The general principles set forth below come
Background | Causes of Paper Deterioration | Temperature & Humidity Mold | Atmospheric Pollutants | Light Ordinary paper is an organic substance composed of cellulose fibers from green plants. Many plants produce fibers suitable for papermaking. Most of these fibers have been used for this purpose at some time in the past. Modern paper, however, is generally made from wood fibers. These are usually chemically purified for book papers or mechanically ground for newsprint. Fine writing papers generally contain linen or cotton fibers. As a consequence of its organic nature, paper is always subject to deterioration if it is improperly made or stored. At the same time, under proper conditions paper may last for hundreds of years. The oldest known extant paper, which dates from around A.D. 100, was found in Western China by Sir Aurel Stein, who conducted excavations there during the early part of the 20th century. Most early paper, particularly paper made from about the 12th to the middle of the 19th centuries, was strong and durable. If they have been properly stored during the intervening centuries, the majority of books published during that period are still in excellent condition. Unless they are of the type designated as permanent/durable or acid-free, most modern book pages, on the other hand, have an expected useful life of less than 50 years and will not last even this long unless housed in a proper environment. Most modern nonbook papers (bond and ledger papers, photocopy papers, onion-skin, and the like) do not have a significantly longer expected life unless they are permanent/durable or acid-free. Cause of Paper Deterioration It is generally agreed by conservators and paper chemists that the primary reason modern book papers deteriorate so rapidly is that they are acidic -- a condition brought about largely by the use of alum-rosin compounds as sizing agents. In the presence of atmospheric moisture that alum-rosin sizing generates sulfuric acid which acts to destroy the paper, a reaction that is accelerated by high temperature and high humidity. A second factor in the deterioration of many modern papers is the use of unpurified wood fibers (groundwood). Such fibers are substantially weaker than those of chemically purified wood pulps. In addition, the lignins that remain in the pulp may degrade to form acids that weaken the sheet. Paper produced from groundwood is also more subject to the deteriorative effects of light than paper made from purified pulps. Newsprint contains mostly fibers from groundwood, and book papers often contain some percentage of these unpurified fibers. Because alum-rosin sizing and groundwood both came into common use for the manufacture of book papers about the middle of the 19th century, conservators, librarians, archivists, and others concerned with the problem often refer to the period from about 1850 to the present as the "era of bad paper." Other important factors in the deterioration of paper include such pollutants in the atmosphere as sulfur dioxide, nitrogen dioxide, and ozone; the invisible ultraviolet radiation in both sunlight and fluorescent light, and the short wavelengths of visible light; and microorganisms (mold and bacteria), which usually grow best under conditions of high temperature and high humidity. In addition, insects and rodents often feed on paper and bookbindings and may cause serious damage to these materials. In most instances, the owners of books and manuscripts can provide protection to their collections by observing the basic precautions set forth below. Temperature and Humidity Extensive research and a wealth of accumulated evidence show that the lower the temperature at which it is stored, the longer paper will last. It can be demonstrated theoretically that for every 10°F decrease in temperature, the useful life of paper is approximately doubled. As a result, a number of modern research libraries and archives have been designed with storage areas in which the temperature can be maintained as low as 55°F. Few, if any, private homes or small libraries would find such a low temperature feasible or acceptable, but the principle is sound -- the lower the storage temperature the longer the paper will endure, all other factors being equal. For most homes and libraries practical considerations dictate a temperature range of 68° to 75°F. In the summer, if a house is not centrally air-conditioned, a room-type air-conditioner may be used to keep the temperature at the proper level. In the winter, the thermostat in the room where books are housed should be set as low as feasible. Books should never be stored in attic areas, which, if not air-conditioned, may reach temperatures of 100° to 150°F on summer days, especially in southern and middle latitudes. Under such conditions, book paper undergoes rapid deterioration. Humidity also has a serious effect on the life of book paper. If too high, it hastens acid deterioration and leads to deterioration by such biological gents as mold and bacteria. If too low, the paper suffers from desiccation (drying out). Present research indicates that most modern books (with the exception of those few that may be printed on permanent or acid-free paper) will last longer if kept at a relative humidity of 40 to 50 percent. Leather bindings require a slightly more humid environment of 45 to 55 percent, while vellum or parchment materials should be stored at 50 to 60 percent. If a compromise is necessary, relative humidity should be maintained as close to 50 percent as possible. In some climates, maintenance of humidity is this range will require the use of a humidifier in the winter and a dehumidifier in the summer. Low winter temperatures will cause moisture condensation on the windows of a room having relative humidity in the 45 to 55 percent range. Double-glazed windows are helpful in maintaining humidity at the right level without condensation. On unusually cold days, however, it may be necessary to accept some reduction in humidity to avoid excessive moisture on interior window surfaces. There is some evidence that regular changes in temperature and relative humidity (cycling) can lead to a weakening of paper, as a result of internal stresses set up in the paper as it responds to these changes. There are no firm data to indicate how serious this effect may be, but Library of Congress scientists do not believe that is results in measurable damage to paper if such changes in temperature and relative humidity can be held to less than 10°F and 15 percent. It is probable that the book structure is more seriously affected by cycling than the paper, but here too the effects are probably minimal if changes can be held within the above limits. Experimental work is under way at the Library to obtain more precise data on the effect of temperature and humidity changes on paper. Every year the Library's Preservation Office receives a number of inquiries from persons who have suffered damage to book and manuscript collections stored in basements which have either flooded or are so humid that the materials have mildewed. Unless a basement is absolutely waterproof and "floodproof," and unless it can be maintained at a relative humidity of less than 60 percent, it should not be used for the storage of valuable books and documents. Rare book collectors who wish