made from lapis lazuli. Artists were waiting for a pigment like this, so within two years of it's discovery it was already being traded across Europe. Prussian Blue is a strong colour that tends towards black or dark purple when mixed into oil paints. Interestingly, the particle size of the pigment creates the exact hue.
Prussian Blue is a complex chemical including iron and cyanide. It's not particularly toxic because the cyanide is bound tightly to the iron. I was surprised to learn about the number of applications where this pigment is used. In medicine, Prussian Blue is used to detect iron in biopsies like bone marrow. It is also the basis for laundry bluing, that is, it's used to add a slight hint of blue to someone's washing to combat yellowing of whites.
Once children's crayons contained a Prussian Blue, but now that has been changed to Midnight Blue. It has been a long time since I've looked at crayon colours – I think the last time was when I melted them to colour wax for candle making.