Blueprint

The process is obsolete, initially superseded by the diazo-based whiteprint process, and later by large-format xerographic photocopiers. It has since almost entirely been superseded by digital computer-aided construction drawings.

The term blueprint continues to be used informally to refer to any floor plan (and by analogy, any type of plan). Practising engineers, architects, and drafters often call them drawings, prints, or plans.

The blueprint process

The blueprint process uses a photosensitive ferric compound. The best-known version utilises ammonium ferric citrate and potassium ferricyanide. The paper of the prospective copy is impregnated with a solution of ammonium ferric citrate and dried. When the paper is illuminated, a photoreaction turns the trivalent ferric iron into divalent ferrous iron. The image is then developed using a solution of potassium ferricyanide forming insoluble ferroferricyanide (Prussian blue or Turnbull's blue) with the divalent iron. Excess ammonium ferric citrate and potassium ferricyanide are then washed away. The process is also known as cyanotype.

This is a simple process for the reproduction of any light-transmitting document. Engineers and architects drew their designs on cartridge paper; these were then traced on to tracing paper using India ink for reproduction whenever needed. The tracing paper drawing is placed on top of the sensitized paper, and both are clamped under glass, in a daylight exposure frame, which is similar to a picture frame. The frame is put out into daylight, requiring a minute or two under a bright sun, or about thirty minutes under an overcast sky to complete the exposure. Where ultra-violet light is transmitted through the tracing paper, the light-sensitive coating converts to a stable blue or black dye. Where the India ink blocks the ultra-violet light the coating does not convert and remains soluble. The image can be seen forming. When a strong image is seen the frame is brought indoors to stop the process. The unconverted coating is washed away, and the paper is then dried. The result is a copy of the original image with the clear background area rendered dark blue and the image reproduced as white lines.

This process has several features:

• the image is stable
• as it is a contact process, no large-field optical system is required
• the reproduced document will have the same scale as the original
• the paper is soaked in liquid during processing, and minor distortions can occur
• the dark blue background makes it difficult to alter, thus preserving
  • the approved drawing during use
  • a record of the approved specifications
  • the history of alterations recorded on the sheet
  • the references to other drawings

Introduction of the blueprint process eliminated the expense of photolithographic reproduction or of hand-tracing of original drawings. By the later 1890s in American architectural offices, a blueprint was one-tenth the cost of a hand-traced reproduction. The blueprint process is still used for special artistic and photographic effects, on paper and fabrics.

Various base materials have been used for blueprints. Paper was a common choice; for more durable prints linen was sometimes used, but with time, the linen prints would shrink slightly. To combat this problem, printing on imitation vellum and, later, polyester film (Mylar) was implemented.