B18 Sculptures with a Patina
Description
Several sculptures with decorative oxide coatings are shown. Then a square of brass is placed in an ammonium polysulfide solution in a petri dish. After a brown color forms, the brass is taken out of the solution and allowed to dry. It is then placed in a dish of fresh copper sulfate where the surface turns black. Several polysulfide forms are present, and several reactions take place.
Equation
Cu + S22- --> CuS22-
Cu + Sx2- --> Cu(Sx)2-
Cu2+ + Cu(Sx)2- --> Cu(Sx)Cu
Reference
MetalsFinishing 50th Guidebook Directory, Metals and Plastics Publications, Inc.,Hackensack, N.J., (1982)p552.