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Whiteprint describes document reproduction using the diazo chemical process. It is also known as the blue-line process. 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 in reproducing architect's and engineer's 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 for many purposes a drawing print that lasts only a few months is sufficient.
The diazo printing process 
There are two components in this process:
- The diazonium salt: a light sensitive chemical
- 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.
Fading prints 
A quirk of diazo blueline prints is that with continued exposure to ultraviolet light, either from natural sunlight or from typical office fluorescent lighting, a blueline copy will fade over a span of months (indoors) or just days (outdoors), becoming illegible. Hence, blueline drawings that are used as engineering working copy prints has to be protected when not in use, and stored in flat files in the dark. Improperly exposed bluelines are more likely to fade at an increased rate since the chemical reaction in the ammonia phase will 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 requires 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 
During the 20th century, elaborate color-coding schemes were somewhat standardized in each engineering shop for indicating changes to blue-line drawings. Revisions were 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 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.