Does Anyone know how to restore its original green color on copper
This recipe comes from a handout titled "Patination and Coloring of
Metals". I gleaned it from my Father's collection of notes after he
got out of casting iron and bronze to take up working with stained
glass. While I have not used this particular recipe I have used many
of the other recipes in this handout and they have been accurate.
However I must caution you to proceed at your own risk. You will need
an accurate scale (i.e. triple beam or electronic jeweller's scale).
Alternately you can multiply all of the ingredients by the same
factor (say x 2) to increase the weights and make weighing more
accurate when using scales that are perhaps not so accurate. I know
this reads like an 12th Century Alchemist's recipe but here it is
just the same.
Bronze - green patina
6 Fluid ounces Water
1.0 grams - Oxalic Acid (wood bleach)
2.0 grams - Calcium Chloride
1.0 grams - Copper Sulfate
4.7 grams - Ammonium Chloride
4.7 grams - Copper Nitrate
Excercise proper precautions (read this as goggles, apron, rubber
gloves, respirator ect...) when working with these chemicals and
compounds as some of these are corrosive or otherwise hazardous.
You cannot be too careful and while you may look like an alien with
all of this gear on you will be a Live Alien with all of your parts
intact! Clean the metal and apply one coat each day until the desired
patina appears. This can take several coats. Next use a toothbrush
to lightly knock down any excess lumps or high spots that could be
abraded if left in place. If brushing results in a significant
change to the patina re-apply the solution repeatedly until the
desired patina is again achieved. Repeat the brushing step. Once you
have the right patina and have removed the high spots you will need
to set the patina with a coating of beeswax or clear lacquer. This
step is important as it will prevent further oxidation of the metal
that could result in the patina changing color over time.
Note; The beeswax or lacquer will likely make the patina darker.
Usually I recommend experimenting on a piece of metal of the same
composition but it sounds like you may be hard pressed to find
another piece of 4th century "bronze".
Note; to make the color brighter leave out the copper sulfate and
oxalic acid. Conversely you can darken the color by adding varying
amounts of these compounds up to the full amount called for in the
basic recipe. I hope this is helpful to you in your quest to restore
and preserve this relic.