These days, when someone talks about a blueprint, a digital printer or plotter creates a document with black or blue lines on a white sheet of paper. However, over a century ago, blueprints were actually blue. How did this process work, and why did it change? We break down its history:


Sir John Herschel developed the process for creating blueprints in 1842, finding a way to reproduce documents with ammonium iron citrate and potassium ferrocyanide.

In this process, initially created just to transfer notes, two papers are exposed to a bright light, and the applied chemicals create a permanent blue compound, called blue ferric ferrocyanide, except for where the paper is covered or has lines drawn. As a result, a solid-blue document with white lines is created.


Believe it or not, architectural firms and builders used this process until the 1940s, when the diazo whitepress and xerographic copiers replaced it with black or gray lines on a white background.

The diazo process didn’t differ greatly, however. Considered “dry” blueprinting, it utilized paper with a ferro-gallate coating, giving it a yellow color. Similar to original blueprinting, this light-sensitive paper was placed beneath an original drawing; the copy, then, was exposed to ammonia vapor. Although initially done by hand, machines eventually automated the process.

By the 1970s, those in the industry found that the diazo process worked faster on documents with blue lines and, ultimately, that it created a clearer drawing. Thus, for an easier-to-read image, drawings started having a solid white background, and blue lines delineated the image. The process, however, changed very little: Instead, only the lines drawn on the original appeared on the copy.

21th Century

By 2006, stricter ammonia regulations complicated the architectural printing process. However, digital files and print technology started emerging in the 1990s, first with pen plotters and then with inkjet technology. Compared to today, both are significantly slower, but at the time, printing a drawing out on inkjet greatly sped up the diazo process.

Throughout the 2000s, the cost of architectural printers declined, making them more affordable for most firms, and pen plotters came back, offering a lower-cost option for lower volumes of drawings. Along with the price, digital files and smaller printers mean more jobs can be completed in less time, all while reducing the amount of space taken up in the office.

Whether you’re looking for an architectural printer or need your construction documents created, Joseph Merritt & Co.’s team is here to assist your project.