Temperature, humidity, light and pollution greatly affect the rate of deterioration of paper-based material. They alter both the chemical and physical rate of deterioration, and the rate of biological attack of paper-based material.

The major constituent of paper is cellulose which is a complex long chain molecule made up of smaller, predominantly glucose-based units. The breakdown of paper, whether it is caused by internal pollutants such as lignin or acid, or external factors such as atmospheric pollutants, is a chemical reaction and bound by scientific laws. One such law, the Arrhenius Equation, shows that rates of reaction approximately double per 10°C temperature rise. In fact, recent calculations by David Erhardt indicate that for cellulose hydrolysis — the most common cellulose chain cleaving reaction (and therefore paper weakening reaction) — the rate doubles for 5°C rise in temperature.1 An increase in temperature also produces a corresponding increase in biological activity i.e. an increase in mould growth. Physical damage is caused by temperature fluctuations which set up internal stresses in both the paper content and structure of bound/mounted material.

Much research has been carried out and many books written on conditions for storing paper-based material, and 18°–22°C is the recognised standard quoted for general storage areas.2 Most small public and private collections will not have the benefit of air-conditioning to control their storage, therefore a careful choice of position within the building should be made. Tim Padfield, in his recent article in the AICCM Bulletin3 advises locating libraries, archives etc. centrally within a building and also gives good practical advice on means of modifying building climate without using airconditioning, e.g. planting trees to give shade to rooves in hot climates, thus reducing radiant heating effects.

Good storage demands both low temperatures and little fluctuation, and in non-conditioned buildings, both these requirements have to be considered when looking at potential storage sites i.e. cool areas are often damp, but a slightly higher temperature can be tolerated if the temperature is stable. Aviod storing material in spaces, garages or out-houses where extremes of heat and cold will be experienced. Basement areas usually have a suitable temperature, but should be checked for high humidity levels and/or flood problems. Cupboard/small utility areas with boilers or water heaters are frequently both too hot and too dry, plus the combustion products of gas fueled units can increase the level of atmospheric pollutants in a storage area to unacceptable levels.

Relative Humidity (RH) is the ratio of the moisture content of a set volume of air at a set temperature to the maximum possible moisture content of the same air and is usually expressed as a percentage:

The higher the %RH, the higher the water content of the air. As the temperature rises, the water content in the air increases but gives the same %RH i.e. at 25°C and 60% RH, the air is considerably “wetter” than at 20°C.

As with temperature, RH affects material three ways — chemical, physical and biological. The physical effects of humidity are obvious. In most cases, an increase in RH produces swelling in material likely to be found in a collection (e.g. paper, wood, leather, adhesives), with a corresponding shrinkage as the RH falls. This “cycling” causes material to distort and eventually can lead to splitting and disintegration of an object. Prolonged low humidity at normal temperature causes a reduction in the internal moisture content of the material, producing shrinkage and embrittlement with associated cracking, flaking and eventual deterioration of the object.

Biological activity increases dramatically with increase in RH. At room temperature and 60% RH, the majority of common fungal spores will begin to grow. Paper, glue, binding and photographic material found in most archical collections provide excellent nutrients for their growth.

Chemical activity, especially hydrolysis reactions which are the main degradation reactions in cellulose products, also increase with increased humidity. Appropriate humidity levels have been identified for the various materials that make up library/archive collections4, and for general mixed storage, a standard of 45–55% has been arrived at. Humidity between 40–60% at 20°C is adequate for paper-based material, provided there is little (10%) daily fluctuation.

It is possible to measure humidity levels without resorting to the purchase of expensive thermohygrographs. Small electronic monitors are available from chemical agencies for around $200, as are sling hygrometers (modified wet and dry bulb thermometers). Also available from Archival supply companies are chemically impregnated strips that will give good indications of humidity levels, at a cost of approximately $1.00 each.

