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XXI

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Bronze Disease

by Bruce Nesset

Also see:
Bronze Disease Treatment

Although a patina (a hard green coating on most bronze coins) is the result of corrosion, a patina actually seals and protects coins. Bronze Disease [BD], however, is to bronze, what rust is to iron. The matrix of copper and tin that forms bronze is attacked by hydrochloric acid {HCl} to form tinchloride and copper. BD sometimes attacks and can severely damage ancient bronze coins.

"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 overtime." 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 was away as much as possible. A coin with bronze disease 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?

or

Bronze disease is a light green powder that comes off easily and often forms pits deep into the coin. In the second example,the reddish-brown is copper that has been freed from the bronze matrix,producing the "scarring" that is characteristic of BD.

The best treatment of BD is prevention. Do not remove the patina on ancient coins. BD will often rapidly attack coins that had a patina removed by over cleaning. Keep your coins in a dry place and use desiccant if you live in a humid area. Unfortunately, if you have a large collection, someday you will probably have to deal with at least one case of BD.

What to Do

  1. With running water and a stiff 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. Prepare a bath of 5 parts baking soda [sodium bi-carbonate] to 8 parts washing soda [sodium carbonate]. You may go by weigh or simple dry measurement, i.e. tablespoons full. Store the mix in an& air-tight container.
  4. Use demineralized or unchlorinated 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 sestertius allow at least 72 hours. After the initial soak, rinse with clean water and give the coin/s a light scrubbing.
  6. Repeat step 5, twice more.
  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.
  9. While this procedure should not effect a true green patina. It will remove any artificial coloring or repatination.


Reference: For further reading - the following is where some of the information used in this article was derived: http://nautarch.tamu.edu/class/anth605/File12.htm

Bruce Nesset

bknesset@yahoo.com

All coins are guaranteed for eternity