The chemical solution of the iron salts (ferric ammonium citrate and potassium ferricyanide) is sensitive to ultra violet (UV) light and will change colour to a greyish blue when exposed to direct sunlight or another UV light source.
A substrate, such as paper or finely woven cotton or silk fabric, is coated with the solution and left to dry naturally in a dark area. The dried coating turns to a greeny-yellow in colour.
Cyanotype printing is a negative process - where light cannot reach the sensitised areas the colour does not develop. Items such as leaves, petals or feathers can be used as a mask, as can an acetate sheet with an image drawn on it in permanent ink, or large photographic negatives.
When the print has fully exposed (times will vary depending on the intensity of UV light), the print is then rinsed in running water until the undeveloped chemical is removed. As the material dries, the print will oxidise and turn into the Prussian blue associated with the cyanotype blue printing process.
The photograph below shows a print I created using an acetate sheet with a design drawn in black fibre-tip pen. To get a clear print it is necessary to keep the acetate sheet stencil as flat and close to the fabric as possible. Clear adhesive tape, or the weight of a sheet of glass or empty picture frame can be used to help maintain contact with the surface. The less clear areas of my print (down the right hand side and bottom) are where my acetate sheet was slightly curled away from the fabric.