Whether a sheet of paper lasts indefinitely or only briefly depends on the materials and methods used in its manufacture as well as on the environment in which it is stored. Since the early observations of Murray and the practical solutions suggested by Sutermeister and Barrow, it has been repeatedly demonstrated that additives which create acidity within paper hasten its deterioration. Acidic species catalyze hydrolytic degradation of the polymeric cellulose molecules, reducing their chain length; even a few chain scissions per molecule cause a substantial loss of physical properties. Mildly basic species such as calcium or magnesium carbonate minimize the acid concentration and therefore the rate of the acid hydrolysis reaction. The cellulose molecule can also suffer hydrolytic cleavage in an alkaline environment. Hence the need for a weakly basic compound to buffer the pH of paper close to neutrality.
Oxidized cellulose structures that may be introduced during the bleaching process can also adversely affect paper. Carboxyl groups in the cellulose matrix have been shown to accelerate the process of degradation, especially in the presence of trace concentrations of copper and iron metals. In such cases, considerable yellowing of paper results, along with a loss of physical properties . Recent work has shown that copper and iron catalysts are much less effective in a nonacidic environment. Thus, the deacidification of paper stabilizes it also against oxidation by metal ions . In the absence of metal catalysts, oxidative degradation does not appear to be a significant factor, since the concentration of aldehyde, ketone, and carboxyl groups does not change appreciably during accelerated aging. Acidic species and metallic contaminants have also been shown to catalyze the cross-linking of cellulose. Residual lignin in wood pulp can accelerate the degradation of paper too. Lignin itself has a fairly stable structure, but chlorinated lignin can cause paper to yellow.
J.A. Chapman was probably the first to demonstrate the importance of environmental conditions, publishing his results in 1919 and 1920. He showed that books stored in libraries in the tropical areas of India deteriorated more rapidly than the same books stored in cooler areas in northern India or in England. Since then a considerable body of data has been developed using accelerated aging to show that high temperatures, as well as high levels of relative humidity, facilitate the degradation of cellulose. A further disadvantage of high humidity7 is an increase in the rate of mold growth, which occurs over a wide temperature range if the relative humidity exceeds 70%.
F.L. Hudson and C.J. Edwards tested a book that had been left in Antarctica by the ill-fated expedition of Robert Scott in 1912 and preserved there until 1959. They compared books of the same time stored in England, observing statistically significant differences tor several physical properties, which suggested that the cooler environment greatly increased the stability of paper.
The magnitude of the preservation problem created by air pollution has been indicated by studies comparing books from libraries in heavily polluted cities with the same books stored in rural areas. Laboratory studies have shown that paper exposed to small amounts of sulfur dioxide and cellulosic fibers exposed to nitrogen oxides undergo substantial degradation. The damage to books takes the form of a phenomenon known as browning, as well as a definite weakening of the edges of the leaves. Degradation from polluted air was probably a more serious problem 50 to 100 years ago, when unvented gas space heaters were common and gaslights were used for illumination. Since sulfur exists as an impurity in most sources of natural gas, there was a steady supply of sulfur dioxide. This was oxidized to sulfur trioxide by metallic impurities in the paper, such as manganese and iron. Sulfur trioxide combines easily with moisture to produce sulfuric acid.
Ozone can conceivably exert a deleterious effect on paper by inducing the formation of peroxide groups in the presence of moisture. Trace concentrations of metal catalysts such as copper and iron can then decompose the peroxide groups to initiate oxidation reactions, which in turn can lead to chain scission. However, there is no direct evidence to show how much of a threat ozone presents to the permanence of paper under ambient aging conditions. In the case of nitrogen oxides too, experimental work is needed to determine tolerance levels.
