A sun print is an image created by placing an object on a photosensitive surface, usually paper or fabric, which is then exposed to sunlight. While the color and contrast of the print will vary depending on the surface used and the exposure time, an image of the object will appear on the surface. The most common form of sun print is the cyanotype, a print made on a chemically treated surface resulting in a distinctive white image on a blue background. Sun printing is simple enough for small children to participate in while still allowing enough creativity to entertain older children and even adults.
Cyanotypes are a type of sun print created on paper or fabric treated with potassium ferricyanide and a ferric salt, which must be protected from sunlight prior to use. Objects to be printed are placed on the printing surface as desired, then pinned or placed under safety glass that must not block ultraviolet light. Intricate, flat objects such as leaves and feathers work especially well. The print is then exposed to sunlight, from 10 minutes on a bright, sunny day up to 30 minutes if overcast, turning the exposed surface blue while leaving the hidden portions white. Next, the surface must be rinsed in water until the water runs clear, taking care not to expose the print to direct sunlight until it has dried fully on a flat surface.
For a simpler version of a sun print, dark construction paper can be used. As with the cyanotype, an object is placed on the paper, which is then set out in the sun. Exposure to sunlight will cause the paper to fade, resulting in a dark silhouette of the object. While this is a very simple and inexpensive method, one that is easy enough for preschoolers to make, it does require patience, and up to six hours of exposure to direct sunlight might be required for this method of sun printing. It should be noted that there is no way to permanently set this type of sun print, and construction paper will continue to fade when left in the sun.
The cyanotype process was discovered in 1842 by Sir John Herschel, an English mathematician, chemist and astronomer who contributed to the early advances of photography. This process which would later be adapted for making blueprints. Herschel's friend Anna Atkins, a pioneer in photography and perhaps the first woman photographer, published cyanotype images in Photographs of British Algae: Cyanotype Impressions, making her book the first to use photographic illustrations.