Suspected silver carbonate tarnish on proof coins from Haifa, Israel

Greetings!

I have a number of coins sent from Haifa, Israel that may have exposed to alkaline aerosols or condensate. See https://link.springer.com/article/10.1007/BF00479411




These silver proof coins have varying degrees of what appears to be a white corrosion layer chemically bonded with the surface, so that the surface is still mirrored and reflective. However, the white corrosion does not photograph well at most angles. The "white" corrosion might be yellowish were it not for the mirrored silver surface.

I accept responsibility for the first coin, so rising it with isopropanol and briefly bathing in in an utrasound for 10 seconds seemed appropriate with no detectable difference in appearance.

Could the corrosion be silver carbonate?

Any other suggestions?

Thank you in advance!

 

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There are some mints that apply a coating on proof coins to prevent toning.
If these coated coins are dipped in chemicals, such as acetone, it will permanently damage the appearance of the coin.

Unfortunately I know this from experience.


I have never found any kind of publication that lists which mints or in which years this technology was used, but after ruining 3 proof coins, I have since stopped using acetone on any proof coinage.

Could be a possibility.

 

Acetone always leaves an ugly residue, regardless of purity.

This is an example of a coin I submitted not noticing it had residue left by acetone:



Such acetone residue can not be conventionally removed from a coin surface by distilled water or by isopropanol because these solvents bead, while acetone covers the coin surface completely.

For example, an acetone dip may be used in conjunction with other solvents to provide a substrate free from residue.

One would need a ventilator hood, a probe sonication device, a temperature controlled nitrogen evaporator, supply of glass beakers, metal clips to fasten the substrate, etc

Don't know if a coin could be mounted in such a way as to survive the probe sonication which provides up to 1,000 times the power of tub style ultrasound.

See technician's question concerning wafers below:
https://www.reddit.com/r/chemistry/comments/8qca7z/why_does_acetone_leave_residues

Such a procedure to provide wafers free from residue is outlined in the response below:
https://www.reddit.com/r/chemistry/comments/8qca7z/why_does_acetone_leave_residues/e0i78gi/

Best regards

 

Could it be residue from whatever organic was dissolved and then redeposited if the acetone evaporated without being thoroughly rinsed?

 

This is true.

Acetone is such a good organic solvent that pure acetone is only theoretically available in the lab where it's distilled.

It takes additional solvents to remove acetone impurities using a probe sonicator.

Tub style ultrasound is not sufficient since the only solvent that may be used in the tub is distilled water. Use of other solvents present a fire hazard. Also, differences in acoustics could burnout the acoustic generator.

Differences in surface tension among solvents make sonication necessary near the target surface.

"Acetone HPLC Grade" has the least residue at 5 ppm.

Thank you for asking.