Photogravure is the production of an etching upon a copper-plate from photographic negatives, and is effected by a combination of light, chemical action, and hand-work. Of the different processes the most usual is that called "Klic's method." A coppered plate is dusted with asphalt, which is partially melted by the application of heat. A negative carbon print is then transferred to it, and the design is eaten in by a solution of perchloride of iron (strong for the thin parts, and weak for the thick parts). Another process is the Collotype method. In this case the impression is taken upon bichromatised gelatine spread upon glass, and this plate is then washed and used as a printing-block.