Cyanotype is an early photographic printing process invented by Sir John Herschel in 1842, known for producing striking Prussian Blue images. The process relies on a light-sensitive solution of iron salts combined with potassium ferricyanide, which is applied to paper, cotton, or canvas. Once dried, the coated surface is exposed to sunlight under a negative to form the image. Due to its limited resolution, working with line negatives is recommended for best results. After exposure, the print is washed in cold water until the yellow washout disappears, and highlights can be enhanced with a brief chlorine bleach rinse or selectively lightened with diluted bleach. Cyanotypes are valued for their distinctive blue tones and historical significance but should be protected from prolonged bright light to prevent fading.