"In the Tower: Mark Rothko," on view February 21, 2010 through January 2, 2011, featured the enigmatic black paintings of Mark Rothko. In preparation for this exhibition, Jay Krueger, head of painting conservation at the Gallery, investigated the layers and materials Rothko might have used to create the black paintings. These austere paintings—each presenting a single black rectangle on a black or nearly black field—are among the most mysterious of Rothko’s career. Black is a frequent, often imposing presence in Mark Rothko’s earlier paintings, from the figurative works of the 1930s and the surrealist-inspired canvases of the mid-1940s to the “multiforms” of the late 1940s. Black was rare in his work for more than a decade, but Rothko returned to it in 1964 for a key commission. That year the collectors Dominique and John de Menil commissioned Rothko to create series of paintings for a Catholic chapel in Houston, today the non-denominational Rothko Chapel. Having recently worked on multicanvas commissions for the Four Seasons Restaurant (1958–1959) and Harvard University (1961–1963), Rothko was already painting in the series format. He worked on the chapel commission from the fall of 1964 through the spring of 1967, producing 14 large paintings and four alternates, many of them direct successors to the black paintings of 1964.