This section is from the book "Cassell's Cyclopaedia Of Mechanics", by Paul N. Hasluck. Also available from Amazon: Cassell's Cyclopaedia Of Mechanics.
One method of blackening brass is as follows. Dip the article in a bath consisting of 1 part of sulphate of iron and 1 part of white arsenic dissolved in 12 parts of hydrochloric acid. When the article has become sufficiently black, rinse it well in several changes of cold water to remove the acid, dry in sawdust, and polish with blacklead; it may then be lacquered with a pale lacquer. Another method, and one more generally adopted, although somewhat more expensive, is as follows. Well polish the article with tripoli, and afterwards wash it well in a mixture consisting of 1 part of nitrate of tin and 2 parts of chloride of gold dissolved in a little water and acid, Remove the article and wipe it with a clean linen rag. A slight excess of acid will increase the intensity of the black. The following method will also be found very good, and is the same as that adopted in oxidising silver articles. Give the article a light silver-plating by deposition, in a similar manner to ordinary cheap electro-plated goods. Then prepare a solution made as follows. Dissolve in a little acetic acid 2dwt. of sulphate of copper, l dwt. of nitrate of potash, and 2 dwt. of muriate of ammonia.
After warming the articles, apply the solution with a camel-hair pencil or immerse in the bath, then expose them to the fumes of sulphur in a closed box. This may readily be done by placing in a tin biscuit-box a red hot iron bowl, such as the bowl off a small lead ladle, in which are a few pieces of sulphur. Hang the articles on a rod across the tin, and close the lid. It will be necessary to do this where there is a fairly good draught to carry off the sulphur fumes.