During the past year, Municipal Archives conservators have been treating the original oversize plans for the construction of the Brooklyn Bridge to make them accessible to scholars and the public. The drawings are very large, very detailed, and in some cases very brittle. Naturally, as conservators, we have a lot of questions: Why is some of the drawing paper so hard and brittle while some is not? Considering that they are eight to twelve-feet in length (and some even longer), how were the drawings made? Are the inks soluble and will they bleed or smudge during treatment with water? A more in-depth look at the manufacturing and drafting processes helps to inform our treatment plan.
Paper
In the early part of the 19th century paper mills mechanized their processes to keep up with demand. Later, chemical bleaching and alum-rosin sizing was introduced into the paper manufacturing process.
The standard type of paper available for drawings of this size was called “Manilla” so named because the abaca fiber used to produce the paper was recycled from ropes from Philippine ships. By the mid-19th century it was made with wood pulp and coloring to imitate the light yellow-brown color of the original. The paper produced was available in rolls and had a hard, smooth surface, perfect for drawing. It was cheaper than white drawing paper and could be cut to almost any length.
“Sizing” is essentially a filler or coating applied to paper that alters the surface texture in order to prevent inks from bleeding into the absorptive fibers. Traditionally applied to the surface of each individual sheet, the modern paper mill could add sizing to the beaten pulp, streamlining the papermaking process. Rosin is an acidic resin that requires alum (a soluble aluminum-containing-product) to adhere to cellulose fibers. The rosin coated fibers resist the absorption of water (or inks containing water) improving the writing surface.
Due to the very smooth surface and internal sizing, water is not readily absorbed into these papers. NYC Municipal Archives.
In addition, rosin sizing improved the formation and strength of each sheet as it sped through the iron rollers of the mills. Rollers heated to higher temperatures would create a smoother and harder surface. A drawing with very fine lines requires paper with these qualities to keep the pen point from skipping and the ink from spreading. According to Cathy Baker in her excellent book, From the Hand to the Machine, “. . . alum was used rather indiscriminately to correct most problems in the manufacture of paper.” [1]
The acidic sizing process, plus the inherent acidity of the wood pulp paper fibers that the drawings are composed of, as well as almost 100 years of less-than-ideal storage conditions prior to their acquisition by the Municipal Archives all contribute to the extreme brittleness that these papers display today.[2]
Raking light highlights the subtle texture of cracking from embrittlement in this drawing). NYC Municipal Archives.
In order to make the drawings available for research, part of our treatment includes washing out acidic compounds to prevent further degradation. The paper must be submerged in and absorb water so that the the acids will wash out. Knowing that there will be resistance to that absorption allows us to modify the treatment to include a solvent in the water bath. The solvent helps break the bond of the resin to the paper fiber, allowing water in and out of the fibers taking the acidic compounds along with it.
Ink
Submerging the paper in water means that we would have to be certain that the drawing will remain in place. Nineteenth century manuals for the practice of draftsmanship recommended that the “best” ink for drawing was “the cheapest” black India ink.[3] India ink is made from a combination of soot, or “lampblack” and gum Arabic with shellac or bicarbonate of ammonium or magnesium added to make it waterproof. The soot is the collected pure carbon formed when oil or tar is burned in a low oxygen environment. Essentially, the collected soot is ground into a very fine pigment and mixed with the water-soluble gum binder. The oldest true inks were made this way.
The first synthetic colored dye, Mauveine purple, was developed in 1856. Because of the immense commercial success of the dye (it could easily color cotton, making colored fabrics accessible to the new middle class) chemists raced to produce a rainbow. The first colors were in the red family—madders and carmines—purples and vermillion-like colors. The chemistry for blues did not arrive until 1863 and was quite expensive. The indigo plant remained the primary source for blue dye until the 1890s.
Three ink colors were used by draftsmen in the Brooklyn Bridge drawings: black for the primary image and shading; red for measurement indications; and blue to highlight certain features like train tracks or architectural elements. If a color was used to indicate measurements, red was the standard. Blues were used to indicate metals such as cast- and wrought-iron and steel.
The subtle red colored lines indicate measurements and the subtle blues indicate the steel railroad tracks on the bridge. NYC Municipal Archives
Professional draftsmen are artists, trained to produce mechanical or technical drawings that fully describe the way something functions or is constructed. A modern draftsman is highly-skilled in perspective drawing and graphic design, and knowledgeable regarding measuring systems, notational systems and engineering. Today, architectural drawings are largely finalized using CAD software. When the Brooklyn Bridge was built, the drawings were produced entirely by hand, using specialized tools and standardized drawing techniques. The draughtsman’s work was relied upon to convey complex details—from the span of the bridge to the angle of the threads on the bolts that held it together—in a common pictorial language that would be understood by the numerous foremen, craftsmen, contractors and vendors involved in the production of the structure.
The drawings were composed using a “T” square, compass, protractor, French curves, set triangles, and metal fountain pens to make straight and precisely curved thin lines in black ink. Shading was done using patterns of lines drawn closely together or by color washes. Patterns for shading and methods for drawing plans were published in guidebooks to provide instructions on the proper way to convey the details of the plans to others. [4]
Although most black inks remain insoluble during aqueous treatments, they must always be tested for water-fastness. Not only because one’s eye is not necessarily the best judge of ink quality, but because water-soluble dyes (aniline reds or indigo blues, or both) were sometimes added to improve the richness of the black.
Colored inks made with dyes were favored for fountain pens because the brightly colored liquid flows easily, but dye-based inks remain water soluble and fade in UV light. Colored inks must always be tested when considering aqueous treatment to prevent bleeding or washing off.
We learned that the red and blue inks of the Brooklyn Bridge drawings are soluble in water to various degrees. Thus, conservators may use special methods of fixing the inks or masking off areas that may be negatively affected by water. Similar to the use of a mordant added to fabric dyes to prevent bleeding, conservators may use fixatives to keep inks stable during aqueous treatment.
A water droplet is placed in an innocuous spot on the ink to be tested. The droplet is blotted with a clean blotter paper. This test shows minimal solubility of the black ink. NYC Municipal Archives.
There is always the expectation of a surprise when carrying out a complicated conservation treatment. That expectation is a motivating factor in planning a well- thought out and researched treatment protocol. Understanding the materials that make up your object contributes a great deal to performing a treatment without surprises.
[1] Baker, Cathleen A., From the Hand to the Machine. Ann Arbor, MI: Legacy Press, 2010. p. 72
[2] You can read about the discovery and storage conditions of the Brooklyn Bridge Plans before they were acquired by the archives here. Link to: https://www.americanheritage.com/treasure-carpentry-shop
[3] Camp, Fredrich T., “Draftsman’s Manual, or How Do I Learn Architecture?” New York: Comstock, 1883
[4] American Technical Society, “Cyclopedia of Architecture, Carpentry and Building,” Chicago: American Technical Society, 1912