Seems like the topic of matching antique brass comes up from time to time
on this group. Brass can be colored chemicly to achieve an antiqued
color or patina. Below is a post I saved from rec.guns many years ago.

The Coloring of Brass
by Royce W. Beal
written on 17 March 1995 specifically for the readers
of the rec.guns newsgroup.
questions should be directed to me at

Read this entire essay before attempting any one
treatment. If you choose to just "cut and paste" part
of this, please make sure you get the safety instructions
and warnings after the recipes. Under no circumstances
do I consider myself liable for any accidents which occur
while using any of these chemicals. Also, I do not consider
myself an expert in this field and am still doing research
for the FAQ. This will be a temporary article. Because I
am still experimenting, I cannot vouch for all of these
colors.
Concentrations and conditions DO matter. (Concentration
is more important than actual volume, so if you want to
use less, make sure that you use proportionately less of
each ingredient) If you want good results follow the
recipes closely. Above all it is important that the brass
surfaces be clean. This means an extra hour or so in the
tumbler for the cases and then touch them only sparingly.
I have tried to collate recipes which will require the
acquisition of the more common chemicals. I have also tried
to steer clear of the really hazardous arsenic and cyanide
salts (which you probably can't get anyway) If you feel
that you've been cheated by this, please refer to the
references section of this report and find the books for
yourself in any well stocked library.
It is my understanding that these are all surface
coatings and should not damage or weaken the brass.
Obviously you will want to do this treatment with unprimed
brass. DO NOT USE METAL UTENSILS (ok maybe stainless steel)
Glass or Plastic containers are the preference. If you are
really worried about what this is going to do to your brass,
refer again to the reference section below.

TIFFANY GREEN:
Copper Sulfate.................8 ounces
Ammonium Chloride..............4 ounces
S
odium Chloride................4 ounces
Zinc Chloride..................1 ounce
Acetic Acid....................2 ounces
Water..........................1 gallon

VERDE:
Copper Nitrate.................16 ounces
Ammonium Chloride..............4 ounces
Acetic Acid....................1 quart
Water..........................1 gallon

GREEN:
Iron ( ferric) Nitrate.........2 ounces ( Fe(III)(NO3)3)
Sodium Hyposulphite............8 ounces
Water..........................1 gallon
(use at boiling temperatu
re, brass can be immersed
or the solution may be "painted" on)

HARDWARE GREEN:
Iron (ferric) Nitrate..........1 ounce (Fe(III)(NO3)3)
Sodium Thiosulfate.............6 ounces
Water..........................1 gallon
(use at 160F)

RED:
Iron (ferric) Nitrate..........6 ounces (Fe(III)(NO3)3)
Sodium hyposulphite............6 ounces
Water..........................1 gallon
(use at 170F will speed up this reaction)

BLUE:
Sodium Hyposulphite............8 ounces
Lead Acetate...................4
ounces
Water..........................1 gallon
(use at boiling temperature)
or
Lead Acetate...................2 to 4 ounces
Sodium Thiosulfate.............8 ounces
Acetic Acid....................4 ounces
Water..........................1 gallon
(use at 180F. This color will change if
not lacquered [DO NOT LACQUER FIREARM CARTRIDGES]
Take your chances with the color change.)

BLUE BLACK:
Copper Carbonate...............1 pound
Ammonium Hydroxide.............1 quart
Water.
..........................3 quarts
(Add the water after the carbonate and hydroxide
have been mixed. There must be excess Copper
Carbonate. Use at 175F. This color can be fixed
(made more permanent) by quickly dipping in a 2.5%
Sodium Hydroxide solution.)

BLACK:
Ammonium Hydrosulfide...........2.25 ounces
Potassium sulfide...............1 ounce
Water...........................1 gallon
(use at room temperature or COOLER for best results)

BROWN:
Potassium Chlorate..............5.5 ounces
Nickel Sulfate..................2.75 ounces
Copper Sulfate..................24 ounces
Water...........................1 gallon
(use at boiling temperature)


SAFETY:
1. NEVER taste any of these chemicals.
2. Keep very far out of the reach of children.
3. Most Nitrates are good oxidizing agents and
should not be stored with anything flammable.
4. Acetic Acid has a VERY strong pungent odor.
Use in well ventilated areas. This acid can
be airborne in vapor form. If you feel that
you have breathed enough of it to feel
uncomfortable, leave the area and drink a
carbonated soft drink. "Have a Coke" Do not
underestimate this chemical.
5. Many of these chemicals may stain your skin or
clothing. Wear rubber gloves and protective
clothing including glasses of some sort.
6. Steam can cause serious burns. Solutions of salts
can actually exceed the boiling point of water.
The steam from these solutions can be very dangerous.
BE CAREFUL WITH STEAM AND BOILING SOLUTIONS.
7. Feel free to change concentrations for experimentation
purposes but do not change the ingredients in any
one recipe.
8. Always be fully awake and alert around chemicals.

CONVERSIONS AND INTERPRETATIONS:
Ounces are assumably troy ounces, even when dealing
with liquids or solutions. Do not use fluid ounces.
1 ounce = 31.103 grams = 480 grains
1 quart =
0.25 gallon = 946.4 mL
1 gallon = 3.785 L

REFERENCES:

Meyer, Walter R. title: Plating and Finishing Guidebook
ninth edition - 1940 pp.72-75 (cited)

Metal Finishing Guidebook twenty-eighth edition - 1960
article by Hall, Nathaniel title: Coloring of Metals pp. 477-479
(cited)

Krause, Hugo title: Metal Coloring and Finishing (not cited)

Hiorns, A. H. title: Metal Coloring (not cited)

Field, S and Bonney, S.R. title: Chemical Coloring of Metals (not
cited)


-Royce