Exclusive to silverstackers electron microscope images of white spot

Following on from this topic http://forums.silverstackers.com/topic-59134-milkspotting-on-perth-mint-2015-kilo-kookaburra.html I've been having a chat with our Chemist/Metallurgist about white spot and he kindly provided the scanning electron microscopy (SEM) images of white spot below (with my non-techy description).

Brainy white spot


Blob white spot


Moon crater white spot


These images show a small selection of white spots we have seen over the years. Note that these are images of the "inclusions" at the centre of the spots, not the actual spot themselves (which is just AgCl crystals as seen surrounding the defect in image 2).

We know the white spot/milky blotch stuff is AgCl, but what makes elimination of them from our process difficult is that there can be multiple causes of the spots as you can see from the images, they are not all the same (although visually they all look like a white spot to the human eye). Note that it is a piece of debris/inclusion that causes the formation of AgCl, which results in white spot. In some cases the debris/inclusion is in the metal of the coin, in others the debris is raised above the coin surface, indicating that it had settled after striking.

Debris can come from such things as poor water quality, cast in crucible material or struck in debris and you get different types of white spot depending on the source of the problem. It is highly likely the recent "outbreak" isn't the same as the defects appearing on older coins. We are certainly ensuring our air and water filtration is up to standard and running as clean a factory as possible.

Regarding suggestions on how to remove the white spot from coins, our Chemist does not recommend this as the formation of the white spot causes the silver in the coin to become consumed resulting in a recessed area, which is apparently obvious in the first image (although I just looks like some brainy looking virus on the coin to me). As a result attempting to remove the white spot could result in making the finish of the coin worse.

Of course I don't expect this will dissuade cleaning experiments and I'm sure all silverstackers look forward to the results of the proposed cleaning methods. You might just want to run them on coins of varying age to ensure your method work on all different types of white spot.

 

http://www.saltlakemetals.com/Solubility_Of_Silver_Chloride.htm

 

Has the refining process changed over the last few years? it could be some residue in the small percentage of non .999 metal & be all through the coins but only visible on the outside.

Could it be flouride or something else added to the water in recent years ?I know it sounds stupid but it seems to grow like rust & even a microscopic piece if debri can start the process.

It seems like only a recent problem something must have changed . It couldnt just mysteriously appear after centuries of no spots & not have a recent cause .

My thoughts are there needs to be a GOOD look at any process changes made in the years when it started to happen imo for example if you have had correspondence with the Canadian mint over the years & adopted something from them that seemed to be a good idea to save money or time .They seem to be the milk spot kings .

Dig deep enough with a bit of commonsense & im sure the cause will be found .I think it may be a case of overlooking the obvious .

Even the smallest change needs to be taken into consideration like supplier or product changes for all products & contacting other non milk spot mints & asking them for the products they use (if they would give you the info is another story )

A thorough methodical investigation from start to finish will get to the bottom of it im sure

 

Flouride in the city tap water..sodium silicofluoride this sounds the most likely to me .

Seems like PM are actually using water filtration hence minimal problems with milk.. RCM and others may not be so diligent.

Good theory anyway .. Na2SiF6 is the sodium salt of fluorosilicic acid. It is a powder or very fine crystal that is easier to ship than fluorosilicic acid

appreciate the images and information, great stuff

 

First off, thanks for all the effort being made to ID the issue, and find solutions to the problem. I must say that I'm concerned about purchasing coins at a high premium over spot, because if and when white spots develop, Bron, you and I both know, the premium is out the window when one goes to sell. Sorry to say, that the beautiful and pristine Perth Mint coin is no different than a maple or an eagle at that point. Sure it is a lower mintage coin, but so what? It's not like there are only a 500 or a 1000 of these minted. When someone in the market wants to purchase these, they want spot free problems...and will pay a premium. Unfortuantely, I'm most likely finished buying Perth Mint product until I know for certain that this problem is correctly solved. I will keep an eye on my 'pristine' horse kilos. They arrived in perfect condition. God help me if they develop spots. I just may be forever done with the PM, sadly...and just go back to stacking eagles, at least I know what I'm paying for, and at a much lower premium. Thanks again.

 

What Zoom % is this electron Microscope? Very interesting pictures and thoeries, thanks for sharing them.

 

Thanks for the detailed explanation! I have more questions to ask:

1. If milk spots are contamination settling on/in the coin, why would the milk spots grow bigger and wider?
2. What are the contributing factors that expedite the milk spot process? Heat? Moisture? Anyway we can prevent milk spots from contaminating the whole coin?
3. I have never seen any .925 or Britannia silver having spots, not even .925 jewelry. Silver Chloride doesn't form on non-.999 fine silver?

 

Wow Thanks for the horrifying pictures! Hopefully one step closer to figuring this out. That is really great the Perth mint is trying to come up with a solution to this problem instead of just saying there is nothing they can do , hopefully other mints will follow their lead.
I am still confused what AGCL is and how it forms, from chlorine? i agree that it is a good possibility that there is something introduced in the newer process causing this as I cant say I remember spots on any of the older silver lunar coins (and I have seen thousands). More chlorine in the water these days? or of a different type ?? Keep up the great work on this Perth Mint and Thanks for sharing the info and pictures Bron!

 

Just stating the obvious, but going back to when the issue first started popping up will certainly help any mint get that much closer to a resolution.

In Perth's case, I've seen at least 2 examples on 2010 coins -- possibly minted in 2009?

Not sure if it goes back earlier.

But if not, what happened differently in 2009? Looking beyond the Perth Mint's walls might also be a good idea.


Are there earlier examples?

 

Why am I keep hearing Tap water been used? In any laboratory practises, Isn't DI water supposed to be use?
Also the word "debris" in poor water quality, Shouldn't all water used are DI water??? or even filtered water.
Also the water filtration need to change as regular as possible.



1for1

Bron

yrh0413

Yes, it can grow bigger, some sort of crystal, given the right conditions and temperatures, this thing can keep growing.

 

There is a world of difference between a laboratory and a factory production line. Labs also have water taps for general use, distilled water is used for analytical work.

 

I have quite a few BU Britannia coins from 2011-2013 have milky spots, pretty bad, comparable to those Canadian Wild life coins.

 

I concur and would ask these same questions.



.

 

Would a sharp blast of filtered compressed air after striking clean off any settling debris?

 

All I know, the first mint that can guarantee against white spots (if possible), and has a decent low Mintage bullion coin, will earn my business for life...and Thems are not trite words...again, not sure if that is even possible.

 

Thanks for the feedback and questions, I'll pass them on but there is a limit to the detail we are willing to get into as that will tip off our competitors as to how to fix the problem.

 

Isn't it this sort of events that make people realize that higher premium coins aren't a good investment?? Pretty to look at but price still comes back to what spot is...

 

As someone who has looked at quite a few Scanning EM pictures those deposits look like they are on the surface not etched into it. As far as the inclusions ? chicken or the egg which came first.
I presume Bron that you know its silver chloride from X-ray Spectrometry which will give you a chlorine peak and I presume a silver peak - hard to assess the silver on a silver coin? I presume the inclusions will also have elemental peaks which will be some clue as to what they are. To date the only methods that have effectively removed these are mechanical and these will damage the underlying coin surface.
So clearly the challenge exists on how to chemically remove these deposits and preserve the coin.

 

The first picture looks kind of like algae. Has the Perth Mint tested the water, and where it comes from?

 

I have absolutely zero techo background in this stuff, just acting as a scribe for our experts in the factory. I'm sure there is a lot more our guys know about the other elements that are in these inclusions and that drives their investigations.