Whiteprint

Whiteprint is the commercial terminology to describe document reproduction using the diazo chemical process. It is also known as the blue-line or blue-line process. There are two components in this process:

1. The diazonium salt: a light sensitive chemical
2. The azodye (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 diazo printing process

The process starts with original documents (those that needed to be reproduced) that have been created on a translucent medium. Such media included polyester films (1980s), vellums (1970s–1980s), linens (1930s–1970s), and translucent bond papers (bonds). Any media that allowed some quantity of light to pass through typically worked well as a master, and the duration of time for which the master would ultimately be required to be maintained largely determined the particular choice of media material. Depending on the thickness of the master and the media type, the intensity of the UV exposure light (see below) was adjusted through an intensity knob that had typically been pre-marked for all the typical media types that were 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) were likewise typically pre-marked in any particular shop, having been optimized based on trial runs.

The original document was laid on top of the chemically-coated side of a sheet of the diazo paper, which was retrieved from a light-protected flat file, and the two sheets were fed into the diazo-duplicator, being pulled into the machine by rotating rubber friction wheels. There were two chambers inside the machine. The first was the exposure area, where the sandwich of the two sheets (the master and the diazo paper) passed in front of an ultraviolet lamp. Ultraviolet light penetrated through the original and neutralized the light sensitive diazonium salt in the areas on the sheet that would soon become the white areas on the copy where it penetrated (where there was no image on the master.) Once this process was complete, the undeveloped image could often be seen as very light yellow or white marks/lines on the diazo sheet. This completed the exposure phase.

Next, the original was peeled from the diazo paper, as the sandwich of master and diazo exited the machine, and the diazo sheet alone was fed into the developing chamber. In the developing chamber, fumes of ammonium hydroxide created an extremely alkaline environment. Under these conditions, the azodyes (couplers) reacted with the remaining diazonium salt and underwent a chemical reaction that resulted in the coupler lines changing color from invisible (or yellow) to a visible dark color. The range of colors for the lines that resulted was usually blue or black but sepia (a brownish hue) was also quite popular. When making multiple copies of an original no more than four or five copies could 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 was often required for the fumes to subside enough to permit making additional copies if no ventilation existed. 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 was not happy with the lightness of the lines, it was also possible to run the blue-line through the developing chamber once more, which often increased the contrast of the lines relative to the base media. Repeated lack of contrast and light prints were also a tip-off that the operator needed to adjust speed and-or amount of ammonia. Likewise, if one was not paying close attention, the master and the diazo print could both go through the developing chamber. If this occurred, one simply peeled the master from the diazo paper, and ran 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.

Fading prints

A quirk of diazo blueline prints was that with continued exposure to ultraviolet light, either from natural sunlight or from typical office fluorescent lighting, a blueline copy would fade over a span of months (indoors) or just days (outdoors), becoming illegible. Hence, blueline drawings that were used as engineering working copy prints had to be protected when not in use, and stored in flat files in the dark. Improperly exposed bluelines were more likely to fade at an increased rate since the chemical reaction in the ammonia phase would continue until the process was complete. Incandescent lighting was often used in areas where the blueline engineering prints were needed to be posted on a wall for long periods, where rapid fading was undesired. This fading process also required reduplication every few months in a typical office for any project using bluelines. Properly exposed blue-lines should not be exposed to the elements but blue-lines kept in flat files or hanging on racks in a cool, dry room often retain the majority of their lines and are able to be scanned into digital format for various purposes.

Document control

Elaborate color-coding schemes were somewhat standardized in each engineering shop for indicating changes to blue-line drawings. Revision control was done in contrasting color on the blue-lines, for example red markup of a blueprint copy by the engineer, then yellow markup on the copy by the draftsman who implemented the changes on the original drawing, then brown markup by the checker, on a check-print (a brown-line). Finally, the architect or engineer, draftsman, checker and supervisor would sign the original drawing, making it a legal document.

An alternative revision control scheme was to use red-lines to indicate additions, yellow-lines to indicate deletions, and green-line tracings to check (i.e., to confirm conductivities, or to add notes to draftsmen for how to make corrections), on copies of blue-line prints that were under engineering control.

In whatever scheme was standardized in each shop, these colored lines were written on the blue-line drawing using colored pencils. Once the modification process was complete, the red-lined blue-line prints would be taken to document control, where the original master would be modified to reflect the changes made during the engineering process. New blue-line prints would then be made of the modified master, and the process continued, until upon completion of the project, the master with all changes incorporated, and authorizing signatures affixed, would be archived in a vault.

See also

• Azo
• Azo compound
• Blueprint
• Ozalid process

References