Rust on my .999 Gold Bitcoin penny ?

[Xavier59]

Hello,

In 2019 I bought from u/chronicsky a .999 gold bitcoin penny.
Recently I was checking my collection and I noticed a lot of rust on the penny

See below:



A few notes:
- The coin had no rust when I received it
- I kept it in a clean capsule and it was not in contact with other coins
- It was sometimes exposed to humidity
- I have searched and not found any discussion around fake bitcoin penny, u/chronicsky has good rating and is still active on the forum

I don't understand why it caught rust. As I understand, .999 gold coin should never catch any rust because gold doesn't get rusty?

1/ Why do you think this happened? If anyone has a gold bitcoinpenny and gold tester, it would be awesome to try it and post your result
2/ Do you have any idea how to clean it? I have tried to gently scratch with my finger but it didn't do anything and as it's pure gold I know it's very maleable so I'm afraid to damage it.

Thanks

[MoparMiningLLC]

My gold bitcoin pennies look the same

[transvestite lamb]

Not sure what the spots are however I can say I have tested many BCP gold and silver coins with my Sigma Metalyics PM Verifier and they have all returned 999.

[YodasRedRocket]

Generally referred to as copper spots. A quick google search will tell you all you need to know. The small percentage of non-gold alloy in the mix.  You can remove the spots with a torch but they will inevitably return. You can slap some silica packs in your coin storage to try to prevent this from happening but the environment starts attacking coins from the second they’re minted.

[Xavier59]

Generally referred to as copper spots. A quick google search will tell you all you need to know. The small percentage of non-gold alloy in the mix.  You can remove the spots with a torch but they will inevitably return. You can slap some silica packs in your coin storage to try to prevent this from happening but the environment starts attacking coins from the second they’re minted.

Thanks. According to the internet, it only develop on .999 coins if there are impurity during the minting process. Which was likely the case here, I don't think BCP run a professional mint 

I didn't have a blow torch so I just went with baking soda instead (I followed this video: https://www.youtube.com/watch?v=NzUcUeWJztI).

This method probably looks evil to the expert collectors but for me it's good enough, I just want a good looking shiny gold coin 

There is almost no copper left just a minute after putting it in the hot wallet.

[YodasRedRocket]

Not just .999 bullion coins, 90% constitutional gold will obviously have copper spots as well. And every maker can have these issues even the US Mint.

[OgNasty]

Generally referred to as copper spots. A quick google search will tell you all you need to know. The small percentage of non-gold alloy in the mix.  You can remove the spots with a torch but they will inevitably return. You can slap some silica packs in your coin storage to try to prevent this from happening but the environment starts attacking coins from the second they’re minted.

Keeping your coins in pristine condition is difficult as you are right that the environment starts attacking them the second they're minted.  You really need the care to start right then.  A large factor in whether your coins quickly develop this sort of spotting is if you handle them with your bare hands.  Your hands contain oils and dirt that accelerate the process greatly.  This is why I often talk about the care that I put into making sure my coins never touch human hands.  If you keep them in their capsules and store them well, they should continue to look great for a long time.  I would recommend that others avoid touching their coins if they want them to look their best over time.  The coins in my collection are all still flawless looking.

Don't take my word for it though.  Here is Grok's take:


Handling gold coins with bare hands accelerates the development of spots (often called "spots," "tarnish," or "finger spots") on high-purity gold coins (especially .999 or .9999 fine bullion coins like Canadian Maple Leafs, Austrian Philharmonics, or modern Krugerrands).
Here’s why:

Skin oils and acids
Your fingers leave behind natural skin oils (sebum), sweat (which contains salts, lactic acid, urea, etc.), and microscopic dirt. These residues are slightly acidic or corrosive over time.

Pure gold doesn’t tarnish, but trace impurities do
Modern bullion coins are 99.9–99.99% gold, with the remaining 0.01–0.1% being trace amounts of copper, silver, or other metals (even if not intentionally alloyed).
When skin oils sit on the surface, they can react with these trace metals or with atmospheric sulfur compounds, forming tiny dark sulfide spots (usually copper sulfide or silver sulfide). These appear as brown, black, or rainbow-colored spots.

Real-world evidence  
Coins that are frequently handled (e.g., passed around at coin shows without gloves) develop spots much faster than coins kept in capsules or handled only with cotton gloves.  
The most notorious examples are Canadian Silver Maple Leafs (which are .9999 silver, not gold) that get “milk spots,” but the same principle applies to trace copper in gold coins. Gold bullion from the Perth Mint, Royal Canadian Mint, and others often shows fingerprint-related spotting within months or years if handled bare.

How fast does it happen?
It can take weeks to years depending on:  
How oily/acidic your skin is  
Humidity and temperature  
How much sulfur or pollutants are in the air  
Whether the residue is wiped off immediately or left sitting

Best practices to avoid spots:
Handle high-purity gold coins only by the edges or with clean cotton/nitrile gloves.  
Store them in inert capsules (Airtite, Quadrum, etc.) or original mint tubes/blister packs.  
If you do touch the surface, some collectors gently rinse with distilled water + a drop of mild dish soap and pat dry (controversial—some say never clean coins), but prevention is far better.