Blueprints are prints made on a specially prepared substrate such as cotton, paper, or Mylar. The material on which the blueprints are made is coated with a light sensitive solution. When exposed, the solution sets into a dark blue color, leaving the unexposed portions white. The result is a traditional blueprint, also called a cyanoprint, a design in white against a blue background. Blueprints are used in architectural designs such as renderings for houses and ships, and the term is also used more generally to refer to a detailed and precise plan.
Contact printing is used to produce blueprints. First, a drawing is made on tracing paper, cloth, or another material which is transparent, allowing light through. The drawing is positioned over material soaked in a blueprint solution, and then exposed to a bright light. The resulting blueprint is chemically fixed and washed to remove chemical traces. It will be resistant to fading and marking, and is usually made on a durable material which can be rolled up and easily transported between offices and job sites.
The solution used to coat the substrate for blueprints is made from potassium ferricyanide and ammonium ferric citrate. It was developed by Sir John Herschal, a British astronomer, in the mid 1800s. The chemicals will react with bright light to form ferric ferrocyanide, an insoluble blue pigment. Potentially, numerous blueprints could be made from a master drawing and easily distributed, at a cost much lower than that of conventional photographic reproduction.
Modern advances in replication technology have allowed other techniques to replace the blueprint, although some firms continue to use blueprints for rendering. When marking up a blueprint, chalks in contrasting colors such as red and yellow are used, so that the markings are clearly visible. The edited blueprint is returned to the person who drafted it for the changes to be made, and then is printed again and checked. Once all parties are satisfied with the blueprint, it will be officially stamped, making it into a legal document which will be filed along with the other paperwork on the structure being built.
Drawing for blueprints and drafting in general requires training and skill. Good blueprints have crisp, clean lines and easy to read text, usually written in unadorned capital letters. High quality blueprints can look like a work of art, as they come from the hands of someone trained in technical drawing, and some building owners hang their blueprints on the wall as an artistic addition to the interior design.