These days we are working on a few blue-prints and blue-lines in our lab. Before starting the work, we tried to discuss the material and technique used in making them. This write-up is the summary of our discussion on the topic.
Both these processes (blue prints and blue lines/white prints) use different light sensitive chemicals for re-producing or copying the original drawings made on translucent paper. These processes were discontinued over a period of time because more stable and easier processes were developed later to replace them.
Here we will share some information regarding these two techniques.
John Herschel, who was a chemist, astronomer, and photographer, developed the process for blueprints in 1842. Herschel had discovered the cyanotype process after a series of experiments. The process starts by taking a drawn image on semi-transparent paper weighed down on top of a sheet of paper or cloth. The paper or cloth is pre-coated with a photosensitive chemical mixture of potassium ferricyanide and ferric ammonium citrate. Once the drawing is exposed to light the exposed parts of the drawing (the background) became blue, while the drawing lines blocked the coated paper from exposure and remained white.
Introduction of the blueprint process eliminated the expense of photo lithographic reproduction and the need to hand-trace original drawings.
White-prints or Blue-lines
Whiteprint describes a document reproduction produced by using the diazo chemical process. It is also known as the blue-line process since the result is blue lines on a white background. It is a contact printing process which accurately reproduces the original in size, but cannot reproduce continuous tones or colors. The light-sensitivity of the chemicals used was known in the 1890s and several related printing processes were patented at that time. Whiteprinting replaced the blueprint process for reproducing architectural and engineering drawings because the process was simpler and involved fewer toxic chemicals. A blue-line print is not permanent and will fade if exposed to light for weeks or months, but a drawing print that lasts only a few months is sufficient for many purposes.
There are two components in this process:
- diazonium salt: a light sensitive chemical
- Azo dye (also known as the coupler): a colorless chemical that combines with the salt to produce color.