After an aircraft designer draws a complete aircraft design on tracing paper, the tracing paper is then placed over a chemically modified blueprinting paper. An ultraviolet light is shone on the two papers, inciting a chemical reaction in the blueprinting paper that turns the entire page blue except where the original tracing blocked the light. The blueprinting paper is then washed and dried to protect the lines from exposure.
Blueprinting paper turns blue because it is coated with a mix of iron ammonium citrate and potassium ferricyanide. Nothing happens to the two chemicals in the dark, but when bombarded with ultraviolet light, the iron ammonium citrate becomes an iron salt and reacts with the potassium ferricyanide to form a compound called prussian blue. Washing the paper removes the rest of the water-soluble chemicals and leaves a white drawing on a blue background.
Blueprinting is a fast and cost-effective way to make a copy of an original aircraft drawing. However, it is fast becoming an obsolete copying process and is now being replaced by the diazo whiteprint process and large-format xerographic photocopiers. Though many modern blueprints are made up of black or grey lines on a white background, people still call them blueprints, mainly because of tradition.