Questions and Answers
Some of the issues involved in caring for easel paintings are covered in questions and answers below. For more detailed advice you can contact the National Gallery Scientific and Conservation Departments via the Gallery’s Information service.
1. What is ‘preventive conservation’?
Preventive conservation means the application of principles of environmental management usually to an indoor context of display or storage for a work of art, with the aim of eliminating or reducing levels of damage that may be caused by environmental factors. In practice, for easel paintings this means ensuring a stable level of relative humidity in the region of 55% for UK conditions, and limiting light exposure as much as possible, consistent with the public viewing requirements for that work of art. It should also mean eliminating all ultraviolet light content from any source of illumination, whether natural or artificial.
2. What factors could damage paintings on display?
Paintings are most affected by excessively high, low or changing levels in the surrounding moisture content of the air (measured as relative humidity, RH). High light levels and any ultraviolet light content from illumination are damaging. Pollutants may have specific chemical effects on certain of the materials of painting, particularly pigments.
3. How is it known that environmental factors can damage paintings?
The National Gallery maintains detailed long-term conservation and scientific records of paintings in the collection. Many of the conservation records go back to the dates of acquisition. The condition of paintings in the Gallery is monitored regularly by conservators. The most common form of serious damage that can occur is loss of paint as a result of flaking. The usual cause for this type of damage is movement of the painting’s support, whether it is a wood panel or a canvas. Dimensional changes in the support, as well as twisting and warping of panels are caused by changes in relative humidity, particularly rapid changes.
It was noted that when the Gallery’s paintings were stored at Manod slate quarry in North Wales during wartime, where environmental conditions deep inside the quarry could easily be controlled to give very stable relative humidity and temperature, naturally occurring damage and degradation were at an extremely low level. When the pictures were returned to Trafalgar Square at the end of the war to an uncontrolled Gallery environment, the pattern of regular flaking resumed, particularly of early Italian panel paintings, and regular remedial conservation treatment was once again required.
Soon after, a recommendation of a government enquiry (the 1948 Weaver Committee report published by HMSO) into the care of the nation’s paintings was that air-conditioning, to control the Gallery’s internal climate, should be introduced as a key measure to help preserve the collection. The conservation records show that this was effective and deterioration of the paintings, particularly panels, diminished significantly as more rooms were equipped with environmental control.
At the same time, the Weaver Committee recommended setting up a permanent professionally-staffed Conservation Department at the Gallery, and the expansion of the Gallery’s nascent Scientific Department, established first in 1934. The first responsibility of both these Gallery departments was to work on caring for the nation’s paintings; this remains a central priority today.
Knowledge that light can damage works of art, particularly the coloured materials used in paintings, drawings, textiles and so on, is very old and based on observation of the works themselves and on more direct experimentation from the early 19th century onwards. The Gallery has maintained a programme of long-term colour measurement on pictures which has shown the necessity to control light levels for preservation. In addition, much research on fading and discoloration of traditional artists’s materials, especially pigments, has been published in the National Gallery Technical Bulletin.
4. Why is it necessary to control atmospheric moisture levels (relative humidity) in picture galleries?
Easel paintings contain moisture in their structures, particularly in the wooden components of panels, in the textile supports of their canvases and in glue-size primings of all types of supports. The level of atmospheric moisture surrounding a picture dictates how that object exchanges moisture with its environment. When the atmosphere is dry (low relative humidity), the painting will release moisture into the surrounding air and the support will tend to dry out, resulting in shrinking, cracking and twisting of the boards which make up wood panels, and to tightening of canvases on their wooden stretchers.
High levels of atmospheric moisture (high relative humidity) also induce dimensional changes in the supports of paintings which take up moisture from the atmosphere under these conditions. These dimensional changes can threaten the integrity of the painted surface, since although the support can move, the paint layers are much less flexible and cannot respond by movement in accordance with an expanding or contracting support. Paint can be lost in these circumstances, and frequently is, without environmental control. The safest means of eliminating these damaging changes is to stabilise the microclimate around paintings, and particularly to stabilise relative humidity at suitable levels. In practice, this is best achieved through the use of air-conditioning systems which control relative humidity, temperature and also, as a side-benefit, generally remove atmospheric pollutants by air filtration.
