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.