Caring for your Collection

Understanding the Environment

When those of us caring for collections hear or read about “environmental conditions”, “relative humidity”, “temperature”, “pollutants” and “dust”; we may sometimes feel intimidated, not only by the words themselves, but also by their implications for the artefacts. However, if the terms are related to the situation in our homes, it will help dispel much of the myths surrounding them, and lead to a greater understanding of how best to provide and maintain a safe environment for the artefacts in our care.

Few of us, even in these days of central heating and synthetic fabrics, would wish to dress in clothes which had not been properly “aired,” and were still damp. This may be dictated by health considerations, but neither would a linen cloth after ironing be put away while still damp, because disfiguring black spots of mould, very difficult to eradicate, may develop. A wooden front door, too, will respond to changes in the environment, swelling in damp conditions, and shrinking in frosty, dry conditions. We know that fabrics and furnishings fade when exposed to too much light, silver tarnishes, and dust is a perennial problem.

Exactly the same problems will confront the curator, the amount of moisture in the atmosphere, light both natural and artificial, pollutants and dust. The remedy is to be found by implementing either a “high-tech” or a “low- tech” approach, the aim being to provide as stable an environment as possible, taking the building, resources in terms of manpower and funding, and the needs of the collections into consideration.

ENVIRONMENTAL CONDITIONS

Temperature

Temperature will be determined to some extent by the comfort requirements of those working in the area and by ensuring that it does not drop to the point where condensation occurs.

Relative Humidity

Relative humidity (R.H.) is the degree, expressed as a percentage, to which the air is saturated at a given temperature. If the R.H. is 50% at 20oc it means that the air could hold as much moisture again before being fully saturated. However, the higher the temperature, the more moisture the air can hold. This is the reason why heating is sometimes used to control the relative humidity. If the temperature drops, the air cannot hold as much moisture and some will fall out as condensation, or dew, noted after a sunny day, followed by a cold night.

So what is the recommended R.H. for artefacts? Generally speaking, a R.H. above 65% is likely to favour mould growth, especially if the circulation of the surrounding air is restricted. To assist in coming to a decision, it is helpful to understand the nature of the artefacts whether organic or inorganic, the environmental conditions to which they have acclimatised, and on their condition i.e. whether or not they have deteriorated and to what extent. A risk assessment analysis should be undertaken. It is essential to avoid a rapid change in the climate to which artefacts have acclimatised. These conditions may not conform to the “ideal” specifications recommended in text books. A historic house, for example will have a seasonal variation in the R.H. It will also have furnishings, carpets, curtains, upholstery, to absorb some of the excess moisture. Assuming the house itself is well maintained, the artefacts will probably be in good condition. However, if central heating is introduced without taking adequate precautions, the increase in temperature will result in a lowering of the R.H. Wooden objects such as panelling and furniture may show severe cracking. Paintings, especially panel paintings, may also be damaged.

Although the conditions in an historic house may not be ideal, a large building such as the National Gallery, is an ‘artificial’ environment. For security reasons, and to maximise the area available for exhibiting paintings, there are few entrance doors, no windows which open, no fireplaces and few hygroscopic materials. But it must be heated somehow, while controlling the R.H., pollutants and dust reduced, and ‘fresh’ air circulated, This is a situation where a “high tech” solution may be the only option especially as lending institutions now require evidence of stable environmental conditions before agreeing to a loan request. In his book, The Museum Environment (1986) Garry Thomson gives the specifications for an air-conditioning plant intended to provide a very stable R.H. of 50% or 55% throughout the year at a temperature (centigrade) of 19o ± 1o in winter and 24o±1o in summer with the concentration of pollutants and dust reduced to very low levels. It would be relatively simple to incorporate such a plant in the plans of a new building. However to install a sophisticated plant, theoretically capable of furnishing such conditions in an existing building might prove to be neither efficient nor cost effective, nor particularly appropriate to the requirements of the building itself or of the collections. The fabric of the building may not be capable of sustaining the ‘plenum’ or slightly positive pressure required to prevent the ingress of untreated air, and the migration of moisture into the stonework may damage it in winter. A plant once installed will require servicing, maintenance and eventual replacement. Allowances will have to be made for the cost on an ongoing basis. The findings of recent research have shown that the installation of an air-conditioning plant may not necessarily provide the level of stability of R.H. as might have been expected. Also, there has been a realisation that to specify a very tight range of R.H. where the variation is 5% or less, may not be achievable, nor sustainable. To find a solution acceptable to both the curator/administrator and the conservator will demand the skill and ingenuity of the architect and engineer. It is vitally important to involve all parties in discussions from the planning stages onwards. It is interesting to note that there are now several examples of where a purely passive approach to environmental control has been implemented in new buildings.