5. What is relative humidity (RH)?
Air can hold moisture in the form of water vapour. The amount of moisture any given volume of air can contain is dictated by the air temperature. Cool air can hold less moisture than warm air. The scale of relative humidity gives a measure of how much moisture is present in the air, independently of temperature. The RH scale (as a percentage) is defined as the amount of moisture (water vapour) in a given quantity of air, divided by the maximum amount of moisture which that quantity of air can hold for that given temperature (multiplied by 100%). It is the level, high and low, and rate of change of relative humidity in air that determines the dimensional responses of paintings, not the absolute levels of moisture in the air.
6. How is relative humidity controlled?
Relative humidity can be controlled in an enclosed space using electrically-powered, free-standing humidfying/dehumidifying units. They do not provide very close control of conditions in larger exhibition rooms, but are useful in emergencies or for buildings in which permanently-installed plant is not technically possible or affordable. For significant collections of paintings which contain fragile works, installed air-conditioning plant is much preferable and provides well-controlled internal climates.
At the National Gallery, as elsewhere with similar collections, it is usual for sensors which measure relative humidity and temperature to be installed in the rooms (other sensors measure light levels) and for the output of these sensors to be used to control the air-conditioning plant so as to provide steady levels of relative humidity and temperature. Air-conditioning plant is energy-intensive machinery; modern improvements, including more sophisticated software control, however, continue to make it more efficient.
7. What effect does light have on pictures?
On exposure to light, pigments can fade, or, in certain cases, darken. Light has an effect on the paint binding medium, particularly for oil paints which will darken, and on surface varnishes, which also discolour over time. Some types of pictorial art – pastel drawings, watercolours, gouache paintings (glue-medium paintings) and certain types of drawings – are more sensitive to light than conventional easel paintings, and light levels should be reduced further for their long-term preservation, or the works should be shown only for short periods of time. Certain types of paper, some of which are occasionally used as a support for oil paintings, are themselves more vulnerable to light than most other supports.
8. How can the damaging effects of light be reduced?
From a conservation standpoint, the paintings would be best left in darkness for as much time as possible; there is, however, a clear clash of interests between this extreme position and public access. Two factors are critical: the overall light intensity (strength of the light) that falls on paintings and the length of exposure to light. For this reason, the National Gallery controls daylight levels in rooms by various means, and also controls the levels of supplementary light from artificial lighting systems. The aim is to limit the annual exposure of the paintings to light to a prescribed level set at a value not to produce visibly detectable light damage to paintings over considerable periods of exposure.
One method of light control for daylight in the National Gallery involves mechanised external blinds, which open and close in response to outside light levels and, for the most recent systems, respond also to the position of the sun in the sky. As a consequence of its highly damaging effects on works of art, any ultraviolet light content of a light source must be rigorously reduced by filtration to extremely low levels, and preferably eliminated entirely. Filters can be applied to glazing (any windows or rooflights) and to sources of artificial light.
9. Is it necessary to control the temperature of picture galleries?
Temperature changes and high and low temperatures are generally less damaging than changes in relative humidity or increases in light level. However, artificially-induced temperature changes without accompanying humidification or dehumidification result in a shift of relative humidity in the air of an exhibition room, and should therefore be avoided. For example, in the winter where external temperatures are low, the outside air can only contain a low absolute level of moisture. If air inside a building is heated artificially without adding extra moisture content to the air (humidification), the relative humidity (RH) level will fall, sometimes drastically. This must be avoided for pictures. A certain amount of winter heating and summer cooling may be necessary, however, for the comfort of visitors.
10. Does pollution entering the galleries affect paintings?
Acidic gaseous pollutants are present in urban atmospheres arising from vehicle exhausts and the burning of fossil fuels, and from some more natural sources. London air is now considerably less polluted than before the Clean Air Act (1956) – for more detail on the history of London pollution and its effect on paintings see Volume 21 of the National Gallery Technical Bulletin. The pollutants are principally sulphur dioxide and the oxides of nitrogen. It is likely that these gases could damage certain materials present in easel paintings, although layers of varnish present on the surfaces of most pictures acts as a partial barrier. Glazing would also diminish the potential for damage. Active carbon filters in the ducting of installed air-conditioning plant removes these gaseous pollutants very effectively; other filters remove dust.
