https://nl.numista.com/235350 spots

Hi all!

I noticed some little spots on this coin, and on other coins in this series.

What are these, and is there a way to remove them? Since these should be 24ct gold, I think rust isn't supposed to happen??

Have a nice weekend!

Tony.c

That's an interesting observation I'd have missed that!

10 Dollars - Elizabeth II (Taj Mahal) - Solomon Islands – Numista

I just googled some for sale on ebay that says they are golden and not gold and can see others with the same description plus states they are magnetic so plated. But there were some done in pure solid gold with certificates, plus gold over laid on silver, pure silver and also the colour coins

7 COINS NEW 7 WONDERS OF THE WORLD,GOLDEN NOT GOLD,CHINA GREAT WALL-CHINA 2007 U | eBay UK

More than one variety created. Some listings I can see in other places say there were only 7000 minted in pure gold. Perhaps the person listing thought theirs was gold but it's not if that is rust.

That ebay link is for the Chinese medals which are plated steel. Did you find any of Solomon Islands coins that were anything other than solid gold. At 1 gram I'd doubt they would have made plated ones.

My bad, saw the silver ones presumed these that came up in the same listing were the same 

DUH! 

1. 99.9% still leaves a lot of room for impurities.  If one of those rises to the surface then gets oxidized, you get a spot.  You can do the math and prove it to yourself.  A coin the size of a U.S. dime (19 mm) has a surface area of 283 sq mm.  So if 0.1% of that is impurities, you get an impurity area of 0.3 sq mm.  That could accomodate 8 impurities that are each 0.2 mm in diameter (i.e. visible to the unaided eye).  Copper impurities could give a brown to black color.

2.  Inadequate “hygiene” through all steps of the minting process can also leave surface impurities (residues from cleaning the blanks, airborne particles, etc.)

tdziemia

1. 99.9% still leaves a lot of room for impurities.  If one of those rises to the surface then gets oxidized, you get a spot.  You can do the math and prove it to yourself.  A coin the size of a U.S. dime (19 mm) has a surface area of 283 sq mm.  So if 0.1% of that is impurities, you get an impurity area of 0.3 sq mm.  That could accomodate 8 impurities that are each 0.2 mm in diameter (i.e. visible to the unaided eye).  Copper impurities could give a brown to black color.

2.  Inadequate “hygiene” through all steps of the minting process can also leave surface impurities (residues from cleaning the blanks, airborne particles, etc.)

Thank you. But then:  older gold circulation coins in .900 don't have those spots. For these modern coins one would expect them being made from modern gold bars, so there would be less impurities?

My point was only to show that 0.999 leaves room for visible impurities.  

I don't have a good answer to your point except that I supose there can also be differences in the type of impurities (i.e. lrimarily silver vs. curous/cupric), etc. 

I did remember reading a paper on the source of these spots in modern coin production, but couldn't find it. Will keep looking