A museum's primary purpose is to ensure that its collections are available for future generations. A popular image is of white-coated conservators working in laboratories to stabilise and repair items that have suffered from damage or neglect. This is remedial conservation. It is, however, more efficient to prevent or reduce decay from occurring in the first place. This is preventive conservation.

Preventive conservation is a process that seeks to prevent, reduce or mitigate the effect of all the factors that, every day, threaten an object's continued survival. It requires an all-embracing approach - a constant assessment of how collections are stored, handled, displayed and maintained. It should involve all people who work for a museum, not simply conservators. Good building repair and maintenance routines are as necessary for long-term survival as the more obvious and immediate issues of object handling, storage and security. Preventive conservation is a strategic enterprise driven by a museum's primary purpose, that cannot be introduced without planning, or its benefits demonstrated in other than the long term.

Deterioration is a continuous, natural process. It can, however, be slowed; indeed, science has suggested ways in which the natural lifespan of most museum objects can be extended. Many of these techniques are based on common-sense and good housekeeping. However, these must inevitably be reinforced by the results of current research, and access to specialist information is vital if an informed approach is to be taken. There is benefit in having formal systems in place, incorporating agreed standards (such as those set by the Museum & Galleries Commission's Standards in the Museum Care of ... Collections). Appropriate and continuing training, to ensure consistency in approach over time and between individuals, is also vital.

Table 1 summarises the major threats to museum collections, how they are caused, and the action that can be taken to remove or reduce their impact.

An intensive museum-wide study of the current situation - a preventive conservation audit -will enable a museum to:

. assess the degree of danger posed by the threats to that individual museum

. establish the priority areas for action

. demonstrate the benefits, in quantifiable terms, of any changes it introduces

The preventive conservation audit should work methodically, ideally gathering data and making consideration in this order:

. the building

. the collection

. the in-house procedures

TABLE 1 - Preventive Conservation - a summary of the threats (and their possible remedies)

The building is the first line of defence, and the audit will start here. A survey can establish how well the building cushions the indoor environment from that outdoors, and isolates the conditions within. If the building has poor features, yet is capable of satisfactory modification, its physical improvement should come before consideration of new internal control equipment that will be expensive to install, and energy-inefficient to operate.

The environment within the building should be surveyed using 24-hour recording devices during a full calendar year. An external sensor, to provide contrasting readings, is essential to judge the efficiency of the building fabric. Such a survey may quickly identify the existence of differing environmental conditions within the building. As an immediate response, the collection may be moved, and the most sensitive items placed in the most stable and easily controlled zones - for example, in rooms near the centre of the building that are usually furthest from the effects of the weather. Any additional control measures that are needed can then be economically and effectively targeted on specific zones, rather than applied indiscriminately throughout the building. It may be that no area within the building can easily be brought to an adequate standard. In such cases, consideration should be given to relocating the museum.

The suitability of the building for museum use depends largely on the nature of the collections that are being kept within it. Ideally, surveys - either of the collections in their entirety, or of samples - will be undertaken to uncover and quantify the current condition of the collection, and suggest priorities for remedial treatment. At the very least, a visual inspection should be undertaken, and regularly repeated. While visual inspection is not ideal as the only form of check - severe damage may have already occurred before it is noticed - it is a start. Over time, it must include inspection of all items, not just those on display. To facilitate this, it is best that most boxed items in store are not wrapped, merely padded to prevent their movement.

Different types of collections need different forms of care. However, it usually impractical to create ideal conditions for each type. Compromise prevails in most situations, with only the most `difficult' material subject to specific provision. Some items, made of composite materials with widely differing requirements (eg wood & iron) pose particular problems, that may never be wholly resolved.

All museum workers need to be aware of the special requirements of museum collections. Light, temperature and humidity levels, pests, pollutants and the use of inappropriate materials, or inadequate handling, all pose threats.

Comprehensive and effective in-house procedures are relatively easy to check. They should be written, and followed. They should feature in the induction scheme for new staff.

Disaster planning for most museums can be achieved by preparation of an Emergency Manual (such as that developed by EmmS) containing contact numbers for the various assistance that can be summoned, and also annotated floor-plans (indicating features that may be vital in case of fire-fighting or salvage), and maintenance checklists.

Deficiencies highlighted by a conservation audit should be tackled in a logical and methodical manner. If the audit is not be wasted effort, adequate resources, both financial and human, should be provided. Many detailed responses and remedies have already been described in Table 1. The following are probably typical responses:

APPENDIX 1 - Temperature & Humidity

Temperature and humidity are key agents of deterioration. Relative humidity (RH) is a ratio of water vapour in the air to the amount it could hold if fully saturated, and is expressed as a percentage. Low levels of relative humidity mean dry conditions since the air is then capable of taking up moisture. High values are recorded when the air is already humid or wet and unable to take up much additional moisture. Temperature with a thermometer; RH is measured with a hygrometer; a thermohygrometer measures both.

Extreme, or rapidly fluctuating, relative humidity poses a major threat, especially to organic materials. Their hygroscopic nature (they can rapidly absorb and release water) means that they quickly expand and contract, generating stresses that threaten release through damage such as cracking. The goal for most types of museum collection is avoiding rapid RH change (no greater than +/- 3% in one hour, +/- 5% in 24 hours) while staying within the range of 40-65% RH.