to maintain the best possible environmental conditions may want to invest in a hygrothermograph which provides a continuous record of temperature and humidity. Others may wish to have an accurate, inexpensive instrument that does not produce a record but does indicate both temperature and humidity, such as a thermo-hygrometer. Mold or mildew attacks on books can occur when both temperature and humidity are uncontrolled. Mold spores are always present in the air and on books. These spores may begin to grow when the temperature is above 80°F, the relative humidity is above 70 percent, and the air is stagnant or nearly so. Some molds can grow at temperatures as low as 40°F if the humidity is right. In cases where severe mold growth has been experienced and the area has not been thoroughly fumigated, mold growth can start again at temperatures and humidities below what are generally considered the threshold limits. Mold prevention requires that both temperature and humidity be kept within the range suggested above. Initially mold may be only a nuisance. If detected before growth has become advanced, the visible evidence of mold (usually the spores) can be brushed away quite readily. When mold is first observed in materials, immediate action should be taken to change the environmental conditions so that it cannot continue to propagate. This may be accomplished by lowering the temperature and humidity. If mold growth is already well advanced, the entire book collection, etc., and the area in which it is housed will require fumigation. Unless these steps are taken promptly the mold will digest the material upon which it has begun to grow. This results not only in the staining and deterioration of the materials involved but in a rapid loss in strength of the paper. Mold can be very difficult to bring under control and serious damage is often caused before it is detected. As in so many instances, prevention is easier than correction, and it will pay the collector, the librarian, the curator, the archivist, or the amateur lover of books to monitor the environment so that conditions favorable to mold growth do not occur. Atmospheric Pollutants Such gases as sulfur dioxide, hydrogen sulfide, and nitrogen dioxide are often major factors in the deterioration of paper. They originate from the burning of fossil fuels or other combustion or chemical processes such as occur in furnaces, automobiles, power plants, and the like and thus are most serious in industrial and urban areas. However, they are difficult to combat anywhere. No economical means of protection are possible in the ordinary home or small library. If it is desired to remove these pollutants from the air supply of libraries, archives, and museums the intake air must be passed through treatment beds incorporated in the air-handling equipment, where special chemicals absorb the pollutant gases. Certain compounds are not so dangerous in themselves but cause damage to paper because of their tendency to combine with atmospheric moisture to form acids. Thus, SO2 is catalyzed by other compounds in the air into SO3, which then united with water vapor to form sulfuric acid. Ozone, another atmospheric pollutant, also causes the oxidation and consequent embrittlement of paper. This gas is often found in the air after thunderstorms and is generated by the interaction of sunlight and nitrogen dioxide from automobile exhaust. A prominent constituent of smog, ozone is also produced by electrostatic filtering systems, sometimes used ignorantly by architects and engineers in the air-conditioning of libraries, archives, and museums and less frequently in residential air-conditioning systems. Dirt and dust particles in the air not only carry with them the adsorbed pollutants mentioned above but may exert an abrasive action on books and papers. Thus, keeping clean filters in the air-conditioning system is an important responsibility of the custodian, whether he be private collector or librarian. The ultraviolet rays in sunlight and fluorescent light promote the oxidation of cellulose and, therefore, can cause rapid and serious deterioration in paper. Although the deterioration caused by visible light is less severe than that caused by the invisible ultraviolet, it too can cause damage to paper, leather, and fabrics. The rays at the blue end of the visible spectrum are more damaging than those at the red end. Light has two effects on paper, both of which result in its ultimate embrittlement and deterioration. First, it has a bleaching action and thus causes some whitening of paper and the fading of colored papers and certain inks. Second, it causes any lignins which may be present in the paper to react with other compounds and turns it yellow on exposure to light. At the same time these visible effects of light are taking place, certain invisible changes also occur. Fibers in the paper are broken into smaller and smaller units until they are so short that they can no longer maintain the bonds necessary to hold the paper together. Unfortunately, the reactions initiated by light continue after the source of damage has been removed. All of this means that paper will last far longer in the dark (other factors being equal) than in the light. Storing book collections in the dark is seldom practical. However, suitable precautions may include storage out of direct sunlight, the use of special ultraviolet filtering materials over fluorescent tubes or windows, or the use of heavy draperies. The use of fluorescent lights is usually limited in the home, so this source of ultraviolet is not likely to be a problem there. Where these damaging rays need to be filtered out of sunlight, UF-3 Plexiglas or Acrylite OP-2 is available as a substitute for regular window glass, or these materials may be taped or otherwise fastened over existing window glass. Valuable manuscripts and similar paper documents such as old deeds, military commissions, diplomas, and the like require similar protection if they are to last. Preferably, these materials should be kept in acid-free file folders in the dark. If possible they should be deacidified and alkaline buffered by a competent paper conservator before storage. However, in those instances where it is desired to display a particular item, it should be cleaned and repaired as necessary and then deacidified, buffered, and framed under UF-3 Plexiglas or Acrylite OP-2. These protective measures should never be in direct contact with the document, however, since the yellow dye used to filter out the UV will discolor the paper. Competent framers are usually prepared to mount and mat documents with acid-free board. The mat separates the document from the plastic. Even when so protected, documents should not be hung where they are exposed to strong light from the out-of-doors or to fluorescent light. Exhibit cases are generally provided with fluorescent lights which emit heavily in the ultraviolet region and thus may cause severe damage to books, manuscripts, and other paper materials unless measures are taken to filter out the damaging rays. For this purpose special filters made of UF-3 or Acrylite OP-2 in the forms of sleeves which slip over standard fluorescent tubes are available. Under some conditions it may be preferable to use the plastic filtering material in sheet form. |
Environmental Protection of Books and Related Materials