Engineers and architects drew their designs on cartridge paper these were then traced on to tracing paper using India ink for reproduction whenever needed. This is a simple process for the reproduction of any light transmitting document. Excess ammonium ferric citrate and potassium ferricyanide are then washed away. The image is then developed using a solution of potassium ferricyanide forming insoluble ferroferricyanide ( Prussian blue or Turnbull's blue) with the divalent iron. When the paper is illuminated, a photoreaction turns the trivalent ferric iron into divalent ferrous iron. The paper is impregnated with a solution of ammonium ferric citrate and dried. The best known is a process using ammonium ferric citrate and potassium ferricyanide. The blueprint process is based on a photosensitive ferric compound. The blueprint process Architectural drawing, 1902 Architectural drawing, Canada, 1936 Practising engineers, architects, and drafters often call them 'drawings', “prints”, or “plans”. The term blueprint continues to be used less formally to refer to any floor plan (and even less formally, any type of plan). It has almost entirely been superseded by digital computer-aided construction drawings. The process is obsolete, largely displaced by the diazo whiteprint process, and later by large-format xerographic photocopiers. Color or shades of grey could not be reproduced. Blueprints were characterized by white lines on a blue background, a negative of the original. It was widely used for over a century for the reproduction of specification drawings used in construction and industry. The process allowed rapid and accurate production of an unlimited number of copies. Xerograph just doesn't have the same ring as blueprint for a shorthand description for a master plan, though.A blueprint is a reproduction of a technical drawing or engineering drawing using a contact print process on light-sensitive sheets introduced by Sir John Herschel in 1842. More recently, the diazo whiteprint process and large-format xerographic photocopiers have largely replaced blueprinting even for these specialized purposes, and many “blueprints” are now black or grey lines on a white background. After carbon copying and copier machines took on that job for smaller documents, architects, engineers and shipwrights continued to use blueprinting to copy their large-scale drawings. The technique was faster and more cost-effective than hand-tracing original documents, and caught on as an easy, inexpensive way to reproduce drawings and texts.

After the paper is washed and dried to keep those lines from exposing, you’re left with a negative image of white (or whatever color the blueprint paper originally was) against a dark blue background. When the two papers are exposed to a bright light, the two chemicals react to form an insoluble blue compound called blue ferric ferrocyanide (also known as Prussian Blue), except where the blueprinting paper was covered, and the light blocked, by the lines of the original drawing. The drawing is placed over a piece of blueprinting paper, which has been coated with a mix of ammonium iron citrate and potassium ferrocyanide from an aqueous solution and dried.

The process goes like this: Someone creates a drawing on translucent tracing paper or cloth. The blueprinting process was developed in the mid-1800s, when scientists discovered that ammonium iron citrate and potassium ferrocyanide created a photosensitive solution that could be used for reproducing documents. It’s because of how those documents are made. Technical drawings of architectural or engineering designs always seem to consist of white images and text on blue paper.

Old blank blueprint paper