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 that accurately reproduces a translucent original in size, but can reproduce only limited tonal range and no color.

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 around the turn of the 20th century 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 print that lasts only a few months was sufficient as a working drawing for many purposes, and the original vellum drawings could be stored and reprinted as needed. Whiteprinting became obsolete around the turn of the 21st Century as technical drawing was replaced by computer-aided design and thus digital pen plotters or large-format printers (still sometimes colloquially called "blueprint printers") became favored.

Two components underpin diazo printing:

In a related sense, the process relies on two properties of diazonium compounds:

In a variety of combinations and strengths, these two chemicals are mixed in water and coated onto paper. The resulting coating is then dried yielding the specially treated paper commercially sold as diazo paper. This solution can also be applied to polyester film or to vellum.

The process starts with original documents that have been created on a translucent medium. Such media include polyester films, vellums, linens, and translucent bond papers (bonds). Any medium that allows some light to pass through typically works as a master; the desired durability of the master determines the choice. Depending on the thickness and type of the master, the intensity of the UV exposure light is adjusted according to media types commonly used for masters in any particular shop. Similarly, the speed control (for setting the speed at which the sheets are pulled through the machine) is likewise typically pre-marked in any particular shop, having been optimized based on trial runs.

The original document is laid on top of the chemically-coated side of a sheet of diazo paper, which is retrieved from a light-protected flat file, and the two sheets are fed into the diazo duplicator, being pulled into the machine by rotating rubber friction wheels. There are two chambers inside the machine. The first is the exposure area, where the sandwich of the two sheets (the master and the diazo paper) passes in front of an ultraviolet lamp. Ultraviolet light penetrates the original and neutralizes the light-sensitive diazonium salt wherever there is no image on the master. These areas become the white areas on the copy. Once this process is complete, the undeveloped image at the locations where the UV light could not penetrate can often be seen as very light yellow or white marks/lines on the diazo sheet. This completes the exposure phase.

Next, the original is peeled from the diazo paper as the sandwich of master and diazo exits the machine, and the diazo sheet alone is fed into the developing chamber. Here, fumes of ammonium hydroxide create an extremely alkaline environment. Under these conditions, the azodyes (couplers) react with the remaining diazonium salt and undergo a chemical reaction that results in the unexposed lines changing color from invisible (or yellow) to a visible dark color. The range of colors for these lines is usually blue or black, but sepia (a brownish hue) is also quite popular. When making multiple copies of an original no more than four or five copies can typically be made at a time, due to the build-up of ammonia fumes, even with ventilation fans in the duplication room. A slight delay of perhaps five minutes is often required for the fumes to subside enough to permit making additional copies if no ventilation exists. Many blueprint shops ran ventilation ducts from the machines to the outside. Smaller and mid-size blueprint machines were often outfitted with neutralizers which absorbed some of the ammonia for some time.