Regional conditions vary widely. When works of art are displayed or stored in or nearby a city, pollution from car exhaust and the burning of fossil fuel endangers them. In the tropics or near-tropics, high relative humidity, sunlight and mold pose threats.

Interior pollution must be considered as well. What will be the immediate surroundings? A new building, for example, with fresh paint and new concrete can be a dangerous place to install works of art, especially photographic prints. The frame can mitigate some of these dangers, but protecting works of art exposed in the world entails monitoring conditions and, if possible, controlling the environment outside the frame as well.

Works of art are especially vulnerable to light, relative humidity, heat, pollution and biological threats. When uncontrolled these factors act in concert. Mold, a biological threat, is everywhere. But only in conditions of high heat and high humidity does mold pose a serious hazard. Oxidation is a constant threat of damage to paper, and acidity in paper has long been a major cause of its disintegration. But high temperatures, high relative humidity, and atmospheric pollution, especially in combination, hasten the deterioration.

Environmental Impacts

The Effects of Light

Dennis Oppenheim Photograph Reading Position for a Second Degree Burn

Dennis Oppenheim, Reading Position for Second Degree Burn, 1970. The UV radiation of sunlight damages all organic materials.

Light degrades organic materials, which include anything that comes from plant or animal sources, as well as plastics and dyes.  Light speeds up oxidation and thus the chemical break-down of these substances. Oxidation is a complicated set of interactions which degrades the polymeric structure of organic materials. Ultraviolet (UV) light, invisible to the human eye, is more energetic than visible light and therefore more damaging.

UV light is especially destructive to papers containing lignins, such as newsprint or other papers made from unpurified wood pulp. It is also a major cause of color fading; some organic colors are especially affected by intense or prolonged exposure to UV sources. Most traditional photographic prints are vulnerable to UV as well.

Sunlight must never fall on a glazed picture—the impact is immediate and dramatic. Localized heating and drying of materials can quickly cause permanent damage.

“The instability of the microclimate in pictures that are briefly exposed to shafts of sunlight through windows is particularly striking, and leads us to emphasize that ambient climate control in a gallery gives no protection against direct sunlight, particularly on objects enclosed behind glass.” Tim Padfield et al, How to Protect Glazed Pictures From Climatic Insult

Sources of Light

Two sources of light.

Usually we are dealing with sunlight, fluorescent lights or various incandescent lights.  Sunlight generates more UV than the other sources; about six times as much as tungsten incandescent lamps. Daylight, even with the UV removed, can be assumed to be about three times as damaging as tungsten incandescent sources. In terms of UV emission, fluorescent lights, both tubes and Compact Fluorescent Lamps (CFLs) fall in between daylight and tungsten lights, and are considered a serious UV threat to art materials. Generally speaking, if tungsten incandescent light is the only light source, UV filtering is unnecessary, but other incandescent light sources, such as quartz halogen lamps, emit significant ultraviolet radiation and should be filtered at the source or in frame glazing. Since tungsten incandescent bulbs are being phased out, all current sources of light should be considered to be possible emitters of UV radiation at levels harmful to works of art. LED lamps, which may be the lighting of the future, do not emit significant UV.

Filtering of UV Light

There are ultraviolet filtering sleeves for fluorescent bulbs and thin UV filtering films which can be affixed to window glass. The effective lifespan of sleeves and films is unknown.

Electro-magnetic Spectrum Showing Ultraviolet, Visible and Infrared Light

The electro-magnetic spectrum. Ultra-violet radiation, shown just to the left of visible violet, is invisible, high frequency, and more damaging to most materials than light in the visible spectrum.

Average Temperature and Relative Humidity Indoors in NYC

Heating an apartment in New York City in the winter results in bone dry air if humidification is not provided. In this example, if the outside air is heated to 80 DEGREES F, then the RH will drop to 12%, dryer than the desert.

