Whiteprint is the commercial terminology to describe document reproduction using the
diazoDiazo refers to a type of organic compound that has two linked nitrogen compounds. The general formula is R
2C=N
2. The simplest example of a diazo compound is diazomethane. The electronic structure of diazo compounds involves a positive charge on the central nitrogen and...
chemical process. It is also known as the blueline or blue-line process.
There are two components in this process:
- the diazonium salt: a light sensitive chemical
- 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
paperPaper is thin material mainly used for writing upon, printing upon or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....
.
Whiteprint is the commercial terminology to describe document reproduction using the
diazoDiazo refers to a type of organic compound that has two linked nitrogen compounds. The general formula is R
2C=N
2. The simplest example of a diazo compound is diazomethane. The electronic structure of diazo compounds involves a positive charge on the central nitrogen and...
chemical process. It is also known as the blueline or blue-line process.
There are two components in this process:
- the diazonium salt: a light sensitive chemical
- 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
paperPaper is thin material mainly used for writing upon, printing upon or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....
. The resulting coating is then dried yielding the specially treated paper commercially sold as Diazo paper. This solution can also be applied to
polyesterPolyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...
film or to
vellumVellum is mammal skin prepared for writing or printing on, to produce single pages, scrolls, codices or books. It is generally smooth and durable, although there are great variations depending on preparation, the quality of the skin and the type of animal used...
.
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 paperBond paper is a high quality durable writing paper similar to bank paper but having a weight greater than 50 g/m2. The name comes from it having originally been made for documents such as government bonds. It is now used for letterheads, other stationery and as paper for electronic...
s (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
UVUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
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
ultravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
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 hydroxideAmmonium hydroxide , also known as ammonia water, ammonical liquor, ammonia liquor, aqua ammonia, or aqueous ammonia, is a solution of ammonia in water...
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 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. 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
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH
3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
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 blueline 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 tipoff 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 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 bluelines should not be exposed to the elements but bluelines 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 blueline drawings.
Revision controlRevision control is the management of changes to documents, programs, and other information stored as computer files. It is most commonly used in software development, where a team of people may be changing the same files...
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, making it a legal document.
An alternative
revision controlRevision control is the management of changes to documents, programs, and other information stored as computer files. It is most commonly used in software development, where a team of people may be changing the same files...
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 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 authorizing signatures affixed, would be archived in a vault.