Environmental Deterioration


Introduction

Museums have an obligation to care for their collections in a way which will ensure that they remain in the best possible condition, for as long as possible. In order to do this museums must ensure that collections are stored, displayed and transported in secure conditions where the risk of damage and deterioration is minimal at all times. The same care should be displayed for collection documentation, both electronic and manual.

Why objects deteriorate

The chemical make-up of most materials is such that they are unstable and change with time. This is referred to as the 'inherent vice' of a material. Most objects begin to deteriorate from the moment they are made. The rate and degree of deterioration depends on the environmental conditions to which the artefacts are subjected. These environmental factors include:

Deterioration can be caused by any single factor or combination of factors whilst objects are in storage, transit or on display.

Temperature and relative humidity

Fluctuations in temperature and relative humidity (RH) can cause many problems for objects. High temperatures can lead to drying out of organic components such as timber and paper and can cause physical damage if the materials in the object expand at different rates. DeterioČration processes such as oxidation are accelerated by increased temperature. At the opposite extreme, freezing can also cause physical damage to some materials. High RH can encourage the growth of fungi, cause swelling of moisture-absorbent materials, cause condensation and a host of other problems. Low RH can lead to shrinkage and the embrittlement of some organic materials.

From a preservation viewpoint it is most important for the temperature and RH to be as stable as possible. This does not necessarily warrant air-conditioning, as often a building is able to regulate itself to a certain degree. A stable environment means ensuring constant temperature and RH levels, as opposed to rapid fluctuations. If constant, temperature and RH levels can be outside of the 'ideal' conditions of 20 degrees C +/- 2 degrees C and 50% +/- 5% RH. If necessary, vulnerable objects can be stored or displayed in their own micro-environment.

The more 'housing' (eg box, display case) that surrounds an object, the more it will be protected from the elements, i.e. if an object is in a building, the building material will protect the object from outside environmental changes. If the object is in a display case, or stored in an acid-free box, it will be further protected against fluctuating humidity within the building. In regard to reducing RH fluctuations, it is advisable to have hygroscopic (water absorbing) materials used in display and storage areas. If materials such as acid-free mount board, washed cotton and linen are used, they will absorb and expel moisture in the air.

As sunlight passes through windows, solar radiation (heat) is trapped, raising room temperature (the greenhouse effect). Eliminating direct sunlight by fitting blinds and curtains to windows will help reduce the daily temperature fluctuations that affect RH stability. If necessary, RH may be controlled by raising or lowering the temperature. This can be achieved relatively easily by using a heater with a thermostat on a low setting (although check with the manufacturer if it can be left on for extended periods of time). The slightly higher temperature will lower the humidity. In summer, high RH may be reduced by opening the windows and allowing air circulation. As a short-term answer to the problem of high RH, dehumidifiers can be placed in rooms.

Try to avoid placing objects on or against external walls as these can be prone to condensation and west-facing walls can be a source of fluctuating high temperatures. More sensitive organic objects should be placed in inner rooms if possible, away from external walls. Place 'spacers' behind frames so they are not in direct contact with walls. This will allow air to circulate behind the pictures and help prevent them from being affected by the changes in temperature and RH conditions of the external walls.

Light and ultraviolet radiation (UV)

Light will damage a wide variety of materials such as dyes and pigments, cellulosic materials such as wood or cotton or proteinaceous materials such as leather and wool. The amount of deterioration caused by light is proportional to both the light level (intensity) and the amount of time that an object is exposed to that light. For example, if the light level on an object is doubled, the same amount of deterioration will occur in half the time. The most harmful forms of light are direct sunlight and fluorescent lights due to their high UV content.

As a general rule, it is best to avoid direct sunlight and very intense lights and to limit the length of time that objects are exposed to light. Avoid storing or displaying objects close to windows, fluorescent lights or on west-facing walls with a lot of sunlight. Where possible it is better to use electric light (such as incandescent) and eliminate direct sunlight by installing blinds, curtains or by blacking out windows. By using electric light you can limit the length of time that objects are exposed to light ie only light displays when visitors are in the museum. At the very least, windows in storage and display areas should be covered with UV filtering film. Always remember, the lower the light level the longer the life of an object.

It is also desirable to avoid placing lights inside display cases. Lights inside a display case increase the temperature of the air in the case and consequently lower the relative humidity, creating a very dry environment. This will cause irreversible damage to the objects. Having lights inside the cases also increases the likelihood of fire.

