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Inherent Vice: Structures

Reproduction Methods

Beginning in the late 18th century, various reproduction methods began to replace hand copying of important documents. In the late 19th century, copying options were expanded and copying of original architectural drawings, plans, and maps also became common. Objects produced using some of these copying processes deteriorate more quickly than others, and some have detrimental effects on other collections if they are stored together. Therefore, identifying particular processes is crucial in making storage and other preservation decisions for these materials.

A complete review of all copying processes is beyond the scope of this lesson, but a few of the most common are described here.

Letter copying process—Invented in 1780, this involved writing letters with a copying pencil and then pressing them against water-dampened thin tissue with a screw press. This process became popular in the 1850s and was used into the 1950s.

Hectograph process—This process, which became popular in the late 1800s, used aniline ink (usually purple or blue) and was used both for copying documents and architectural drawings. Writing was transferred to a gelatin pad from which copies were made. Various other processes for copying documents (e.g., mimeographs, Photostat, spirit duplicating) were invented in the late 19th and early 20th centuries, but many of these were not stable. See Louis Nadeau's Office Copying and Printing Processes (PDF) for a summary of processes.

Blueprints can fade on exposure to light or alkaline materials.

Blueprints—Invented in 1871, this process was used primarily to copy architectural drawings, plans, and maps. A translucent original was placed over sensitized paper containing iron salts, and then exposed to light. After exposure, the original was removed and the copy was washed to remove the unexposed compounds under the lines of the original drawing. The result was an image of white lines on a blue background. Blueprints are sensitive to alkaline environments, and light exposure. Similar processes include Vandyke prints (which are sensitive to sulphur and other pollutants and often brittle), and Ferrogallic prints (which are alkaline and light sensitive and can stain adjacent prints).

Diazotypes—This process became popular in the 1920s, and by the 1950s it was used to copy office records as well as maps, drawings, and plans. A translucent original was placed on paper coated with diazonium salts; this was exposed to ultraviolet light, which attacked the salts and turned the exposed area off-white. Depending on the specifc type of compound used on the paper, the lines on diazo prints can be various colors. Diazo prints are often processed using ammonia, which causes them to give off alkaline vapors that may harm other materials. The residual chemicals remaining in the print (diazo prints are not washed) also cause oxidation and discoloration of the print itself, particularly on the edges. Sepia prints, a type of diazo print, can create greasy stains or pink stains on adjacent materials. Diazo prints are light sensitive.

Electrostatic copying—Also known as xerography, this technology was introduced in the late 1940s and became popular in the 1960s. It makes a copy of the original by forming an image with toner powder using electrostatic charge and fusing the image to paper using a solvent, heat, or light. This is the most common copying process used today. If electrostatic copies are made on good quality paper and well-bonded to the paper, they are very stable. However, electrostatic copies of maps or plans made on matte or transparent Mylar are not permanent because the toner does not bond well to the Mylar.

See Lois Olcott Price's The Fabrication of Architectural Drawings to 1950 for more information on reproduction methods for architectural drawings and other materials.