11. Does the Gallery have a recommended environmental specification for paintings?
The Gallery’s current specification is a modified form of a set of environmental recommendations for so-called ‘Class I Museum Conditions’ published by Garry Thomson, who was Scientific Adviser to the Trustees of the National Gallery from 1960 to 1985. They are contained in the second edition of 'The Museum Environment' (Butterworths, London, 1986). Refinements have been made by David Saunders in ‘The Environment and Lighting in the Sainsbury Wing of the National Gallery’ (ICOM Committee for Conservation, Vol. II, 1993, pp. 630ff) and other Gallery experts. In summary the current specification is:
Light level: 150 ± 50 lux (UV radiation content now specified as less than 10 μW/lumen; formerly 75 μW/lumen), annual light exposure limit: 650 kilolux hours; Relative humidity: 55 ± 5 %; Temperature 21 ±1°C (winter); 23 ± 1°C (summer).
12. Is active environmental control in museums responsible use of energy?
This is a matter of definition. It is widely accepted that it is essential to expend energy for environmental control in certain vital contexts: hospital operating theatres for example or institutions such as Kew’s Millennium Seed Bank at Wakehurst Place. For unique and fragile works of art such as Old Master paintings, their future preservation is only guaranteed by providing closely-controlled environmental conditions.
In order to display a collection to best advantage for the greatest benefit to the public, it is necessary to provide Gallery conditions that are safe for the more fragile works in the collection. This enables works of differing sensitivities to be hung together. There are currently no methods of passive environmental control that can meet these requirements fully in a large historic building open to the public for long periods of time. Only ducted air-conditioning systems can provide safe conditions for long-term preservation of paintings collections, and these systems inevitably consume a proportion of the Gallery’s energy budget.
13. What measures does the Gallery take to reduce its energy use?
With the help of the Carbon Trust, the Gallery has recently embarked on a major energy-saving plan designed to reduce energy use by 43% over a four-year period. At the heart of this is the installation of a new, highly efficient combined heat and power plant (CHP) which will bring about total energy saving of 15%.
At the same time, starting in 2011, a programme of installing new light-emitting diode (LED) lamps in the Gallery rooms was begun. LED lights have much lower energy consumption than incandescent lamps and are also more energy efficient than fluorescent lights. They have long operating life-times which lowers maintenance costs. LEDs provide satisfactory colour rendering for paintings and are suitable for the conservation requirements of Old Master pictures. At the completion of this new lighting project, the Gallery will save a further total 5% of its total energy budget. Other energy saving measures will contribute to the overall target figure of 43% reduction.
14. Will future technology solve our problems of effective preventive conservation?
This is hard to predict. For newly-built projects it is possible to improve both the energy performance of buildings and their effectiveness in maintaining relatively stable internal conditions of microclimate naturally. These building characteristics become less attainable in parts of the world where the external climate is extreme. Passive control of conditions is also more difficult to achieve when a building such as the National Gallery receives large numbers of visitors.
For the present, and near future, it is difficult to envisage the provision of stable ‘Class I Museum’ conditions without ducted air-conditioning plant. Many types of collections, however, are less sensitive than Old Master paintings, and less stringent environmental conditions for preservation are possible. For paintings, where active environmental control is not possible, various designs of enclosure can help maintain relatively safe conditions for their protection. However, these inevitably act as a barrier to the best visibility and may require individual monitoring to ensure the safety of the contents.
15. Where can I get more advice on the care of Old Master paintings?
Queries on this subject may be directed to the Scientific and Conservation Departments of the National Gallery via the Gallery’s Information service.
16. Where can I find further published information on this subject?
The standard work on this subject is G. Thomson, 'The Museum Environment', 2nd ed., Butterworths, London, 1986.
For refinements to the Gallery’s specification, see David Saunders, ‘The Environment and Lighting in the Sainsbury Wing of the National Gallery’, ICOM Committee for Conservation, Vol. II, 1993, pp. 630ff.
For advice on the preservation of more generalised categories of cultural material, see David Grattan and Stefan Michalski, ‘Environmental Guidelines for Museums – Temperature and relative Humidity (RH)’ [External link] available from the Canadian Institute for Conservation (CCI). The CCI classification deals with general classes and combinations of materials which results in placing easel paintings in the category defined as ‘very high vulnerability’.
17. Where can I get more advice on other kinds of works of art, cultural objects and collections?
A new document ‘Specification for managing environmental conditions for cultural collections’ (PAS 198:2012) [External link] will be available from the British Standards Institute. This deals in general terms with collections of cultural objects of diverse types, and focuses on reductions in the use of energy in preservation strategies.