Blueprints

Also called a cyanotype. Developed in the 1840s by John Herschel, blueprinting uses a wet process to produce an image of white lines on a cyan or Prussian blue ground. To make a blueprint, a heavy paper (or more rarely drafting linen) support is impregnated with potassium ferricyanide and ferric ammonium, placed under a translucent original drawing, weighted with glass, and exposed to ultraviolet light. After sufficient light exposure, the glass and original drawing are removed and the blueprint paper is washed to reveal a negative image. This same process, using an intermediary reprographic drawing, could also be used to produce a positive blueprint—blue lines on a white ground—however, this more expensive and time-intensive method was far less commonly employed.

The major disadvantages of the blueprint process, however, included paper distortions caused by the wet process which might render scale drawings less accurately, as well as the inability to make further copies from the blueprints. Nonetheless, for its efficiency and low cost, the blueprint process, further simplified and mechanized by the turn of the 20th century, became the most widely used reprographic process from the mid-19th century through the first half of the 20th century.

In archival settings, because the process involves ammonium, the resulting prints should not be stored in contact with other papers that have a buffered reserve, nor should blueprints be de-acidified, as the resulting chemical interactions can cause irreversible image loss. Blueprints are also highly light-sensitive and should not be exposed to ultraviolet light for long periods of time.

Pellet prints

Invented in 1887 by Henry Pellet, the Pellet process uses a wet process to produce an image of cyan or Prussian blue lines on a white ground. Essentially, this process produces a positive image, while a blueprint produces a negative one. To make a Pellet print, a paper (or more rarely drafting linen) support is coated with ferric salts suspended in a gelatin emulsion, placed under a translucent original drawing, weighted with glass, and exposed to ultraviolet light. As with the blueprint process, after sufficient light exposure, the original drawing is removed, the paper washed in a ferrocyanide bath, and then rinsed in an acidic bath to reveal a positive image. This process required fewer steps than creating a positive blueprint, and was thus more widely employed during the late 19th and early 20th centuries.

In an archival setting, Pellet prints should be treated and stored under the same conditions as blueprints.

Van Dyke prints

The Van Dyke process, invented by F. R. Van Dyke in 1901, created an intermediary print—a white line on a dark brown ground—that could be used in any of several other processes, such as blueprinting, to create a positive print, i.e. a dark line on a light ground.

Using a translucent vellum support, the paper was prepared with a coating of silver salts. The vellum was then united with the original drawing, exposed to ultraviolet light, and later washed in a sodium thiosulfate bath.

In an archival setting, Van Dyke prints are relatively rare, as they were created for temporary purposes and often discarded after the final positive prints were made. Because of the nitrates used in preparing the paper and the preferred thin paper itself, Van Dyke prints are often extremely brittle and susceptible to damage. Van Dyke prints should be stored separately and, when possible reformatted before the image degrades unacceptably.

Diazotypes

By the middle of the 20th century, wet-process reprographic techniques such as blueprinting, Pellet, and Van Dyke printing were largely superseded by various dry-printing processes. The most common of these is the Diazotype process, refined in the 1920s, which used paper supports sensitized with diazonium salts, a coupling agent, and an acid stabilizer to produce a dark line on a white ground. The Diazo positive print was considered more readable than a negative blueprint, and the dry process eliminated the image distortion of wet paper.

As with other earlier reprographic processes, a translucent original drawing was placed over a sheet of the sensitized paper and exposed to light. However, the next step exposed the paper to an ammonia gas. This alkaline gas catalyzed a reaction between the diazo salts and the coupling agent to produce an image that fixed in the paper over several days. Typically these prints have blue or dark purple lines on a mottled cream-colored background, although line and ground colors can vary.

A related process is the sepia Diazo print, which produced either a positive or negative print in dark brown and light tones. The negative versions of these prints were most often produced as intermediaries, like the earlier Van Dyke process, to allow corrections and revisions without disturbing the original drawing. In the negative printing process, additional resins and oils were sometimes added to the paper support to increase translucency. Positive sepia prints, generally made on opaque paper, were typically used as an alternative to positive blueline Diazo prints.

Both blueline and sepia prints were often poorly and cheaply processed, resulting in undesirable residual chemical content. Off-gassing of sulfurous compounds, image fading, and yellowing of the paper support are common signs of degradation and are not reversible. Diazo prints are also highly light-sensitive and can fade to illegibility within a short period of exposure to ultraviolet light.

In archival practice, Diazo prints are the most common reprographic format encountered in late 20th-century architectural collections. However, their inherent fragility and fugitive images, as compared with blueprints and earlier processes, makes their care problematic. Diazos—particularly sepia prints, which readily transfer color to adjacent papers—should be physically segregated from all other types of media. Exposure to light and pollutants in air should be minimized, and wherever possible, original drawings or reformatted prints should be kept for reference.

Rolled storage

The most common form of storage for architectural drawings—both for drawings in active professional use and in archival environments—has traditionally been in rolls. While this allows for efficiency in the use of space and ease of retrieval, potentially damaging situations can arise from a casual approach to roll storage. For reprographic drawings on paper supports, rolling can stress paper fibers and make unrolling for examination more difficult. Small rolls can be easily crushed and ends can be creased and torn without additional protective wrapping and support.

Flat storage

In circumstances where fragile, rigid, or otherwise atypical media makes rolled storage unfeasible, storage in flat boxes or flatfile drawers can be the best choice. Acid-free and lignin-free portfolio boxes, ideally no more than four inches deep, can be cost-effective and allow more flexibility in arrangement on shelving. Flatfile furniture should meet the minimum requirements of archivally-sound construction—powder- or enamel-coated steel units with no rust or sharp edges that could damage materials while stored or moved in and out of the drawers.

Drawings should be grouped and identified for ease in retrieval, preferably within folders that are cut to fit the full dimensions of the corresponding container. As with rolled materials, the potentially damaging chemical interactions of print processes should be considered when grouping drawings in folders. Wherever possible, for example, blueprints should be segregated from diazotypes, and sepia diazo prints should be stored alone to the extent possible.