Most of the raw materials which are processed to make surfaces on which artists paint, draw, or print—such as canvas or paper—are in their natural state made up mostly of water. Such hygroscopic materials have an affinity for moisture. Their moisture content continually changes, always seeking equilibrium with the relative humidity of the environment.

The term “relative humidity” (RH) means the amount of water vapor in the air expressed as a percentage of the maximum amount that the air could hold at that temperature. In the winter, if no water is added to interior air, the RH decreases as the temperature rises. Cold outside winter air becomes parched in heated, un-humidified apartments and houses. In our temperate climate, indoor RH may soar to 80% or higher in summer and fall below 15% in winter, literally as dry as a desert.

The Impact of Humidity on Art Materials

Artwork Displays Gouache that is Cracking and Flaking

The cracking and flaking of gouache shown here was probably caused by expansion and contraction of the paper support due to fluctuations of relative humidity.

High RH speeds those chemical reactions which cause deterioration of paper. In high RH mold flourishes. Some dyes fade faster. Paper and mat boards swell and if constrained in a frame, become wavy or wrinkled, often permanently.

The effects of low relative humidity are what we would expect. If a plant is dried it shrinks, twists, and becomes brittle; so does paper.

Relative humidity for paper should be kept between 30-50%, and the RH should remain as steady as possible.

Controlling Relative Humidity

In many exhibition areas, in museums as well as homes, relative humidity rises and falls in daily and seasonal cycles. Daily cycling in particular should be eliminated whenever possible. As papers expand and contract with changes in humidity the fibers are stressed and eventually the paper will be damaged. Cycling can also weaken the bond of the medium to the paper support, and it may cause the paper to become distorted, sometimes bringing it into contact with the frame glazing, which can damage the work.

During the summer air conditioning does a fairly good job of controlling relative humidity. Air conditioning has the added bonus of removing dust and some gaseous pollution from the air. Additional drying of the air may need to be done by de-humidifiers.

Contours Show Indoor Relative Humidity Across US on Average January Day

Indoor relative humidity on an average January day. The contours show the RH that results from heating cold outdoor air to 20 d. °C (68 d. °F) without humidification. Throughout most of the US the humidity would range between 10 and 20%–desert conditions. From Garry Thomson, “The Museum Environment”

In the winter, in regions where heating is necessary, indoor relative humidity will drop. Therefore water must be added to the atmosphere by a humidifier. Keeping the humidity level relatively constant throughout the season should be the goal and 30% RH is a good target.

The safest practice is to maintain two layers of humidity control: the room itself, and the frame envelope in which the artwork is enclosed. Within the frame envelope some mitigation of changes in RH is achieved by using hygroscopic materials, such as rag board, or in more demanding instances, by including silica impregnated materials in the frame envelope.  Humidity indicator cards can be installed in the frame backing under a Mylar window, so that the RH inside the frame can be monitored.

Humitector™ Maximum Humidity Indicator

The Humitector card indicates maximum humidity spikes as well as current RH.

Drying Wood Frame Contracts and Opens Miter Joints

When buildings are heated in winter, air becomes dry and so do materials. As the wood of this frame dried it shrank across the grain, opening the inside of the miter joint.

Heat makes materials age faster. In fact, there is a rough rule, the Arrhenius equation, which states that for every increase of 18 degrees °F (10 degrees °C) the activity rate of most chemical and physical processes doubles. For paper the situation is even worse: those activities which lead to the breakdown of paper double with every increase of about 10 degrees °F. Oxidation and hydrolysis reactions speed up, as do the rates of physical processes, such as evaporation of water, which leads to embrittlement. Mold growth and other biological activities also accelerate.

These are aging processes. In conjunction with the other four factors (light, high RH, pollution and biological agents) high temperature increases the degradation rate of organic materials.

Heat is also an indirect damaging agent in that changes in temperature affect changes in relative humidity.  A spot light trained on a work during the day, for example, will heat up the interior of the frame, and lower the RH inside the frame. In tests we performed at Bark Frameworks we found the differences to be significant. At night when the light is switched off, the RH will quickly rise, causing hygroscopic materials to swell. The next morning they will contract again. The dangers of humidity cycling have already been described.

