A blueprint is a paper-based reproduction of a technical drawing, such as an architectural plan or an engineering design, historically used to create accurate copies of original documents. The term itself refers directly to the distinctive white-on-blue appearance that characterized these reproductions for over a century. This unique color scheme is the direct result of the cyanotype photochemical process, which was developed in the mid-19th century. Understanding the chemistry explains why the background is a deep blue color while the drawn lines remain stark white.
The Chemistry Behind the Blue Color
The traditional blue color is achieved through the cyanotype process, which was invented by the English scientist Sir John Herschel in 1842. This method relies on a photosensitive solution applied to paper, consisting primarily of two iron compounds: ferric ammonium citrate and potassium ferricyanide. The paper is coated with this solution and then dried, resulting in a light-yellow coating that is sensitive to ultraviolet (UV) light.
To create a copy, the original drawing, made on translucent paper, is placed directly on top of the coated paper and exposed to a strong UV light source. Where the light hits the coated paper, a photoreaction occurs, reducing the trivalent ferric iron ($\text{Fe}^{3+}$) in the citrate to divalent ferrous iron ($\text{Fe}^{2+}$). The lines of the original drawing block the light, preventing this chemical change in the areas directly beneath them.
The image is then developed by washing the paper with water, which forms the blue pigment. The divalent ferrous iron reacts with the potassium ferricyanide to create an insoluble compound known as ferric ferrocyanide, or Prussian blue. This deep blue pigment remains fixed to the paper fibers, forming the background of the print. The unexposed chemicals protected by the original drawing’s lines are washed away, leaving those areas as the white of the paper.
Why the Cyanotype Process Was Adopted
The cyanotype process was adopted by architects and engineers from the late 19th century due to its practical and economic advantages. Before its introduction, reproducing large technical drawings required expensive photolithographic techniques or the time-consuming process of hand-tracing the original design. The blueprint method allowed for rapid and accurate production of copies, eliminating the need for manual labor.
The process was simple, requiring only the two chemical solutions, paper, and a source of UV light, making it accessible for any drafting office. By the 1890s, the cost of a blueprint was approximately one-tenth that of a hand-traced reproduction, making it a cost-effective solution for construction and manufacturing. The resulting Prussian blue pigment is stable and color-fast when properly processed, ensuring the working copies were durable for use on construction sites and in factories.
The Transition to Modern Printing
The traditional blueprint process began its decline in the early 1940s with the introduction of the diazo print, commonly referred to as the whiteprint or blue-line process. This successor technology produced blue or black lines on a white background, which was easier to read and annotate than the white-on-blue format. The diazo process was also simpler to execute and involved fewer toxic chemicals than the cyanotype method.
Whiteprints remained the standard for technical drawing reproduction for several decades, but they were not permanent and would fade if exposed to light for extended periods. The ultimate replacement for all chemical-based reproduction methods came with the rise of Computer-Aided Design (CAD) software. CAD allowed engineers and architects to create and store designs as easily editable digital files.
Today, large-format digital plotters and xerographic copiers have rendered the chemical reproduction of drawings obsolete. These modern devices print directly from digital files, producing high-resolution, full-color plans instantly. While the physical process is no longer used, the term “blueprint” remains in common use as an informal reference for any detailed plan or design.