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This
article has been contributed by Bruce Nesset. His knowledge of bronze disease,
and its remedies, have saved several of my "infected coins". Every
collector should be aware of this problem, and heed Bruce's excellent advice
in this matter. Thanks Bruce!!!
Bronze Disease
Bronze Disease [BD] is to bronze metals, what rust is to iron based metals. The matrix of copper and tin that forms bronze is attacked by hydrochloric acid {HCl} to form tinchloride and copper.
quot;The first step in the electrochemical corrosion of copper and copper alloys is the production of cuprous ions. These, in turn, combine with the chloride in the sea water to form cuprous chloride as a major component of the corrosion layer:
Cu -e >> Cu+
Cu+ + Cl- >> CuCl
Cuprous chlorides are very unstable mineral compounds. When cupreous objects that contain cuprous chlorides are recovered and exposed to air, they inevitably continue to corrode chemically by a process in which cuprous chlorides in the presence of moisture and oxygen are hydrolyzed to form hydrochloric acid and basic cupric chloride (Oddy and Hughes 1970:188):
4CuCl + 4H2O + O2 >> CuCl2 0 3Cu(OH)2 + 2HCl
The hydrochloric acid in turn attacks the uncorroded metal to form more cuprous chloride:
2Cu + 2HCl >> 2CuCl + H2
The reactions continue until no metal remains. This chemical corrosion process is commonly referred to as 'bronze disease.' Any conservation of chloride-contaminated cupreous objects requires that the chemical action of the chlorides be inhibited either by removing the cuprous chlorides or converting them to harmless cuprous oxide. If the chemical action of the chlorides is not inhibited, cupreous objects will self-destruct over time." 1
Referring to the equations and comments in reference (1) above, BD is "contagious" through the green cuprous chlorides and care should be taken to wash away as much as possible. [Note: This is contrary to what I have said in the past, and should be considered to be hazardous to other bronze coins.]
Now that you have been hit with the technical details, what are you really
looking for?Look at this more advanced case (below). The reddish-brown is
copper that has been freed from the bronze matrix, and will produce the "scarring" that
is characteristic
of BD.
What to Do
With running water and a stiff nylon brush, scrub the entire surface free of "green fuzz", allow to dry.
Use a magnifying light and a sharp needle to remove and open any obvious green spots still visible. With care these will not be obvious later.
Pre-pare a bath of 5 parts baking soda [sodium bi-carbonate] to 8 parts washing soda [sodium carbonate]. You may go by weight or simple dry measurement, i.e. tablespoons full. Store the mix in an air-tight container.
Use de-mineralized or de-chlorinated water for better results.
Mix 2 tablespoon of the soda mix to three cups of water; add coins and heat the mix to boiling, reduce heat for a minimum of 5 minutes. Set aside and soaking to continue for an extended period of time. This should be at least 24 to 36 hours, for thick coins like sestercius allow at least 72 hours. After the initial soak, rinse with clean water and give the coin/s a light scrubbing.
Repeat step 5, twice more.
After the last soak and scrub, dry the coins, and soak in 100% isopropyl alcohol, for about 20 minutes. This will help draw out more water from the coin fabric. Dry thoroughly, and seal with a paste wax, well rubbed into the coin.
It is recommended that all bronze coins be checked periodically, as I have found "clean" coins suddenly break out 2 years after purchase.
While this procedure should not effect a true green patina. It will remove any artificial coloring or re-patination.
Reference: For further reading - the following is where some of the information used in this article was derived:
Bruce Nesset
