Also called brown prints, this was the earliest form of photo print making, with the negatives being made of glass. Blue prints were made this same way w its the formula that came out blue. They used watercolor paper (or something close) to soak up the formula. Then you would wait as the sun slowly developed (as you watched) the print. When fully developed you just hosed it off and stopped the developing process. Then and now (as you’ll see with mine) you could add color with watercolors or in my case colored pencils. It has a unique place in the history of photography and straddles both photography and drawing and painting.