I've lost track of who first asked this question now, but the following formula
contains one ingredient which we all know and love.
It's from a book which I suspect would appeal to many alt.proc. persons, being
full of fabulous formulae. The following formula has been well tried and tested
over the years by hundreds of jewellers and students, and gives a black
semi-gloss finish. Temperature is critical, as is starting off with a very
clean and grease free piece of brass.
50gm sodium thiosulphate
12.5gm ferric nitrate
1 litre water
then:
20mins hot immersion (60-70 degrees centigrade - no hotter).
There are other formulae for blackening brass, and some with v. nasty
ingredients (ie with lead acetate).
OK, the book is called "The colouring, bronzing and patination of metals" by
Richard Hughes and Michael Rowe, pub. by Crafts Council, Great Britain, 1982.
good luck and cheers
Catherine