Recently I have been presented with the problems of repairing or changing the color of Indian made polished brass objects, usually to brown or grey tones. In one case the object was a Shiva which was about three feet high and across.
Warning: This procedure should be undertaken with appropriate precautions; goggles, gloves, protective clothing, adequate ventilation.
It was cast brass in a hollow core technique and had a number of parts separately cast and then brazed together and the surface finished. This was not immediately evident and occasioned some slightly irritating moments when pickling liquids (which I now do not recommend using) were trapped in several different sections and had to be first driven out through tiny holes with heat and then neutralized and rinsed.
The Shiva had been partly ground at by the owner which had removed a darkened copper plating on the cast brass and revealed the bright yellow core. The request was for a grayish patina, fairly bright with a hint of bluish green. As nothing would match reactions on both surfaces and in order to start with a uniform metal type I decided to plate the object. The same technique worked well later on a pair of 5′ brass cobras destined to be part of a lighting company’s offering. They asked me to change the patina on the snakes from a tan brown to a glossy reddish black with blue hints.
The first time I mixed up a plating solution and used a (DC) wired brush and rectifier to plate with. While this worked I found that precipitating the copper on the cleaned brass surface was easier using a contact plating technique and steel wool as the precipitating agent. Zinc would also be electro-chemically active in this regard.
I found that the fineness of the steel wool made a difference. The finer the steel wool the faster it precipitated the copper. Because it had a great deal of surface area most of the available copper ions were used up in reacting and plating out onto it leaving little behind in the solution to be precipitated onto the brass.
The best grade I found that caused the plating reaction without using up too much of the copper solution on itself was medium grade Bulldog® steel wool. Wearing goggles, rubber apron and gloves I would pour some of the plating solution onto a steel wool pad and slowly wipe the wool with slight pressure over the surface of the brass. Several passes were necessary to effect a good plating. It was very important to be clean beforehand; to have properly cleaned the base metal and rinsed it well. A good deal of rinse water was also used after applications of the copper plating solution.
In my case having got the plating on I sprayed on a dilute liver of sulfur solution onto the plating and rinsed afterwards to blend coloring areas and keep the pace of coloration uniform. One can brass brush the surface with soapy water in between coloring applications to achieve a more glossy result. When the color was correct I placed the objects inside a polyethylene tent (a garbage bag will work) with some household ammonia in a pan in the bottom of the tent. The fumes affect the surface giving blue hints which gather in the recesses of the work. This occurs starting at about one hour and continues with time. I generally use six hours or so. As the contact plating is thin it is possible that too long an exposure would use up the copper metal and begin attacking the brass. Brass in ammonia fumes for long periods of time may crumble when bent even with hands or fingers and becomes very weak and brittle. The surface can then be waxed or sealed. In one case the client requested no protection, he wanted the patina to advance further slowly with time. In the other I used a clear auto enamel which produces a shiny, more metallic surface.
All safety warnings apply. Always add Acid to Water!! Goggles/Gloves!
Put about 800cc water into plastic or glass container after marking the 1000cc level on it. Add the copper sulfate and stir to dissolve. Slowly pour a thin stream of acid into the swirling water. Heat is evolved-be aware of this. Rinse the acid container with distilled water and top up the mixture with it to the 1000 ml level. This solution can also be used as an electroforming solution for growing copper. Remember, acids are dangerous.
A dust mask is suggested around chemicals. Work cleanly. Copper salts are toxic and irritant and should be handled with care. Dispose of properly.