If relative humidity is controlled, temperature control is generally less crucial. Ironically, however, since the human body is far more sensitive to heat and cold than to humidity, it is temperature control that is more often seen as the priority in public buildings. Public areas are usually kept between 17-19^oC (63-66^oF). For collections care, a range of 15-25^oC is acceptable for most collections, while stores can be kept at lower temperatures (there is no lower limit, provided humidity is still controlled) to both save energy and reduce decay rates.

Light can create serious irreversible damage to museum collections. Light is a form of energy and can cause fading as well as deterioration in the materials from which an object is made. Most objects are to a greater or lesser extent affected by light, although metals and ceramics not to the same extent as other materials. Light is measured in lux (or lumen per square metre. 10 lux equals the amount of light produced by 1 candle at a distance of one foot (therefore 10 lux = 1 foot candle).

Light produces damage in proportion to its intensity and the exposure time of the object. Thus a light of 500 lux will theoretically cause the same amount of damage in one year as a light one-tenth the strength (50 lux) will produce over ten years; or, if two 100 watt lamps are put in place of one, then the same amount of damage will occur in half the time. Thus short exposure to a high lux (eg 2500 lux caused by photographic or laboratory lights, or 200 lux for short exhibitions) need not cause undue damage over the total life of an object, provided this high exposure is compensated by a proportionate period of time in a lower than normal illuminance, or in total darkness.

Thus while it is usual to reduce light-levels in museum displays, this need not be the only response - reducing the time during which an item is exposed to light is equally legitimate. Fitting display lights with a timer, or, if continuous lighting while on display is required, limiting the overall time an object is on exhibition in a given year, is appropriate. For the most light-sensitive objects, it may be useful to establish annual lux-hours exposure limits. Lux-hours are a measure of exposure (illuminance x time).

This example assumes that:

. no light falls on the object when its 8 hour display day is over (probably only a reality when a regime of covering a display case);

. that the lux of the light sources is constant.

In reality:

. if any natural light is involved, its illuminance will vary with the time of day, weather and season;

. the precise illuminance of many artificial light sources declines over time, as the filament ages (and dust settles!);

. most museum exhibits are illuminated well before (and often after) public admission times, as lights are turned on, blinds raised and curtains drawn back, to facilitate gallery cleaning, out-of-hours events etc.

The effect of any measures to reduce visible light levels is to darken a room, a change all the more apparent to the human eye if made too rapidly. Measures such as the gradual reduction of lighting levels in preceding spaces, giving visitors' eyes time to adjust, can usually compensate. Neutral-density window film reduces the amount of light entering, but still allows the visitor to look out, while from the outside the window appears darkened. Widely used on coaches and railway carriages, such films can offer an effective alternative to blocking or blinding a window.

Most light sources also emit some ultraviolet (UV) radiation. This is light beyond the limit of human vision, at the violet end of the spectrum, and is also damaging to most museum objects. Since UV is not needed to view objects, and can readily be reduced through the use of filters, removal of the UV element, whether from natural or artificial light sources, should be the target. A meter is used to measure the proportion of UV in the light source in microwatts per lumen (m W/lumen) - a maximum acceptable reading is 75m W/lumen, although filters should be able to reduce this to less than 10m W/lumen. A periodic check is needed to test the continuing efficiency of UV filters (especially of window coatings and film), as this declines over time.

The radiant heat from lights, especially in enclosed cases, often poses as much of a problem as the quantity & quality of the light that they issue. Low-voltage bulbs often emit great heat. Fluorescent tubes are generally the coolest, and hence best for mounting inside an enclosed case. Even so, they need diffusers or sleeves to counter relatively-high UV emissions.

All museum spaces should be regularly set with traps to detect the presence of pests, hopefully before their numbers are a problem. Simple `window' traps placed along skirting boards (the usual insect run) can be very effective, and are cheap and long-lasting. They are most likely to detect pests in May - July, the months when most adult forms migrate to breed. By carefully noting which traps have caught what insects, the source of the problem can be identified. It will frequently be a hole in the building fabric, permitting ingress; alternatively, an item already infested with eggs or larvae may have been introduced to the store or display.

APPENDIX 4 - Pollutants

Pollutants in the museum atmosphere require identification and measurement, and, if they pose a threat to the collection, exclusion. Sources of pollutants are both internal and external.

External sources for both gas and particle pollution include vehicle and factory emissions, and burning fossil fuels. Within the museum they may derive from building works, items already in the collection (eg cellulose nitrate film, or collections in formaldehyde), cleaning agents, or materials used to construct display cases, or provide storage.

Of particular concern (but relatively easy to correct) are materials used to construct museum displays. In the confined atmosphere of a display case, levels of harmful pollutants can cause chemical changes, even without physical contact. For example, hydrogen sulphide, exuded by wool, leather, parchment and certain adhesives, will tarnish silver, polished copper and photographic prints & negatives; sulphur dioxide will harm paper, leather and some types of stone, while organic acids (eg acetic acid), contained naturally in wood, are catalysts for much corrosion, particularly of lead.