In small local museums and libraries only general guidelines can be given, but in a temperate climate such as ours it should not be too difficult to keep the relative humidity within the danger limits of 40%-70% in accordance with Garry Thomson’s recommendations for a Class 11 situation. However within this range, some artefacts such as metals will have more specific requirements which will have to be met. If the ambient conditions are not suitable, sensitive material may be placed in a sealed case provided with a micro-climate at the recommended R.H. In some situations the use of dehumidifiers or humidifiers of the evaporative type may help.

It is very important to monitor the temperature and relative humidity in a museum/gallery and library, constantly if funds allow, regularly if a less expensive, non-recording instrument is purchased. Only by so doing will it be possible to know what the conditions are, and take measures to change them if necessary.

Pollutants

Silver is tarnished by exposure to sulphur dioxide and moisture in the air. Other pollutant gases include the oxides of nitrogen and ozone. Activated carbon filters may be incorporated into an air-conditioning plant to reduce these levels. They are expensive and will require renewal. There is a low-tech approach which involves the use of archival storage materials, sealing the backs of frames to retard the ingress of pollutant gases, choosing a storage area where, for example, the concentration of sulphur dioxide is lower than in other areas in the building, avoiding the use of materials in a display case which out gas.

Dust

A range of filters of varying degrees of fineness may be incorporated into an air-conditioning plant. But whether or not air-conditioning has been installed, there are numerous good-house-keeping techniques which can be implemented to reduce the level of dust which staff and visitors generate. These include providing good mats at the entrance, adequate cloak room facilities, A routine dusting regime should be maintained. This may entail training staff in good housekeeping techniques to avoid damaging artefacts.

Lighting

Most cultural material which is organic in nature is susceptible to damage caused by light, natural or artificial. Textiles, works of art on paper, especially watercolours and miniatures, and natural history specimens are the most vulnerable, paintings rather less so. The most obvious way in which this damage is manifested is in fading or colour change, but fibres such as textiles may be weakened and paper degraded.

Daylight, and some sources of artificial light such as fluorescent tubes, have an ultra-violet (uv) component, rays of short wave length which are considered to be very damaging and so must be excluded. However, this is not the only problem. The strength or intensity of the light is also important or, more accurately, the intensity of the light and the duration of exposure to it.

Glass does not afford complete protection against ultra-violet radiation. This may be achieved by the application of a special ultra-violet absorbing film to the windows or display cases. For “glazing” framed items, the use of ultra-violet absorbing Perspex or Lexan might be an option. However, pastels must not be ‘glazed’ with Perspex or Lexan as the static would lift off the pigments! Fluorescent tubes can be fitted with an ultra-violet absorbing film. This film does not last indefinitely and will require renewal. Tungsten incandescent bulbs emit small amounts of ultra-violet radiation, tungsten-halogen and metal halide emit a lot and must be used with the appropriate filter. The recommended level for ultra-violet content has been reduced to less than 10 microwatts per lumen.

There is a ‘high-tech’ approach involving the use of sensors to operate the opening and closing of blinds to control the amount of sunlight entering a gallery/museum. For other institutions a good-housekeeping or commonsensical approach will be perfectly adequate. However, efforts must be directed at preventing sunlight entering through overhead glazing, rather than trying to control it once it has come in (heat gain must be avoided). The use of blinds on windows will reduce the amount of daylight coming in. There is a range of acrylic films available which will reduce the total amount of light, as well as excluding ultra-violet radiation. Their application to period glass in not recommended Remembering that the duration of exposure is also important, display cases might be covered except when viewing, or the light switched on by the viewer, the exhibits changed frequently – the scope for ingenuity is unlimited! It is useful to avail of the flexibility provided by spot-lights on tracks, especially if fitted with dimmer-switches. However, not only must precautions be taken to ensure that the spots are not a source of ultra-violet radiation, as stated above, they must not throw heat forward on to the exhibit, but allow it to dissipate out at the back of the lamp. Lighting within display cases is hazardous, but the use of fibre optic lighting is a possibility as it does not emit uv and does not produce heat. However, it is more adapted to giving depth and “moulding” three dimensional artefacts than prints and drawings. The bulb providing the light must also be properly installed as it could otherwise be a fire hazard.

Aesthetic considerations to be taken into account when choosing lighting relate to colour rendering and colour temperature It is useful to have instruments to measure the intensity of light expressed in lux, to monitor the ultra-violet content of light sources and the efficacy of ultra-violet light absorbing films, which may deteriorate rapidly.

IPCRA

Irish Professional Conservators' and Restorers' Association