Low humidity can be a problem in modern air-conditioned buildings as the mechanism for controlling temperature can reduce the humidity in the air unless specific humidity control units are fitted. This humidity control is not normally considered necessary for general office accommodation and storage. In storage areas that have continued low humidity levels, it is possible to increase the humidity by two basic means — atomising water, or evaporation. The atomising unit introduces water droplets into the airflow and, if malfunctioning, can cause excessive humidity levels. The cold evaporative units rely on streams of air to evaporate cold water and lose efficiency at levels in excess of 60% RH, therefore reducing the potential damage caused by malfunction. Both types of units are available from air-conditioning companies. On a domestic scale, plants, bowls of moist pebbles or shallow dishes of water will relieve temporary problems i.e. during winter heating.

Permanently damp storge areas can be controlled using dehumidifier units that work as a refrigeration process; these cost upwards of $600 and have to be regularly emptied. Good air flow in such areas help prevent mould formation, and protective storage (i.e. wrappers and boxes), is essential. Structural modification of the area i.e. damp-proofing floors, sealing windows and vents, new damp courses, also provides long-term improvements.

Dirt and gasses can cause considerable damage to the paper-based collection. Dust and dirt produce physical damage by staining and abrasion, and also harbour fungal spores. Atmospheric pollutants such as ozone, nitrogen and sulphur oxides chemically react with the materials that make up archival collections causing pigment fading, paper embrittlement and discolouration. These pollutants also cause mutiple problems in photographic material including silvering out and image fading. Chemical pollutants can also be produced by poor storage materials such as polyvinyl chloride (PVC) enclosures, pine or oak boxes, highly acidic board boxes, etc. Ozone and other complex organic molecules are also produced inside archives, primarily by photocopiers and negative ion generators which are used to “clean” air in offices, smoking areas, etc. Some air-conditioned offices also have electrostatic precipitators in their plant to remove dust and debris from the air. These generate totally unacceptable levels of ozone and are not recommended for use in archives, galleries, museums or libraries.5

It is virtually impossible to control atmospheric pollution in non air-conditioned buildings, therefore it is important that storage materials are of the best archival quality in high pollution areas. The use of archival materials will buffer against outside pollutants and reduce to a minimum interior pollution. It is also recommended that photocopiers are kept in a well ventilated area away from collections. Where art works are on display in non air-conditioned buildings in highly polluted urban areas they should be properly protected with sealed backing board, acid-free mounts and perspex or glass glazing.

Light is made up of a range of wavelengths from high energy ultra violet through the visible spectrum to the lower energy infra red light. All wavelengths of light cause damage to paper-based material — infra red produces heat damage, and visible light causes colour fading. The ultra violet wavelengths catalyse (promote and accelerate) a common form of chemical breakdown in cellulose and therefore paper, increasing brittleness and yellowing.

Light levels are controlled by modifying lighting. 50 lux is the recommended level for display of photographs and paper-based images, and up to 150 lux for oil paintings.6 The correct choice of light fitting can reduce both light levels and UV content. Low power bulbs and tubes are now available, as are cool beam spots for display work. UV modified fluorescent tubes are produced by Philips Sylvana and UV shielding sleeves are also available from archival suppliers. Direct natural light can be restricted by the use of blinds, curtains and UV absorbing film on window glass. UV radiation damage to works on display can be reduced by the use of UV filtering perspex in framing and display cabinets.

Light levels can be measured using a meter. These are available from photographic, archival or chemical suppliers. Meters for measuring UV radiation are also available but are very expensive and are available from major scientific instrument suppliers or overseas archival supply companies only.

Biological infestation, be it fungal or animal, can be kept under control by good housekeeping. A regular cleaning programme coupled with suitable environmental conditions should virtually eliminate the possibility of mould growth on a collection. Good cleaning and good habits, i.e. no food or drink in collection areas, will also significantly reduce the risk of insect or rodent infestation. Regular cleaning removes insect pests, their eggs and larvae and the scaps of organic matter they feed on. Maintaining humidity levels below 60% will virtually eliminate the possibility of fungal growth.

