The blueprint, or cyanotype, is a photo reproduction technique from the nineteenth century to make print from negatives. It has been used a reproduction technique for technical drawings. Drawn on transparent paper these could be copied to paper, hence the name blueprint for a technical drawing. For that purpose it was still used not too long ago. But for reproduction of photos it has long been abandoned. Firstly because it results in photos which are blue, a very deep blue, and nothing other colour. And secondly because the prints look rather harsh. It lacks subtle tones. But it has some distinct advantages as well: it is very simple, does not use toxic chemical and it can be used to print on any kind of surface which absorbs water. It also gives a photo an old atmosphere. If you have photos of an old theme it might be an idea to print them as a blueprint.
But first and foremost making a blueprint is fun. You paint a sheet of paper with a chemical solution by hand, put it in sunlight covered with a negative and wash it clean by hand. If you ever enjoyed printing photos in a darkroom it like that, without the fuss. The result is a hard blue print. Often watercolour paper is used to print a blueprint, but wood or textile can be printed as well. Blueprint has made a bit of a comeback with digital photography. You want to do something with digital photos. Having them printed professionally is not nearly as much fun as making something yourself. Combining digital photography with nineteenth century chemistry is a nice combination.
The paper (or other material) is made sensitive to ultraviolet light with a solution of blue dye and potassium dichromate. The potassium dichromate breaks down under ultraviolet light and binds the blue dye to the paper. After making the paper photosensitive you print a negative on the computer on transparent paper. Then you expose the paper, covered by the negative, to sunlight. After exposing the paper you wash off the excess solution (from the dark parts of the negative) from the paper using water. That is the process in brief. You can give a print more of a dynamic range by printing it in two steps. First you paint the paper with a more diluted solution, resulting in a faint print of the darker parts of the photo (lighter on the negative). Then you make a second print on the same paper using a less diluted solution. The result is a duotone which shows more details in the darker parts of the photo as well.
The process is described extensively on alternativephotography.com. There are also descriptions of many other old photography techniques. You can also follow a workshop in nineteenth century photo technique at Huis Marseille, the photo museum in Amsterdam.Geotag (location) for: