What are the chemicals in solargraphic paper?

They sell paper with a blue coating in kids stores. It comes in a black bag. You put it in the sun with various things on top of it and where the sun hits (UV-light), it turns white. When you wash the paper in water, the reaction becomes fixed. What is the formula?

Sounds like cyanotype. Formulas for that are all over the photo sites. The Formulary sells kits.

PE

Yup, the solargraphic kits seem to be the very simplest form of cyanotype: potassium ferricyanide plus ferric ammonium citrate. They're nearly harmless and easily mixed from scratch.