“Thaw: Fifteen Years of the Marat Guelman Gallery,” at St. Petersburg’s Marble Palace, is a tribute to the ingenuity and taste of the gallerist who changed the rules of Russian art. The show’s title refers to a second political thaw in Russian expressive freedom (following the earlier one under Khrushchev) that has characterized the last decade and a half; during this same time, Russian art made the leap from the arms of the state to those of the private gallery. Cemented by Guelman’s donation of thirty works to the State Russian Museum, the exhibition reads as a formal historicization of this