Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 387 of 707

Sulfur and Arsenic-Based Solutions The traditional way to color a brass surface is to oxidize it with one of these solu- tions. The sulfur-based method is often called "liver of sulfur" and utilizes a mix- ture of polysulfide salts to form a black or brown copper sulfide deposit on the sur- face. It works better on copper than it does on brass and has the inherent disadvantage of having a strong sulfur or "rotten egg" odor. In addition sulfur has several oxidation states and can form a variety of nonreactive polysulfide com- pounds, which greatly reduce its efficiency and tank life in a production operation. In actual practice the bath can be somewhat erratic in its oxidizing power from one batch to the next and requires frequent dumping as it is not considered a replen- ishable product. It generally is considered unsuitable for production scale use. Arsenic-based solutions form a black arsenic oxide on the surface and operate at room temperature; however, they carry a significant toxicity risk for the user and must be handled with extreme caution. In addition to the two methods above there are other solutions that can be used on a small scale to color brass and bronze. These methods utilize a variety of chemicals to form colors on several metallic substrates and are designed for use by individual artisans rather than in production-scale antiquing operations. Copper/Selenium Room-Temperature Oxidizers Since selenium is directly related to sulfur on the Periodic Table of the Elements it undergoes many of the same reactions as sulfur. Consequently, these selenium-based solutions can be used to deposit a black or brown deposit on brass, bronze, or cop- per at room temperature and offer several advantages over the sulfur method. Most importantly, the selenium has fewer oxidation states than sulfur. This means that the solution is easier to control, with all the selenium going into reacting with the brass surface rather than forming nonreactive side compounds. The result is a bath that can be titrated and replenished and operated as a permanent bath in the line with no dumping necessary. This feature gives the finisher greater control over the operation of the bath in terms of reaction speed, the color of the finish produced, and the operating cost of the antiquing step. The only caveat that must be observed is the fact that brass-plated parts will car- ry a cyanide residue on the surface, which must be neutralized prior to immersion in the antiquing solution. This is accomplished by momentary immersion in a weak (2- 5%) sulfuric acid solution to neutralize and remove the cyanide from the surface. Skipping this step will result in low-level contamination of the antiquing solution by cyanide, which will tend to chelate or complex the copper content and reduce the effec- tiveness of the bath or disrupt the normal chemical balance. In practice selenium-based oxidizers have proven to be the preferred way to blacken or brown a brass surface, due to their ease of operation, lack of fumes, dependable operation, and low cost. A variety of colors can be achieved, ranging from golden brown to medium and chocolate brown and black, depending on dilu- tion level and immersion time in the solution. Since these baths are quite safe to work with it is usually easy for the operator to perform the necessary mainte- nance and operate the system without undue hazards. Heated Caustic Oxidizers These baths operate at 240O F and utilize caustic soda and sodium nitrate to oxidize the copper at the surface to a black cupric oxide. Since they react exclusively with the copper at the surface, a copper-rich surface favors the formation of a black deposit in the shortest time. Consequently, many brass parts are "dezincified" prior to blackening. This is done by immersing the parts in a warm caustic bath 386

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