The sketches for Marc Chagall's stained-glass windows at the Centre Pompidou-Metz are revealing of his artistic process. On some boards, each piece of glass' color is numbered, showing the complexity of the nuances. Chagall sometimes worked by superimposing several colors and adding acid to alter them before firing. He also painted tiny gray additions which produced the desired reflections. Brigitte Simon and Charles Marq, master glassmakers and heirs to a workshop created in Reims in the 17th century, were entrusted with the creation of the stained-glass windows designed by Chagall.