Paintings and their Environment
Questions and Answers continued
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).