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