Drawing Sun prints ?

In the days before dye line copying of drawing tracings, you'd clamp the tracing and treated (?) paper in a glass frame and put it in the sun. I suppose the theory was that it used the UV rays (weak) to make a 'contact' copy. Similar to the dye line copying but with weaker natural UV.

There was also a photographic technique in the early days of photography, where a glass negative was positioned in front of some chemically cleaned steel and the suns rays would make a 'positive' on the steel. The UV would increase corrosion (rust?) in the clearer areas. I think they were then varnished to stabilise the process and also to darken the corrosion areas. (S'easier with a digital camera...)

B****y hell you're making me feel old

Mart

In message , GeoffH writes

If you coat a sheet of paper with a strong solution of Potassium ferrocyanide (I think it was ferro and not the usual ferri) and dry it in the dark, the paper is light sensitive, if very slow. Exposed to sunlight in a printing frame under a tracing, the image appears as a darkening of the dark blue-green coating. When washed in fresh water, the unexposed areas are washed away and the image shows as a bright blue colour - hence the origin of the name "blue prints." Its possible that Pot.bichromate was also involved, but I'm too lazy to go and look it up, inless someone really would like to try the process.

I've still a piece of this paper in one of my atlases(!), but I doubt if it will work after about55 years!

Dave. retired prof. photographer.

p.s. pot.ferro and pot.ferri are realatively safe chemicals, despite the word "cyanide" in their name! Pot.ferri is a normal photographic bleaching agent.