Patina, or something worse?

bosox

I have some 19th century bronze coins that, after years of being boxed up, have developed a few small spots of the hard, greenish-blue stuff. There is no pitting underneath. In researching the web I'm pretty sure it is copper carbonate (the normal progression of copper corrosion is from brown copper oxide to blue-green copper carbonate) and not the cuprous chloride (powdery bronze disease). Questions for all:

Will it get worse if the coins are now in saflips?

Should I try to clean it off? If so, how?

How much do minor spots of it adversely affect value?

Thanks

theboz11

I think the cure is a long soak in olive oil to lift the offending substance. I use a Rose thorn and a microscope to loosen the oxidation, acts waxy when cleaning. Not sure about acetone or xylene on bronze?? Works good on copper.

Askari

Acetone shouldn't hurt bronze and will help inert the corrosion. Reholder them in something inert and store them in a humidity/temp-controlled environment. The olive oil soak may lift the corrosion, but there will still probably be a spot where it was -- and the soak can take a long time (weeks to months - you have to keep checking and changing it out). I haven't used the olive oil method myself but the ancients collectors could give you more precise instructions.