The thin section technique was superseded by the now-common liquid-peel technique in 1928.[7][50][53] In this technique,[55][56][57] peels are obtained by cutting the surface of a coal ball with a diamond saw, grinding the cut surface on a glass plate with silicon carbide to a smooth finish, and etching the cut and the surface with hydrochloric acid.[54] The acid dissolves the mineral matter from the coal ball, leaving a projecting layer of plant cells. After applying acetone, a piece of cellulose acetate is placed on the coal ball.