Whiteprint

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:

In a variety of combinations and strengths, these two chemicals are mixed together 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 media 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 using an intensity knob typically pre-marked for 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) pass in front of an ultraviolet lamp. Ultraviolet light penetrates the original and neutralizes the light sensitive diazonium salt wherever there was 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.

...
Wikipedia