Exposure of library and archive collections to light is not usually a serious factor except for items on exhibit. Moreover, window glass filters out ultraviolet radiation below about 330 nm. Light of wavelengths higher than 310 nm cannot cause direct photolysis of pure cellulose. However, some dyes and related compounds, lignin, and metal ions can absorb light in the near ultraviolet and visible regions of the spectrum, and in their excited states induce photo-sensitized degradation of cellulose. Such reactions can proceed by various mechanisms, some of which require the presence of oxygen or moisture, or both.
Materials that are associated with paper records have their own sets of problems. Very few specimens of ink writing have resisted decay and disintegration since antiquity. Those which have survived were made from pigments closely related to such materials as bitumen, lamp black, cinnabar (red mercuric sulfide), minimum (red lead), gold, and silver. More recent historic documents were written in iron gall inks, which were made of gallic and tannic acid, ferrous sulfate (copperas), and gum arabic. In their varied and often uncertain preparation, other acids such as sulfuric, hydrochloric, and acetic acids were also added, as were other metallic sulfates, including alum. The excessive acidity of such inks is evident in many an old document, where the ink is seen to have eaten through the paper. The proportions of the ingredients and the extent of acidity determine not only the color, but also the lightfastness of the ink; badly made iron gall inks not only fade easily, but may also be water-soluble.
Some historic documents present interesting examples of fading. George Mason's draft of the Virginia Declaration of Rights (now at the Library of Congress) was evidently written in two inks. One of the inks has faded considerably, while the rest of the writing is as clear today as it might have been in 1776. The Declaration of Independence also appears highly faded. This most-treasured but much-abused document probably lost most of its ink when the printer William J. Stone pressed a moistened sheet of paper against it for a copper engraving plate that he prepared in 1823. The Treaty of Ghent (1814) was written in iron gall ink on parchment. It is remarkable that even without significant exposure to light, the text of the treaty has faded across several pages to such an extent that it is barely legible. An example of fading of fountain pen ink is presented by the instrument of Japanese surrender (1945), which bears signatures in various inks, some as clearly visible as they were on the day they were written, while others have faded to various extents. This document was on exhibit for some time, and the fading was in all probability induced by light.
Leather is plagued by the same enemies as paper: acid from within and environmental conditions from without. Leather that has been affected by acid from the tanning process, air pollution, and poor storage conditions develops a surface which appears to be crumbling into small reddish particles, or "red rot." Parchment, which is also made from animal skin, is more resistant to the same influences, as it ends up in an alkaline condition at the end of the manufacturing process. Its alkaline content neutralizes acidic pollutants in the atmosphere. Specimens of parchment kept in a cool, dry place are known to have survived almost 2,000 years. However, substantial drops in humidity can cause parchment to suffer severe distortions through uneven shrinkage. It can lose its suppleness through dehydration and become stiff and brittle.
As we will see, there are several approaches available to a custodian concerned with the preservation of archival collections, such as conservation treatment to restore and photoduplication to minimize the use of rare and delicate documents. But the custodian's primary effort should be directed toward providing the right environment for the storage, use, and exhibit of the collections. A good environmental purification and control system stabilizes temperature and relative humidity at desired levels, while filtering out sulfur and nitrogen oxides and particulate matter. Lower temperatures and relative humidity benefit most items in a collection. Environmental conditions in storage areas usually represent a compromise between the requirements of the collections and human comfort.
The Declaration of Independence and the Constitution, both on permanent exhibit at the National Archives, are preserved mainly by environmental control. Each leaf of these documents is sealed within a glass enclosure in an atmosphere of humidified helium. A filter absorbs all ultraviolet radiation, as well as the more energetic portion of visible light. Furthermore, the intensity of light is restricted to 3 foot-candles per hour. At the Library of Congress, the Gutenberg Bible (one of three existing copies printed on vellum) and the Mainz Bible are on display inside specially constructed enclosures that provide a protective environment of filtered air at a constant temperature of 10°C and a relative humidity of 50%. These exceptional items, on which exceptional care is lavished, illustrate the importance of environmental controls.