Buzz
Architectural or engineering technical drawings often appear as white images and text on blue paper. But why is this the case?
The reason behind this comes down to how these documents are produced. The blueprinting process was invented in the mid-1800s when scientists found that a combination of ammonium iron citrate and potassium ferrocyanide created a light-sensitive solution ideal for document reproduction.
Here’s the procedure: An artist first draws an image on translucent tracing paper or cloth. This drawing is then placed on blueprinting paper, which has been coated with a mixture of ammonium iron citrate and potassium ferrocyanide, which is dried after being applied. When both papers are exposed to intense light, the chemicals react, forming a blue compound called ferric ferrocyanide, or Prussian Blue, except where the original drawing blocked the light. After washing and drying the paper, you’re left with a negative image where the lines of the drawing remain white against a dark blue background.
The blueprinting technique proved faster and more affordable than manually tracing original documents, making it a popular and economical method for reproducing drawings and texts. After carbon copying and photocopy machines took over for smaller documents, architects, engineers, and shipwrights continued using blueprinting for their large-scale drawings. More recently, the diazo whiteprint process and large-format xerographic photocopiers have largely replaced blueprinting, even for these specific uses. As a result, many so-called 'blueprints' now feature black or grey lines on a white background. However, the term 'xerograph' just doesn’t have the same ring as blueprint when it comes to shorthand for a master plan.