The simplest approach to conservation lighting is to:

  1. Eliminate as much natural light as possible as it contains the highest UV content and is difficult to control.
  2. Choose a low wattage light source with little or no UV radiation, either:
    • incandescent light bulbs
    • tungsten halogen lights with UV filters, or
    • low UV emission fluorescent tubes with UV filters.
  3. Position lights well away from objects and never inside display cases.
  4. Only light displays when visitors are in the museum.

Other points that you should consider:

Insects and pests

Insects and pests can cause a great deal of damage to objects. Damage may be caused by the insects or pests eating materials, marking them by leaving droppings and regurgitated matter, boring into organic matter, making nests and so on. Insects and pests often thrive in environments with high RH or temperature and in poorly cleaned areas.

It is recommended that objects and their surroundings be kept clean and separate from food areas. Regular inspections of storage and display areas is important. Avoid storing objects in damp and cluttered spaces. An Integrated Pest Management (IPM) program will provide a simple, cost effective and safe means of long-term pest control. If insect infestation occurs, consult a conservator as soon as possible to implement an appropriate eradication plan.

Pollutants

Pollutants can be broadly classified into two categories, particulate and gaseous. Particulate pollution (such as dust, dirt and fluff) can form an unsightly surface film and encourage fungal growth, corrosion, abrasion and insect attack. Particles can adhere to the surface of varnish and paint or become entangled in the fibres of textiles; an object will then require complex methods of cleaning that can be harmful and expensive. Raw concrete, plaster and textiles ie some carpets, curtains and soft furnishings can generate particulate pollutants and should be avoided where possible (concrete and plaster can be sealed).

Sometimes collections may be exposed to salt from sea air. Both organic and inorganic materials on display will be adversely affected by this. Metals in particular, will react by forming irreversible compounds which will cause them to deteriorate rapidly. Well fitted display cases or archival quality boxing and/or wrapping and good housekeeping will eliminate problems with particulate pollutants and sea air.

Many of the woods, glues, paints and other materials often used in the construction of buildings, cases, frames or objects themselves can emit harmful vapours such as organic acids and formaldehydes. These substances can become quite concentrated and seriously damage the chemical structure of materials such as wood, paper, textiles, ivory, bone, and especially metals. Gaseous pollutants from industry such as sulphur dioxide, nitrogen dioxide and ozone can also seriously degrade many materials, as can carbon dioxide from motor vehicles. Photocopiers produce a high level of ozone and should never be positioned close to objects.

Display and storage cases should never be constructed from untreated wood products, particularly chipboard, MDF and plywood. Materials more suitable for case manufacture are coated solid woods and metals.

Fungi

Under suitable conditions fungi will attack organic materials such as wood, paper, leather, cotton etc. The type and degree of deterioration can vary greatly, ranging from slight discolouration to total destruction. Fungi will generally only germinate, grow and produce spores when the RH is higher than 60%. By keeping levels below this, fungal attack can be avoided. Storage and display areas should be kept well ventilated and free from dust and dirt. Avoid damp areas such as basements and inspect items regularly for evidence of mould attack. If an attack is noticed, move the object to a dry environment and call a conservator for advice regarding elimination of the problem with air flow. Always wear a dust mask when handling objects with live mould growths.

Human influences

Objects are often extremely delicate due to construction methods and materials or as a result of deterioration. The simple act of touching an object can lead to deterioration, as damaging amino acids, moisture or salts from the skin can transfer to the object. Vibration resulting from handling can also cause much damage.

Handle objects as little as possible. If handling is necessary, wear clean cotton gloves or at least wash and rinse hands thoroughly. Do not use handles or other protrusions for lifting or holding objects as these are often the weakest point. Lift objects by cradling the base with one hand and supporting the object with the other.

As many of the materials and techniques used by general framers can cause extensive damage to museum objects, framing should be solely entrusted to a specialised conservation framer. Contact a conservator for referral.

Conclusion

Develop a disaster preparedness plan to eliminate or reduce potential risks to the collection, including fire and flood, theft and burglary, vandalism and accidental damage. In addition to prevention, also develop a comprehensive recovery plan that details skills and resources that can be accessed in case of an emergency or disaster. Establishing a disaster preparedness network with other local museums reduces the workload and cost of developing plans and resources.

Further Reading