Image of Bus and Person during London Smog of 1952

The Great Smog of 1952, London. An impenetrable “pea soup” of mist, soot, and sulphur oxides from coal burning. London’s air is much cleaner today, but even at reduced levels, pollutants pose a risk to works of art housed in cities.

There are three specific kinds of pollutants which are most damaging to works of art.

  • Sulfur oxides, particularly sulfur dioxide, SO2. They are generated by the burning of fossil fuels in power plants and boilers. Sulfur oxides ultimately form sulfuric acid. In combination with light SO2 is especially dangerous for paper and textiles. SO2 levels have been trending lower in recent decades in American cities, though not in many other parts of the world.
  • Nitrogen oxides, especially NO2.  They are generated by automobile exhaust. Like SO2, NO2 forms an acid in the presence of water, nitric acid, which is a strong acid and an oxidizing agent. These emissions have also dropped in US urban areas in recent years.
  • Ozone. (O3).  One of the effects of sunlight on car exhaust is the production of ozone – an element of photochemical smog.  Ozone breaks chemical bonds in almost all organic materials.  It is also a powerful oxidant, and a cause of fading of pigments and dyes. Ozone levels are also gradually trending lower.

Glazing that blocks ultra-violet light is generally recommended, though it should be noted that even protected from UV, framed works are highly vulnerable to damage from sunlight. We now have access to non-reflective acrylic glazing. It’s coating also renders it non-static, a decided advantage over conventional acrylic.

These three gases have always occurred naturally in our atmosphere. Despite some recent improvement though, combustion in automobiles and power plants is a far greater source. And these pollutants are most heavily concentrated in cities, where works of art are concentrated as well.

Within the closed environment of frames and display cases, pollutants can be especially damaging to works of art. Woods emit acids, especially tannic, formic and acetic acid. Composite wood materials such as plywood and particle boards also emit pollutants from glues. Off-gassing from wood finishes represents yet another source of interior pollution.

Particulate pollution, such as dust, soot, and dirt, poses another threat.  Not only will particulates soil and abrade art materials, they form a welcoming environment for molds and other organisms.

Foxing Mold Stains on Frame

These stains are sometimes called foxing. Causes are uncertain, but probably fungal growth, encouraged by high humidity, is a main factor.

Mold spores are always in the air.  In order to grow, however, they need specific conditions.  They like it warm (above 75 degrees °F), moist (above 60% RH), and an area that is dark, with little air circulation.  A basement is a good nursery for mold – or the area behind a frame on a damp wall.

Insects, of course, are drawn to all sorts of organic materials.

PEM2 datalogger and a Recording Hygrothermograph

Two instruments for recording temperature and humidity: A PEM2 datalogger from Image Permanence Institute and a recording hygrothermograph.

The frame envelope is only the protective membrane closest to the art work. If conditions in exhibition and storage spaces are not controlled, framing practices will be ineffective and protecting art.

  • In the exhibition space light levels should be monitored and controlled, and UV sources should be filtered.
  •  Sunlight should never fall directly on glazed frames.
  • Ideally, rooms which house art should be designed for constant relative humidity.
  • Air conditioning should not be shut off when people are away in summer, and any seasonal fluctuation of heat and relative humidity should be gradual.
  • The presence of micro-climates within exhibition or storage space should be determined—outside walls may be cooler than interior walls for example, which can raise the RH in the immediate vicinity as much as 10%.
  • Other local conditions should be checked as well. Hanging artworks in the airflow from an air conditioner may cause condensation on the inside of the glazing.

The list could go on. Simply stated, climate control requires attention, and climate control is critical to the long life of works of art.

Interior temperature and humidity can be monitored with traditional recording hygrothermographs or digital monitors available from the Image Permanence Institute, whose data can be computer logged.