A later refinement of the lithographic method, chromolithography produced colored prints on limestone blocks. The process allowed a wide range of colors to be used, from delicate to deep shades. In this technique, multiple stones were used; one of each color, and the print went through the press as many times as there were stones. The problem for printers was keeping the image in register; making sure that the paper lined up exactly each time it went through the press so that each color would be in the correct position and the overlaying colors would merge correctly.