"
Bronze disease" is caused by a layer of copper(I) chloride under the
patina or crust that slowly oxidizes in the presence of air and moisture to produce acids that eat holes in the metal underneath and dissolve the
patina above. You can see these effects pretty clearly here. The best way to treat it, if you want to preserve the
patina, is to deprive the coin of air and moisture i.e dry it out thoroughly with heat and then wax it up. In the extreme case, when there is no
patina worth preserving, then you can strip it down to the bare metal as was done here. The holes and corrosion left in the copper by the BD are pretty obvious but it is unlikely to go further since the copper(I) chloride layer has been stripped away along with the
patina.
The problem then is what to do with the coin if you don't like the bright metal surface that is left? Re-creating a
patina once you have removed it is more-or-less impossible. There are various paint-like materials (Jax?) that I know little about. My own patented antidote is to rub graphite into the porous metal surface and then wax it down. It can always be removed with alcohol and it might create a more uniformly dark and attractive appearance though I wouldn't expect it to materially "improve" the coin.
regards, Dave
