An impressive book on finishing metals is Richard Hughes and Michael Rowe's 1982 The Colouring, Bronzing, and Patination of Metals.
It's a big book. It begins with a 13 page Historical Introduction which covers metal finishing in ancient China, Greece, and Rome, throughout history up to the present day. The surface enrichment of coins is briefly mentioned. There is an extensive bibliography.
The next section is a 23 page discussion of how to actually color metal objects. This is followed by 16 color plates. The plates are quite beautiful in the 1991 Watson-Guptill edition that I borrowed from the New York Public Library. (I have not seen the other editions.)
The bulk of the book is a 250-page section of chemical recipes to get the various color effects. These are historical recipes, which the authors personally carried out “with the exception of a few potentially very hazardous recipes”. Modern chemical names are used for the ingredients. The recipes are dense, four or five to a page, arranged by the desired color of the finished product. For anyone carrying out the recipes, there is an 18 page section on Safety.
The patination technique used by Italian antiquities forgers that I mentioned in March that used Copper Sulfate, Nitric Acid, and Ammonium Choride did not appear in the book (or if it did, I missed it in the hundreds of recipes). Recipe 3.213 (the 213th recipe in the Copper section includes Copper Sulfate and Ammonium Chloride, but used Ferrous Sulfate instead of Nitric Acid. There were several recipes with Nitric acid, such as 1.40, 1.120, 2.29, and 3.132, but not many (perhaps because of its poisonous and corrosive nature). The recipes that did include Nitric Acid combined it with Copper Nitrate rather than Copper Sulfate.
It would be fun to experiment with the patina recipes, perhaps on modern copper coins, but I lack the time to embark on such a project and it probably isn't a good idea to keep such chemicals around the house. If anyone reading the blog has skill in this field and would like my to link to their site or to guest-post here on the topic of producing patinas just let me know.