Our society is becoming more and more aware of the problems of excessive use of chemicals in the home and environment. Conservators are also investigating more passive, non-chemical means of insect and fungal control. These are less harmful to the conservator, user and collection material.7 Examples of this are the use of low temperatures and carbon dioxide fumigation. There is some debate as to temperature and duration of freezing time, but a domestic freezer set at fast freeze for three days should prove adequate. The suggested parameters are -29 to -40°C and 1–3 days.8 Material to be frozen should be bagged or wrapped before freezing. A 60% concentration of carbon dioxide is also proving effective, and studies using nitrogen, low oxygen concentration and vacuum techniques are showing promise.9 Gamma radiation, as used to sterilize medical supplies etc., is also an effective means of eliminating insect and fungal life. This completely sterilizes the material, killing dormant reproductive material as well as active insects or fungus. This technique is quick and cheap, but research has suggested it may increase the ageing in paper-based material.10

These techniques are non residual and material must be cleaned and regularly inspected on return to the collection. There is no point in treating material and then returning it to humid or infested storage conditions as reinfestation will be rapid.

All collections are continually put at risk when infected new donations are brought in. This is especially the case when dirty unsorted boxes from poor storage (e.g. garden sheds, attics) are donated to historical societies, museums or libraries. Such material should be assessed for infestation before introducing it to the collection area. Once silverfish or mould have spread, the damage is done Clean and sort in a separate area and then send for fumigation if necessary.

If you are in a rodent-prone area, it is possible to install proofing strips available from the larger pest control companies to the doors and to seal all plumbing, electrical conduit entrance and exit points. Use chemical bait stations to remove existing pests; a pest-control company should be consulted since many of the “off the shelf remedies are no longer effective as rodents have developed immunity.

Silverfish are the most commonly occuring insect pests in paper-based collections. They “graze” on paper size, fillers and adhesives and will also digest cellulose and linen book cloth, causing notching to edges, delaminations and lacunae (areas of loss). Silverfish are also extremely difficult to eradicate once they have taken hold of a collection. Regular cleaning programmes will reduce the problem, but a severe, building-wide infestation will need professional chemical treatment. Infected rooms will need to be thoroughly cleaned and then floors sprayed with a suitable insecticide. Cislin 10, a pyrethrum-based chemical, is the preferred option as it is water-based and does not leave a powdery residue. This chemical has low mammalian toxicity and low allergenic reactivity with a residual of approximately 12 weeks. Two visits will probably be required to ensure eradication of the insects, and regular inspections of the area should be carried out. A thorough cleaning of all storage units and infected materials will be required to eliminate the insects as spraying the area does not kill the insect eggs which hatch and cause rapid reinfestation.

A localised silverfish problem can be dealt with by the non-professional in the following manner:

Book mites can also be a problem in paper-based collections. Almost invisible to the naked eye, they cause skin irritation and rashes when the material is handled. They are difficult to remove but can be controlled by reducing humidity and thoroughly cleaning the collection as for silverfish infestations.

Fungus or “mould” is the other common biological problem encountered in collections. The paper, glue, binding material and photographic emulsions found in general archive and library collections provide excellent nutrients for fungal growth. Damage to material is rapid and, in the main, irreversible. Staining is caused by complex organometallic compounds secreted by the fungal mycellium into the paper fibres. The black, green, yellow or white powdery residue found on the surface are the spores and fruiting bodies. Removal of this does not remove the vegetative part of the fungus. Massive fungal activity in a collection may require professional treatment, such as carbon dioxide fumigation or gamma radiation. However, localised small problems can be dealt with by the non-professional in the following way:

Both insects and damp threaten paper-based material when placed in long-term storage. To protect your books etc., the following guidelines should be observed: