THE GERMAN ARTIST SIGMAR POLKE, who died in 2010, has never had the name recognition of his peer Gerhard Richter, but a blockbuster at MOMA, opening on April 19, could change that. With more than three hundred figurative and abstract works, straddling painting, photography, printmaking, drawing, cinema, and sculpture, “Alibis: Sigmar Polke, 1963-2010” encapsulates, to the extent that it’s possible, the quicksilver career of the reclusive titan, whose oeuvre was one long experiment. Arsenic, snail slime, and pulverized meteorites were just some of the materials Polke used on canvas, in addition to paint. But for all his forward thinking, Polke was steeped in history, lifting motifs from Goya and Hogarth, referencing the French Revolution, and revisiting Renaissance-era alchemical processes. While his images could be unnerving (notably, spectral paintings of concentration-camp…