"Although the majority of paintings today are painted on canvas, it is not the best choice for painting—in fact they lead to cracking early in the life of a painting.
For over a hundred years, most of the causes of cracking have been explored: humidity and temperature, expansion and contraction, stress, and paint embrittlement. The symptoms were obvious—cracking and paint loss—but the causes were not clearly understood. In 1982, Marion Mecklenburg and other scientists at the Smithsonian Institute, reported the first systematic explanation of painting mechanics, and especially that of canvas paintings, while other researchers at the Canadian Conservation Institute (CCI) soon followed.
(see CCI video below for issues with canvas and relative humidity)
Canvas is hygroscopic, meaning that it readily absorbs and releases moisture from the environment. As humidity in the air changes, canvas absorbs and releases moisture to maintain equilibrium with these changes. Canvas swells and contracts at much different rates than paint layers, producing mechanical stress on paint as the environment changes. (The environment constantly changes due to changes in temperature and relative humidity.) Although these changes may appear to be small, over time they place much stress on paint, which leads to cracking, cupping and paint loss.
Canvas is a very flexible material that is also biaxial, so it expands and contracts at different rates according to its axes—the weft and warp direction of the yarns. The layers in paintings that carry most of the tension (varnish, paint, ground and size), on the other hand, are composed of amorphous or semi-amorphous polymers that are much less flexible, and expand and contract to a much lesser degree and equally in all directions. These stiffer materials are subjected to much stress as the canvas expands and contract, resulting in cracks and then finally loss of adhesion and cupping (illustrated by the animation). Shock and vibration, low temperature, low humidity, high pigmentation, and aging increase both the stiffness and the likelihood of cracking of these layers."
This video demonstrates how very low relative humidity causes cracks to form and expand in an oil painting on canvas. This video was created by the Canadian Conservation Institute.
A video created by the Canadian Conservation Institute demonstrates how very low relative humidity causes cracks to form and expand in an oil painting on canvas. An oil painting, several years old, on sized canvas, with no backing board was placed in a chamber at 50% relative humidity (RH) and keyed out tightly so that any changes in tension would be emphasized.
This video below emphasizes the need to maintain paintings in a normal range of humidity, 40% to 60%, and temperature, around 70º F (21º C). However, this is difficult for the artist to control once a painting is sold and delivered. Painting on a rigid support—especially one that is not affected by changes in the environment (i.e., temperature and relative humidity)—is the best practice to prevent cracking and cleavage of paint.
On Screen: An oil painting, several years old, on sized canvas, with no backing board was placed in a chamber at 50% relative humidity (RH) and keyed out tightly so that any changes in tension would be emphasized. The painting was photographed in normal light (lit from the front with a light source that emits no heat). Then it was photographed again with transmitted light (lit from the back, with a light source that emits no heat) so that light shines through any cracks. [Two photographs of the same painting with different lighting are shown side by side.]
The relative humidity set point in the chamber was dropped in three steps to simulate a drop in RH to very dry conditions:
1. 50% to 35%
2. down to 25%
3. down to 5%
[A graph appears showing conditions for the time-lapse sequence with time on the x-axis and percent relative humidity on the y-axis, showing that the painting spent 3 days at 35%, then 4 days at 25%, then 6 days at 5%.] Let’s watch the time-lapse…The time-lapse sequence begins.
On day 2, at 35% RH, new fine cracks have opened up.
On day 5, at 25% RH, a pattern of concentric cracks indicative of a previous blow to the painting, and
cracks likely due to heavy impasto are visible.
The time-lapse sequence ends.
The painting is shown as it looked before the relative humidity drop. As a comparison, the screen transitions to the painting as it looks after the relative humidity drop. After the relative humidity drop, with normal light, the cracks are not visible. But with transmitted light, the cracks become apparent.
[Two photographs of the same painting with different lighting after the relative humidity drop are shown side by side] Remember….avoid very low relative humidity for paintings, as it will expand pre-existing cracks and can create new ones.
To learn more about different types of crack that may form in paint: