The sensitized paper is then exposed to light. Where the areas of the sensitized paper are not obscured by the drawing, the light makes the two chemicals react to form blue. The exposed paper is then washed in water. This produces a negative image, with the drawing appearing in white against a dark blue background. The semi-transparent original is placed on top of the sensitized paper and a copy of the same size as the original is made by direct contact.
Light destroys the diazonium salt.
The chemicals on the paper acquire color only in the areas not exposed to light. This diazotype method produces dark lines on a white background, and is the popular method used today for reproduction of large-format drawings. The reason people still use blueprints is because it is an inexpensive process. Inc. All rights reserved.