My favorite treatment to produce a black patina on bronze, brass,
copper and even silver involves the use of selenium dioxide solution.
This is a material that is used by gunsmiths and others in industry
to blacken a variety of metals. It is commonly used to produce or
repair the “blue” finish on firearms. I use it on castings of
Si-bronze sculpture to produce a range of shades ranging from grey
through brown to jet black. It can be bought at places such as
sporting goods stores, Walmart (sometimes), gun dealers orgunsmiths.
The last I bought was made by Birchwood Laboratories, Inc. and
marketed as “Brass Black” in a plastic bottle containing three fluid
oz. (90 ml). This amount will cover a lot of surface. Expect to pay
about $5.00 in the USA. If a single coating doesn’t give a dark
enough color, put additional coatings on. It must be applied to clean
metal! There may be some color variation depending upon alloy
composition but this is true of most any chemical patina. On silicon
bronze it is very good and black. On sterling silver it produces a
dark brown to black patina. The patina coating binds covalently and
is quite durable. It is best to coat the final product with a good
hard wax (carnuba or some other) as is wise with any patina that you
wish to stabilize.
It must be noted that selenium dioxide and selenous acid are toxic
and should not be ingested. I am careful to keep it off my skin.
Knowing my own bad habits, I generally wear rubber gloves when using
this and most of the other solutions I use for patination.
I hope it works for you. Gerald Vaughan