Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 388 of 707

(180° F—not hot enough to blacken the surface) to dissolve most of the zinc out of the brass surface, leaving a copper-rich surface behind. At this point the part has a color, which is quite pink and is reactive enough to be blackened by the subsequent oxidizing bath. These heated oxidizers can produce good quality black deposits and can be controlled by titration and/or boiling point. They do present an inherent danger to the operator because of the high operating temperature. This process is an electrolytic blackening operation, which produces a black nick- el sulfide coating on the surface. The finish is very hard and durable and, in many cases, produces a true black color, which the other methods cannot match. It is used most often as an extension of the brass plating operation. Since the parts are already racked for electrolytic deposition of the brass, they are ready for a second electrolytic operation—in this case blackening after thorough rinsing. The bath can be operated as a permanent plating bath in the line with periodic titration and replenishment and excellent tank life. Many experienced platers find that they can mix their own solution using commodity chemicals rather than purchase a pre- blended proprietary product. When operated in this way the operating costs can be quite low. Black Nickel Coating Black nickel works best on racked parts. Bulk or barrel handling methods work less well and usually result in more difficulty in achieving a uniform deposit, due to the continual interruption of electrical contact between the parts. As a result the black nickel finish is best suited for use on high value parts, which are rack plated. Verde Green Patina Also called "verdegris," this finish is a soft, pale-green color, similar to that seen on the aged copper roofs of older buildings. Actually, the authentic green patina formed on these roofs is a mixture of many different copper compounds including oxides, car- bonates, sulfates, sulfides, and more. The composition is directly related to the puri- ty of the air in the area. For example some copper roofs are more black than they are green due to a higher concentration of sulfur in the air from a coal-burning power plant in the vicinity. Others are greener owing to a concentration of nitrates in the air from automobile exhaust. Consequently, the color varies widely. Artificial green patina solutions are, in simplest terms, mildly acidic corrosive cop- per solutions. They work by slowly tarnishing or corroding the surface of the brass or copper substrate and forming some of these same green or bluish colored copper compounds. These finishes can be quite attractive when properly applied. They have, however, two inherent disadvantages: the finish takes several hours to form and it is only loosely adherent to the metal surface. Consequently, the green patina solutions sold commercially tend to be workable only in small volume process lines where the finisher can afford to let the parts hang and corrode as they dry. And because the finish is loosely adherent it depends on the lacquer topcoat to provide the adhesion to the substrate to form a clean final finish. HIGHLIGHTING AND BURNISHING THE FINISH Once the parts have been colored or oxidized to the desired finish they are ready to be highlighted or burnished. This operation can take several forms depending on the final appearance requirements of the part. The essence of the operation is the removal of some or most of the colored finish to reveal portions of the underlying base metal in order to make it appear worn. In other words the colored finish is pol- ished off the high points or highlights of the parts and allowed to remain in the 387

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