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A question for our resident chemist Fluttershy


HighlandTiger

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I'm really trying to get the definitive answer as to why RCM coins are prone to milk spotting. Searching online comes up with very varied answers. Yes we all know it is residue left over from a poorly designed cleaning process, but why do some coins look fine and spot later.

 

I would like to know the chemistry behind it. After all for a coin to look fine one minute and then milky the next, then some chemical reaction must have been initiated. I would like to know what initiated that reaction. Some people on the internet believe that it "just happens" and some that it is due to exposure to air, most probably the moisture in it, that kick starts it off.

 

As a chemist, what is your take on it?

 

 

 

 

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Another puzzling addition is that not all RCM coins have or develop milk spots.

Luckily most of mine are clean.

It is puzzling why some good coins develop marks later.

I have to assume the Mint has multiple pressing stations and not just one so perhaps one or more stations are prone to producing spotted coins and others not. I read the problem relates to poor rinsing of a borax wash. Maybe this is not in the final stamping but in the manufacture of the blanks ? If some blanks are contaminated then the final pressing that generates some heat might bake the borax or whatever substance onto the silver ?

Fluttershy is hopefully able to comment more scientifically but it clearly remains a known issue at the RCM which does not appear to be readily solved.

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I'm probably totally wrong here but I've often wondered if it's something to do with Canadian mint coins being 9999 fine, I know stainless steel has to be a certain percentage carbon to not corrode or become brittle, just a thought really, nature doesn't seem to like anything too perfect.

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As for why some coins do and some don't I can offer a logical explanation.

 

If it is true (And I have read it widely, so tend to believe it) that is is residue left over from the cleaning process, (of the mint equipment) then one would assume that those coins minted early on after the cleaning process are more likely to have contamination and therefore spot. Those minted at then end of a run will probably not be contaminated and therefore not spot. This is only a logical deduction, but makes sense to me!

 

Whether or not that contamination requires a catalyst to initiate or feed the reaction that causes the spots (like oxygen from the air for example) is a very good question. I dare say that not knowing the nature of the chemical containment would make this a difficult question to answer.

 

Will sealing them of from the air help? Honestly, I don't know. But they will have been exposed to air when the tube was opened, and when they were transferred into whatever sealed package is used. Also there will also be a tiny amount of air in contact with the coin unless you vacuum seal it, so it may not help. Experimentation and time are the only ways to know.

 

From previous reading, I would expect a coin to spot regardless of how well kept it has been, because the problem starts at the mint. The coins are exposed to air for a long time after minting before they are ever sealed by the final recipient.

 

Excited to hear what Fluttershy has to say from a scientific point of view! :)

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Wow, what a build up, and I`m sorry to disappoint but I also have been searching for an answer to something very similar, and as yet am lead increasingly to the conclusion it`s Not a chemical reaction at all, but rather a Metalurgical/Physics issue.

 

I`v been working with Fine (999 and also 9999) silver, and am curious as to Why when annealed, the metal goes from shiny silver to a pure matte White color just like milk spots!

there are no White salts of silver that can be made with the parameters given, considering propane air, the Oxides, Hydroxides, Carbonates and even nitrides were considered, non are white!

so Im fairly confident that it`s not a chemical thing.

 

now taking into account Borax or Boric acid/alcohol fluxes and protective coatings, I use these also, and i`ts very likely that some could well get trapped (stamped into) the round, or even be present in the ingot that the rounds are cut from before being stamped, I have also moticed this when rolling out sheets from ingots that I`v cast, and there`s all sorts of problems that can happen with Air inclusion and Borax flux inclusion that isn`t apparent until the metal becomes thin enough, this can be on the first pass or any number after that to not at all!

heat treating the metal after each few passes will even blow out bubbles into the sheet, so we know from this that metal Can include flaws that won`t show until later when something chages.

