Whiteprint - The Diazo Printing Process

The Diazo Printing Process

There are two components in this process:

1. The diazonium salt: a light sensitive chemical
2. The azo dye (also known as coupler): a colorless chemical that combines with diazo to produce color

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 of media material. Depending on the thickness of the master and the media type, the intensity of the UV exposure light is adjusted through an intensity knob that had typically been pre-marked for all the typical media types that are used for masters in any particular shop. Similarly, the speed control (for setting the speed at which the sheets would be pulled through the machine) are 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 the 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 through the original and neutralizes the light sensitive diazonium salt in the areas on the sheet that soon become the white areas on the copy where it penetrated (where there was no image on the master.) Once this process is complete, the undeveloped image 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. In the developing chamber, 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 coupler lines changing color from invisible (or yellow) to a visible dark color. The range of colors for the lines that result 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 outside. Smaller and mid-size blueprint machines were often outfitted with ammonia neutralizer absorbers which would absorb some of the ammonia for a period of time.

If one is not happy with the lightness of the lines, it is also possible to run the blue-line through the developing chamber once more, which often increases the contrast of the lines relative to the base media. Repeated lack of contrast and light prints is also a tip-off that the operator needs to adjust speed or amount of ammonia. Likewise, if one is not paying close attention, the master and the diazo print can both go through the developing chamber. If this occurred, one simply peels the master from the diazo paper, and run the diazo sheet through the developer once more to more fully develop the lines.

Diazo printing was one of the most economic methods of document reproduction of large engineering and architectural drawings.