Drying cracks. Drying cracks usually occur when portions of a paint film shrink as they dry, causing them to break into little "islands." Between the islands, the underlying layers of the painting structure are revealed; they are often a different color than the islands. Portions of an image with this problem are often described as "alligatored." English portraits of the 18th and 19th centuries frequently have extensive drying cracks caused by the once-popular use of asphaltum, a semi-translucent black material that produces a wonderfully rich, "deep" look — until it begins to age. Although alligatored paint can be very ugly and obtrusive, it is often well adhered and stable. Conservation treatment cannot eliminate drying cracks, but can make them much less distracting.

Mechanical cracks and cupping. Mechanical cracks are caused by physical stresses on paint films that have dried enough to become brittle. Many different stresses can cause mechanical cracking, including flexing and shrinkage of the support material (i.e., canvas, wood, etc.) A network of mechanical cracks is sometimes referred to as a craquelure. Mechanical cracks are extremely common in paint films of many different types and are not always a cause for concern. However, as a cracked paint film ages, the platelets of paint between crack lines will often begin to curl, a phenomenon called "cupping." When a painting exhibits extensive cupping, it is sometimes said to have a "quilted" appearance. In addition to being visually disturbing, cupping can be a precursor of flaking problems. Conservation treatment cannot remove cracks from a paint film, but it can reduce or eliminate cupping and the threat of flaking.

About the photos. The upper two photos at right are detail shots of a much larger painting. Paintings by this artist are thickly painted and frequently develop wide drying cracks. The painting on the bottom exhibits cracking and cupping, which are very common for oil paintings on canvas.

Drying cracks before inpainting
Drying cracks after inpaintingCupping between crack lines creates a quilted appearance