Anna Perkins

Preventative Conservation

Overview

Preventative conservation, a passive form of conservation, involves controlling external factors to extend the stable condition of a work of art. By controlling temperature, humidity, light, and pollution, damage to paintings can be limited, preventing or delaying future active conservation treatments. Preservation of artwork is the picture owner’s responsibility, rather than conservators, according to conservator Barbara Appelbaum, so while conservators make recommendations for display and storage conditions, they do not have control over whether their specifications are practically followed.¹

In his book describing the common types of damage and defects that pictures can have, George L. Stout writes, “If all pictures could be kept in air-conditioned, shockproof rooms with constant temperature and relative humidity, many of the ailments reviewed here would not develop.”²

Barbara Appelbaum, a conservator, conservation handbook author, and former vice-president of the AIC (American Institute for Conservation), explains that beyond the “moral, social, or scholarly” reasons for art preservation, “Objects in deteriorating condition lose monetary value as well as their aesthetic and research value.”¹ The less intact material a painting possesses, the less legible the artwork is to the viewer, but also the less material is left for future generations to appreciate or analyze. Appelbaum cautions, “Once such damage is done, no treatment actually reverses the damage….As with people, prevention is always preferable to disease – even a curable one.”¹

Appelbaum offers this basic advice for the stewards of art collections: examine and photograph your artwork, maintain your building (heating systems, water pipes, wiring, etc.), hire a conservator for a conservation survey, and plan for natural disasters (fires and floods).¹

Temperature and Humidity

Rather than always keeping a painting at a cool temperature to delay oxidation reactions, it is more important to keep the temperature of a painting constant. Relative humidity (RH) is the measure of moisture in the air expressed as a percentage, and how much moisture the air can hold depends on the temperature.¹ Fluctuations in temperature are inherently accompanied by changes in humidity, which can cause “[s]plitting, convex warping of the support, blistering of the paint layer and cleavage of the priming layer,” according to Moss.³ Likewise, Appelbaum writes that “[i]nappropriate or widely fluctuating relative humidities are implicated in the deterioration of almost every kind of object, organic and inorganic, wood and textiles as well as metals and glass.”¹

Appelbaum describes that a rise in temperature will draw water molecules out of an object and into the air, and a fall in temperature will force water out of the air and cause it to condensate on an object.¹ These fluctuations happen naturally throughout the day, as the daytime is hotter than the night, people come and go, and bright lights that generate heat get turned off and on.¹ She explains that relative humidity is hard to measure accurately, since body heat and steam can affect measurements, but she recommends hygrometers and data loggers, despite their low accuracy at very low and very high relative humidities.¹

Therefore, climate control measures should be put into place. Appelbaum explains that there is no “ideal” temperature and relative humidity for storing works of art, since climate is so disparate across the world. Rather, it is ideal to keep a painting in the conditions it has been accustomed to. Appelbaum writes:

The popular textbook on preventative conservation, Garry Thomson’s The Museum Environment (1978), was based on the English climate, which is not similar to most of the United States. Appelbaum notes, too, that “unlike Americans, the British are not addicted to the massive use of central heating.”¹ Because England faces high RH, 70-90%, year round, Thomson focused a lot on mold prevention.¹

High humidity can bring with it the risk of mold growth. Oddy, also an English conservator, states, “[T]he presence of micro-organisms…in the air is inescapable. However, moulds[sic] will only grow if the RH is greater than about 65% for prolonged periods of time.”3 Thus, mold growth “is not common under normal conditions” (about 30-50% RH).

By contrast, in much of the United States, the RH can drop to 10-30% during the winter. Thus, the British literature that recommends maintaining an RH of 50-55% is impractical for America, as targeting this condition would “require incredible amounts of water being added to the air at a tremendous outlay of energy,” according to Appelbaum.¹ The recommendation of 50-55% RH is meant to keep British collections from mold growth, but this high of a humidity is not universally necessary to maintain. Appelbaum states, “There is little or no evidence that gives these numbers magic qualities of preservation.”¹

In practice, it is difficult to control humidity fluctuations. According to Oddy, “Nowadays, the technology to control the RH and temperature within a building is available, but it is expensive to install and run. It is also difficult to install in old buildings which do not have service ducts to take the necessary trunking – and most museums are housed in old buildings.”⁴ Air-conditioning, in practice, can be faulty or weak, and it is usually calibrated for the comfort of people, not paintings, according to Appelbaum and Moss. Moss writes, “It is not unusual to find that custodians turn off the air conditioning” nightly, even if the museum contains valuable paintings.³

Another option to controlling environmental conditions, if a painting is particularly fragile or valuable, is a display case. Appelbaum praises the exhibition case’s ability to “keep[] out dust, gaseous pollutants, insects, heat from lights, curious fingers, and air currents” along with mitigating relative humidity changes.¹

Light

Limiting the amount and type of light in the environment of a painting is also important to its longevity. Appelbaum writes, “Deterioration from light is cumulative and irreversible. No conservation treatment can restore the damage done by light.”¹ She notes that light is often a disagreement between curators, who prefer bright lights to best showcase the art, and conservators, who recommend dim lights to minimize light damage.¹ A conservator would, at least, recommend for the lights to be turned off at night, when no one is viewing the art.¹

As described in “Damages and Defects” (link), UV light is particularly damaging to oil paintings, both against the binder durability and the pigment color. The oil binder can crack and discolor, and the pigment can fade or darken. Appelbaum states, “The linseed oil…hardens faster with ultraviolet irradiation, and becomes brittle more quickly.”¹ In The Art of the Conservator, Andrew Oddy explains how exterior light affects the color of a work of art: “[W]hen an object is exposed to bright sunlight, some of the compounds which make up the object may absorb enough energy to cause the molecular structure to break up, and the color changes as a result. Light energy can also be absorbed by other compounds which can then react with oxygen, water, and other chemicals present in the air in small amounts.”⁴

While UV light is particularly damaging, other forms of light, including visible light, can also cause damage to a painting. Appelbaum cautions that all forms of light are harmful and accelerate aging, while noting that higher frequency light like UV and blue visible light are more damaging, and since we cannot see in the UV or IR range, these frequencies are not necessary for viewing a work of art and should be filtered out.¹ Plexiglas or glass with a UV protective coating can be used to filter out UV light, but they overlap a little with blue light and therefore cause a slight distortion in color to the viewer.¹

Pollution

Air quality is another factor in the preventative care of a painting. While it is harder to quantify damage caused by pollution, there is evidence of more acidic archival materials in urban areas than in rural ones, and the detrimental effects of certain pollutants is similar to that of light, according to Appelbaum.¹

Oddy writes, “One aspect of the environment which is having an ever-increasing effect on museum collections is air pollution. As a result of burning fossil fuel, the air is full of traces of numerous chemicals which are harmful to a wide array of museum exhibits.”³ Appelbaum describes the effects of some common pollutants that affect paintings:

Filtration systems to minimize gaseous pollution, like fossil fuel exhaust, and particulate pollution, like dust, along with regular cleaning of the storage space, can reduce a painting’s risk of damage. Storing a painting in a display case, as with humidity, temperature, and light, can minimize pollutant concentration near a painting.¹ Indoor conditions are typically less polluted than outdoor conditions, but the use of carpet or textured walls, as opposed to non-absorbent windows, inside a museum can help absorb pollution.¹

Pests

Pests can consume, stain, or otherwise degrade works of art when the storage space has an infestation, which may be due to neglect. Appelbaum stresses the need for preventative care against pests, since fumigation with pesticides involves toxic chemicals hazardous to human health.¹ Hygiene measures should be taken, like regular vacuuming and inspecting corners of rooms, such that fumigation is a last resort.¹ Insects usually prefer high humidity and can be kept at bay by maintaining a low relative humidity in the storage space. Many insects also prefer low light, an opposing force to the low-light conditions recommended to reduce degradation of paintings.¹ See “Damage and Defects” for more information on the damage that insects and rodents can cause on paintings.

Other Factors

Vandalism, fires, and human contact should be protected against. While security measures and fire safety seem obvious, it is less obvious that touching and handling should be avoided. Oddy states that “people can also adversely affect objects just by touching them.”3 Finger oils can cause a localized darkening of the oil paint film if left to oxidize. Handling brings with it the risk of dropping or scratching a work of art.⁴

References

Damage and Defects Filling Materials