Sun printing is a process by which the light from the sun is used to help develop an image. Have you ever left a few pieces of paper in direct sunlight for a period of time and, when rearranging them later, noticed that the color of the pages on the top of the pile is different from those on the bottom of the pile? Perhaps, if the pages were scattered a bit, the bottom sheets even seem to show the shadow of the top sheets. Maybe you have noticed a similar effect when taking down items that had been posted on a bulletin board for a long period of time. If this sounds familiar to you, then you are already familiar with the basic concept behind sun printing.
There are a few different technical processes for sun printing which create images that are a bit more specific and intended than the blurred shadow of an overlaid piece of newsprint. One of the most common forms of sun printing is a process similar to that which is used to develop photographs. This process relies on the use of gelatin, a dilute solution of potassium dichromate, and exposure to sunlight. The result of this process is a negative plate that can be inked and used in a hand press to make as many prints as are desired.
It is also possible to take a negative of a photos, place it over a sheet of black and white photographic paper, and develop the image via sunlight exposure. This process can take anywhere from a few minutes to a few hours. The defining factor is the type of photographic paper that is being used. Once the image is produces on the paper, it must be fixed and washed according to standard photographic procedures.
Yet another type of sun printing is the process by which cyanotypes are created. To create a cyanotype, you must treat a piece of paper with a solution of potassium ferricyanide and ferric ammonium citrate. Then place an object on top of that paper. It is preferable for the object, such as a leaf or piece of lace, to be rather flat. When the paper, with the object on time, is exposed to sunlight, the UV light from the sun will cause a chemical reaction that will result in a print of the object which was placed on top of the paper. The image can be fixed to the paper by washing it with water.