Information about Whiteprint

Whiteprint is the commercial terminology to describe document reproduction using the diazo chemical process. It is also known as the blueline or blue-line process.

There are two components in this process:

1. the diazonium salt - a light sensitive chemical
2. the azodye (aka coupler) - a colourless 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. It is important to note that 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 media. 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 duplication 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 duplication shop, having been optimized based on trial runs conducted previously.

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, rotating at the speed set previously. 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 (ie. where there were no drawing or image marks on the master.) Once this process was complete, the undeveloped image could often be seen in very light yellow or white marks/lines on the diazo sheet. This step completed the exposure phase.

In the next phase, the master (the original) was peeled from the diazo paper, as the sandwich of master and diazo came slowly out of the machine, and only the diazo sheet (not the original) was then re-introduced into the machine into the developing chamber, it too being pulled in by friction rubber wheels pulling on the diazo paper to set a fixed total exposure time. In the developing chamber, fumes of ammonium hydroxide were introduced to create 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 colour from invisible (or yellow) to a visible dark colour. The range of colours for the lines that resulted was usually blue or black but sepia (a brownish hue) was also quite popular. In engineering offices, when making multiple copies of an original for a team of design reviewers to each have their own blueline copy, no more than 4 or 5 copies could typically be made at any one time, due to the rapid build-up of very strong ammonia fumes, even with ventilation fans running full-tilt in the duplication room. A slight delay, of perhaps 5 minutes, was often required for the ammonia fumes to subside enough to permit making additional blueline copies. If one was not happy with the lightness of the lines, it was also possible to run the blueline through the developing chamber once more, which often increased the contrast of the lines relative to the base media. Likewise, if one was not paying particular 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 develop the lines over the full span of the blueline copy.

Diazo printing was considered to be one of the most economically viable methods of document reproduction of large engineering-type drawings until environmental and health safety concerns of working in close proximity to ammonia over long periods of time forced the migration to xerography during the mid-1980s.

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 (in an office environment) or over just days (if outdoors) to total illegibility. 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. Incandescent lighting was also often used in working areas where the blueline engineering prints were needed to be posted on a wall continuously for long periods of times, and rapid fading was not an acceptable option. This fading process also meant that repeat duplication business would have to occur every few months in a typical office environment, for the duration of any engineering project using blueline copies.

Elaborate color-coding schemes were somewhat standardized in each engineering shop for indicating changes to blueline drawings. Revision control was done in contrasting color on the bluelines, 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, thus 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 connectivities or whatever, 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 coloured lines were written on the blue-line drawing using coloured pencils. Once the modification process was complete, the red-lined blueline prints would be taken to document control, where the original master would be modified to reflect the changes made during the engineering process. New blueline 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 final authorizing signatures affixed, would be archived into a vault, protected against theft, fire, and flooding.

See also

• Diazo
• Azo
• Azo compound