Eliminating the mineral acids and substituting the organic acids, such as acetic, oxalic, citric and carbonic, we can produce coatings of great permanence. The artistic effects brought about are dependent primarily upon the method and means of pro cedure. Applying a solution of any one of these organic acids to the surface of the bronze by means of a brush seldom brings about a pleasing or attractive patina. Usually streaky deposits result. The point to bear in mind is to imitate the natural forma tion of the patina as closely as possible. We recommend the following procedure.
The bronze, whose surface has been carefully cleaned (see BRONZE) so as to expose the virgin metal or only a thin film of oxide, is placed in a gassing chamber. This chamber must be large enough so as to prevent the bronze from coming into con tact with the walls. The chamber is closed on all sides, preferably hermetically sealed. The "gas" is derived from a dish of strong acetic acid situated at the bottom of the chamber. Moistening the surface of the bronze before introducing it into the chamber will hasten the formation of the copper acetate on the surface. The time of exposure is usually three to four days at room tempera ture. Then follows a gassing in ammonia fumes, using the same chamber and substituting a dish of aqua ammonia, concentrated, for the acetic acid. Then the third gassing is with carbonic acid taken from a cylinder. The procedure is repeated in the same rotation if a comparatively thick patina is desired. Irregular
patches of blues and greens and bluish greens are produced, strikingly similar to those met with in nature. The bronze is then carefully and very slowly dried in an oven which is gradu ally heated up to 232° F allowing 12 to 24 hours to attain this temperature, depending upon the thickness of the patina; the thicker the patina the slower the heating.
Finally, the bronze is sprayed with a dilute solution of bees wax in benzol and after ro to 15 hours' drying in the air is care fully rubbed with a soft dry cloth. Lacquers are not as desirable as the bees-wax as most are not durable and fail to bring out the desired artistic effects characteristic of bees-wax. Since bees-wax is at times bleached with chlorine, the unbleached wax is to be specified.
To produce the red patina (cuprous oxide) gassing alternately in ammonia and iodine vapours may be used. We find, however, that the action is comparatively slow and quicker results are ob tained by submerging the entire bronze in a suspension of precipi tated chalk in water to which has been added 2 to 3% of iodine as tincture of iodine The bronze remains submerged for a period of three to eight days, depending upon the depth of colour de sired. It is then dried and finished as above. The slower the patinating the more artistic the results is a good general rule to follow. (C. G. F.)