 

Silver can Age harden as well as Work harden.

an example of age hardening of Alu: 

 

"Whilst working in military research centre in Neubabelsberg in 1901, Wilm discovered age hardening, in particular age hardening of aluminium alloys.[2] This discovery was made after hardness measurements on Al-Cu alloy specimens were serendipitously found to increase in hardness at room temperature. This increase in hardness was identified after his measurements were interrupted by a weekend, and when they were resumed on the Monday the hardness had increased.[3]"

 

Silver is very often put into a tumbler to work harden before it can be worn or used, and here`s the Really interesting thing, you can also work harden silver in an ultrasonic cleaner, I found this out by accident too.

so it`s my opinion that simply Age and/or work hardening through vibration and small temp changes would be enough over time to change the crystaline structure of the metal enough to expose a flaw that was just under or in the surface.

these Little things all make tiny little chages to the metal when they happen, and even though silver is a soft metal it Does harden and quite easily too, tapping on a desk can start it off, you don`t have to dent or even distort the metal to make it change hardness (crystal structure).

 

the Other possibility I can think off, is that Borax flux when heated is perfectly clear like glass and so wouldn`t show on the silver surface of a coin if it`s thin enough.

Pickle is used to clean this off in everyday manipulation of silver, this wouldn`t happen on a sealed coin. this clear borax glasslike or not, Could in theory regain some of its water of crystalisation and turn opaque again.

Pickle (acid) gets rid of it very quickly, but apparently if Can loosen up a bit if left in water for a Very Long time, some of the borax beads that happen when casting silver have started to change color from when i first made them 2 months ago and just left them outside, and theyr`e not as shiny glass like either, so maybe that`s another mechanism?

 

so in short I don`t have a definitive answer to this either, only a few observations made personaly and accompanying hypothesis.

 

I never thought i`d complain about having all 10 maples I recently bought being in perfect condidtion, I could have done with having at least one with these milk thingies on it.

 

or Maybe!!! just Maybe! that`s why it`s called the "Spot" price!? :D

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I found this:
 

 

And then

 

 

 

 

Don't know how much of that is true but it certainly is very interesting.

 

 

Fluttershy - great response! :)

 

Interesting what you say about silver work hardening. I wonder if they anneal the coin between cutting the blank from the rolled out ingot and minting it? I would expect the rolling of the ingot and the cutting of the blank to create some hardness at least? If so they would have to be cleaned before minting, I presume, could this be where the containments are introduced? Although I must admit that I had always believed that it was from the cleaning of the dies, which is why not all coins get effected - late run coins the dies will have lost all their containments!

 

Who knows!? But speculation is fun! :)

 

Sorry - edited to add a Source for those quotes!

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Found this, which I now remember I read a while back, and that is where I got the impression it was the dies - I just rembered that last detail a bit wrong.

 

The planchet is the blank disk the coins are minted from.

 

I it seems they probably do anneal them before striking. I guess we have our answer! :)

 

It is chemicals left on the planchet after the annealing process.

 

 


The following is an email letter from a RCM distributor concerning the problem. To summarize, the RCM's position on "milk spots" on their silver coins is : "Don't like it? Buy something else."

Hello,

Here is the short explanation. We just had a meeting with the Mint about this 10 days ago.

The white stains (or "milk spots") result from the planchet (flan) cleaning and preparation process. Some Silver Maple Leaf coins have them (SMLs), some do not.

This is the Mint's official position: The coins are bullion coins. They are not collector coins. They are sold as one ounce of silver. The Mint knows that there is a problem. The problem has existed since 1988, when the SML coin was first introduced. The Mint says that there is nothing that they can do about the problem.

Our experience is that some SMLs have them and some do not. We do not know what we are going to get when we open the boxes of 500 ounces of silver that come from the Mint. We have to take what we get; we can not return them. We do not have the time to sort them, etc.

We ship out what we get. Per the terms of our invoices, we do charge a restocking fee for returned bullion items.

I am sorry about the problem, but we have no control over it.

