As mentioned above with Gaddi's huge Madonna Enthroned with Saints and Angels polyptych (multipart) altarpiece, wooden panel paintings are by their very nature prone to insect infestations, and the damage they cause is often visible in the unpainted structural parts. In addition, the passage of time causes numerous changes to these early paintings on wooden supports.
One of the Gallery's very oldest pictures, the 13th-century Byzantine Enthroned Madonna and Child in Gallery 1, a gift of Mrs. Otto H. Kahn, still has its original engaged frame; it definitely shows the harmful effects of the more than 700 years that have passed since its creation. With great age often comes a pattern of fine cracks visible both on the frame and surface of this painting, and in many others of similar date. This cracking derives from the different rates of expansion and contraction of the picture's wood support, gesso ground, and paint layers as they dried and experienced changes in temperature and humidity over the centuries. The continuity of many of the cracks from the flat surface of the painted image directly onto the frame is a clear indication that the engaged frame is original to the picture and has never been removed.
In addition, by looking at the side of this painting we can see not only the multiple layers of wood that make up the decorated panel and its frame, but also the edges of a device—called a cradle—that conservators of the late 19th and early 20th centuries used to try to stabilize the complex structures of these early pictures. Because wood expands and contracts with changes in temperature and humidity, panel paintings are intrinsically prone to warping and the resulting damage it can cause to their painted surfaces.
Before the advent of modern climate control, cradles were meant to prevent such warping. How did they work? These devices consisted of a gridwork of wooden strips whose vertical pieces ran with the grain of the painting's wooden planks and attached to them, while the horizontal pieces were held against the back of the picture in grooved mortises cut into the vertical slats. Theoretically, if the painted panel began to change shape, the carefully fitted horizontal members of the cradle would bind up in their slotted grooves and resist the warping. In reality, however, this method proved less than effective and often entailed thinning down the painted panel's original wooden planks when they were installed, so the method was abandoned. The cradle on this Byzantine-style Madonna was applied to the painting in the early 20th century, before it was accessioned to the National Gallery of Art.
The inevitable expansion and contraction of the various layers in wooden panel paintings often led to a gap at the edge of the picture where the engaged frame separated (or was removed) from its painted panel. When this happens, we frequently can see part of the unpainted margin of the panel originally covered by the frame. A ridge of thickened paint and ground, called a barbe, marks this outer edge of the painted surface where the engaged frame separated or has disappeared. A barbe (French for "beard") of slightly raised, jagged paint, and gesso is visible along the lower edge of Agnolo Gaddi's Madonna and Child with Saints and Angels in Gallery 2; along the outer edges of Neroccio de' Landi's Portrait of a Lady (c. 1485) in Gallery 8; and in Bernhard Strigel's 1527 portraits of Hans Roth and his wife Margarethe Vöhlin in Gallery 35A (especially along the left margin of the former). When a barbe exists on a panel painting that no longer has its engaged frame, it is likely that the original dimensions of the painted image are still intact and have not been cut down.