Matisse began Back, his largest sculpture to date, in spring 1908 in his studio in Paris and worked on it for over a year, keeping the clay wet in order to do so. In June 1909, as he prepared to move to his new studio in the Paris suburb of Issy-les-Moulineaux, he cast the relief in plaster, thereby destroying the original. Now working in plaster, he changed his methods, adding fresh material to the hard, rewetted surface and removing hardened forms with chisels, hammers, and rasps. Matisse returned to Back several times over the next twenty-one years, each time beginning with a plaster cast of the previous state. He also preserved a plaster version of each state, and, cast in bronze, they are the works we know today—Back (I), Back (II), Back (III), and Back (IV).
from Matisse: Radical Invention, 1913-1917, July 18–October 11, 2010