Here's what the NGA web page says about the recently (2008) acquired painting. "Katz beautifully captures the glow of weak sun on leaf and water and the contrasting textures of soft grass and rough bark. A delicate craquelure on the tree trunk emerged during the course of painting, and Katz took advantage of it to convey the bark itself. His color choices, such as the tan sky and the white reflection of the black shore, are both memorable and puzzling, leaving the viewer to wonder whether it is 4:30 a.m. or p.m. The former is not out of the question: Katz has said that he wants to explore times that few people have seen, and in Lincolnville, the sun rises that early in the summer. In fact, Katz recalls that the painting was based on oil sketches he made in the afternoon, although the title leaves the time of day ambiguous."
I got out my well worn copy of The Artist's Handbook of Materials and Techniques by Ralph Mayer and looked up "black." Sure enough, full strength oil colors with high absorption, especially lamp and ivory black, should never be overpainted with pigments of lower oil content. If layering is necessary, Mars black is a better choice. Blacks are notorious for causing unwanted cracking as the paint dries, even when thinly applied. Most painters won't be lucky enough to have a team of NGA restoration experts holding their paintings together, so if they want the paintings to last, they should make sure not to put thin over thick.
Other pieces on view in the East Building, Concourse Gallery Lobby with the Katz painting included:
Robert Gober "The Slanted Sink," 1985, plaster, wood, steel, wire lath, and semi-gloss enamel paint, 2006.37.1
On Kawara "Title," 1965, acrylic on three separate canvases, 2006.40.1.1
Robert Morris "Untitled," 1967/1986, steel and steel mesh, 2005.142.28
Barnett Newman "Achilles," 1952, oil and acrylic resin on canvas, 1988.57.5
Tony Smith "Die," model 1962, fabricated 1968, steel with oiled finish, 2003.77.1