I hope that this helps,

John

John Winkelmann
ANA Life Member

Talisman Coins
Official Distributor for the Royal Canadian Mint,
for the Mint of Poland (in Warsaw),
for the Perth Mint of Australia,
and for Monnaie de Paris (The Paris Mint)

9051 Watson Road, Suite 103
St. Louis, MO 63126
UNITED STATES OF AMERICA

Source

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Wow, what a build up, and I`m sorry to disappoint but I also have been searching for an answer to something very similar, and as yet am lead increasingly to the conclusion it`s Not a chemical reaction at all, but rather a Metalurgical/Physics issue.

 

I`v been working with Fine (999 and also 9999) silver, and am curious as to Why when annealed, the metal goes from shiny silver to a pure matte White color just like milk spots!

there are no White salts of silver that can be made with the parameters given, considering propane air, the Oxides, Hydroxides, Carbonates and even nitrides were considered, non are white!

so Im fairly confident that it`s not a chemical thing.

 

now taking into account Borax or Boric acid/alcohol fluxes and protective coatings, I use these also, and i`ts very likely that some could well get trapped (stamped into) the round, or even be present in the ingot that the rounds are cut from before being stamped, I have also moticed this when rolling out sheets from ingots that I`v cast, and there`s all sorts of problems that can happen with Air inclusion and Borax flux inclusion that isn`t apparent until the metal becomes thin enough, this can be on the first pass or any number after that to not at all!

heat treating the metal after each few passes will even blow out bubbles into the sheet, so we know from this that metal Can include flaws that won`t show until later when something chages.

 

Silver can Age harden as well as Work harden.

an example of age hardening of Alu: 

 

"Whilst working in military research centre in Neubabelsberg in 1901, Wilm discovered age hardening, in particular age hardening of aluminium alloys.[2] This discovery was made after hardness measurements on Al-Cu alloy specimens were serendipitously found to increase in hardness at room temperature. This increase in hardness was identified after his measurements were interrupted by a weekend, and when they were resumed on the Monday the hardness had increased.[3]"

 

Silver is very often put into a tumbler to work harden before it can be worn or used, and here`s the Really interesting thing, you can also work harden silver in an ultrasonic cleaner, I found this out by accident too.

so it`s my opinion that simply Age and/or work hardening through vibration and small temp changes would be enough over time to change the crystaline structure of the metal enough to expose a flaw that was just under or in the surface.

these Little things all make tiny little chages to the metal when they happen, and even though silver is a soft metal it Does harden and quite easily too, tapping on a desk can start it off, you don`t have to dent or even distort the metal to make it change hardness (crystal structure).

 

the Other possibility I can think off, is that Borax flux when heated is perfectly clear like glass and so wouldn`t show on the silver surface of a coin if it`s thin enough.

Pickle is used to clean this off in everyday manipulation of silver, this wouldn`t happen on a sealed coin. this clear borax glasslike or not, Could in theory regain some of its water of crystalisation and turn opaque again.

Pickle (acid) gets rid of it very quickly, but apparently if Can loosen up a bit if left in water for a Very Long time, some of the borax beads that happen when casting silver have started to change color from when i first made them 2 months ago and just left them outside, and theyr`e not as shiny glass like either, so maybe that`s another mechanism?

 

so in short I don`t have a definitive answer to this either, only a few observations made personaly and accompanying hypothesis.

 

I never thought i`d complain about having all 10 maples I recently bought being in perfect condidtion, I could have done with having at least one with these milk thingies on it.

 

or Maybe!!! just Maybe! that`s why it`s called the "Spot" price!? :D

 

Great answer Flutters, thanks for that. (though I'm not sure my brain with its B grade Chemistry O Level from 30 years ago, managed to follow it all :D )

 

But we are still no nearer to either finding the answer to why some spot and some don't or if it is possible to prevent or delay coins from spotting in the future. I guess the search for an answer continues.   

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