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 tin-chloride and copper.
?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. The ?fuzzy green? is easily
dislodged and if it can get into crevasses of other bronze coins, it will begin the cycle as moisture is
absorbed.
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
1.With running water and a nylon brush, scrub the entire surface free of "green fuzz", allow to dry.
2.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.
3.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.
[note: There is a commercially prepared version, if you don't want to make your own, contact me,]
4.Use de-mineralized or de-chlorinated water for better results.
5.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 Sestercious allow at least 96 hours. After the initial soak, rinse with clean
water and give the coin(s) a light scrubbing.
6.Repeat step 5, twice more. For the repeat, the water need not be boiled, although warming to at least
120 degrees F or about 55 degrees C, does help dissolution of the embedded salt.
7.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.
8.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. Some artificial colorants are wax based, and will come off coins being cleaned.
Reference: For further reading - the following is where some of the information used in this article was derived, see the
footnote 1.
http://nautarch.tamu.edu/class/anth605/File12.htmBruce Nesset
