Close up view and chemistry of the milk spots

[UraaniBarbaari]

Hello,

Spent few hours yesterday evening after work looking at the white spotting in more detail. Please have a look if you are interested - I think it´s pretty cool. I´m no chemist and don´t quite understand the process so perhaps there is someone here who can comment more in depth. There should be link to the full PowerPoint at the end of this post. The ppt is pretty quickly put together.

 

Milk_spots.pptx

[Pete]

Crickey - fantastic stuff !!

If you can expand your findings then I urge you to publish your data in an appropriate journal or coin magazine as there is so much talk about milk spots & stains on silver without any real conclusions. This would make a nice undergraduate university science project to spectroscopically analyse the oxides and then try and establish why chorine is present in the first place. To the naive ( me perhaps ) suggests the planchets are rinsed in salt water before pressing and any NaCl residue is baked onto the surface. Perhaps the water filtration / purification system in the Mint has agents like bleach in the water to ensure no bacterial growth.There is always the crazy possibility that a worker in the Mint wears contact lenses and uses eyedrops that sometimes contaminate the blanks. Great piece of work and SCIENTIFIC !!

[sovereignsteve]

Great stuff T, keep up the good work

[UraaniBarbaari]

4 hours ago, Pete said:

Crickey - fantastic stuff !!

If you can expand your findings then I urge you to publish your data in an appropriate journal or coin magazine as there is so much talk about milk spots & stains on silver without any real conclusions. This would make a nice undergraduate university science project to spectroscopically analyse the oxides and then try and establish why chorine is present in the first place. To the naive ( me perhaps ) suggests the planchets are rinsed in salt water before pressing and any NaCl residue is baked onto the surface. Perhaps the water filtration / purification system in the Mint has agents like bleach in the water to ensure no bacterial growth.There is always the crazy possibility that a worker in the Mint wears contact lenses and uses eyedrops that sometimes contaminate the blanks. Great piece of work and SCIENTIFIC !!

Thank You Pete!

It´s pretty cool what you are able to see just after few hours of research. There would be much more to see and say about this, but I´ll likely have very little time to take this much further. What I will do at least is that I´ll get a neat Perth Mint coin to have some data to compare. Also if I get a chance I will produce some "element maps" to show the distribution of different elements. If it works it should show the textures pretty nicely.

4 hours ago, sovereignsteve said:

Great stuff T, keep up the good work

lol, thanks Steve!

[augur]

Thanks @UraaniBarbaari for assessing the composition of the spots on your silver Maple Leaf ? 

NaCl is obviously omnipresent but the further presence of K and Ca could indicate the use of not deionised water to rinse the planchets. MoS2 could be used as a grease for minting equipment. Only Cd is a mystery to me and apart from a use in leather tanning I am not sure of the source of contamination. 

Possibly the MoS2 acts as a crystallisation germ when it dissociates and aids the formation of AgS which the further oxidises to AgO and AgCl. 

The replacement of MoS2 grease of minting equipment with ceramic grease might help to reduce or prevent the white spot formation?

[Roy]

Good stuff!

Is MoS2 the formula for molybdenum or moly grease...that lovely black stuff? 

[Pete]

33 minutes ago, augur said:

 Only Cd is a mystery to me and apart from a use in leather tanning I am not sure of the source of contamination. 

Cadmium ( electroplated ) was often used to protect steel from corrosion so perhaps there is a small trace of Cd in the coin press machines that is contaminating the coins. Maybe one press is worn and that's why a few coins are affected and not them all. All speculation of course.

PS - If we knew the precise chemical composition of the milk stain would that permit any of our "chemists " to advise an appropriate solvent ( checking Wiki silver chloride would dissolve in Ammonia ) that would wash the milk stains and spots away ? Silver Chloride ( AgCl ) is a white substance that does not dissolve in water and is a good contender here as the milk pox affecting our coins.

[Roy]

But wouldn't that affect the price of silver....prices would rocket overnight!

Said 'chemist' would probably be found in the desert.

Oops..wrong thread! 

 

[UraaniBarbaari]

8 minutes ago, Pete said:

Cadmium ( electroplated ) was often used to protect steel from corrosion so perhaps there is a small trace of Cd in the coin press machines that is contaminating the coins. Maybe one press is worn and that's why a few coins are affected and not them all. All speculation of course.

PS - If we knew the precise chemical composition of the milk stain would that permit any of our "chemists " to advise an appropriate solvent that would wash the milk stains and spots away ?

I saw few particles with Fe, Cr, Ni and Cu ( so stainless steel from the press) but didn´t record the assays and don´t remember if they had any Cd. I think Cd was present in small amounts only in one assay. It seems to give false peaks close to silver peaks so an artifact in those EDS images?

If the white hazing is just oxidized Ag then how would you be able to "deoxidize" it?

[augur]

5 minutes ago, Roy said:

Is MoS2 the formula for molybdenum or moly grease...that lovely black stuff? 

Yes, the measurements showed the presence of Mo (Molybdenum) and S (Sulphur) so moly grease or MoS2 could be the source. Mo can also be found in hardened steel (dye) but then we should find also other traces of Fe, Co, Cr etc. 

[Roy]

Fascinating.

As a layman, isn't it relatively easy to deoxidise? I'm thinking of rust?

[augur]

14 minutes ago, UraaniBarbaari said:

If the white hazing is just oxidized Ag then how would you be able to "deoxidize" it?

 

11 minutes ago, Roy said:

As a layman, isn't it relatively easy to deoxidise? I'm thinking of rust?

 There are many different ways to "deoxidise" or reduce silver chloride to elementary silver but: There is a structural change in the surface so that the morphology won't be the same. The area can remain dull at best or turn black at worst. Would be worth to attempt electrolytic and/or chemical reductions (e.g. glucose/NaOH) but I wouldn't get my hopes too high...

 

19 minutes ago, UraaniBarbaari said:

I saw few particles with Fe, Cr, Ni and Cu ( so stainless steel from the press) but didn´t record the assays [...]

Well under those circumstances Mo could also be from the dye steel, still the presence of S would make the MoS2 source more likely. 

[UraaniBarbaari]

39 minutes ago, augur said:

 

 There are many different ways to "deoxidise" or reduce silver chloride to elementary silver but: There is a structural change in the surface so that the morphology won't be the same. The area can remain dull at best or turn black at worst. Would be worth to attempt electrolytic and/or chemical reductions (e.g. glucose/NaOH) but I wouldn't get my hopes too high...

 

ok, interesting. Would asetone dissolve the silver chloride? 

[augur]

9 minutes ago, UraaniBarbaari said:

Would acetone dissolve the silver chloride? 

Even worse than water; equally alcohol doesn't work. Ammonia dissolves AgCl but again the surface is altered as you would dissolve a piece of the coin. 

[Roy]

Urine?! 

[tbone]

Looks like mold!  Milk spots are a huge problem with most mints imo, more than they care to believe.  This was one of several reasons I stopped collecting higher priced silver BU/proof coins.  I'll stick with gold and 90% I think

[UraaniBarbaari]

2 minutes ago, tbone said:

Looks like mold!  Milk spots are a huge problem with most mints imo, more than they care to believe.  This was one of several reasons I stopped collecting higher priced silver BU/proof coins.  I'll stick with gold and 90% I think

Absolutely, gold is better in many ways imo.

But why you rarely see spotting on Perth Mint coins? Something in their process obviously that produces better quality.

[augur]

Although I would love to see @Roy to give his silverstack a golden shower, AdBlue is a better source of urea, which can dissociate into ammonia  

Elementary silver is black. You will know this effect from photoplates where the exposure of AgCl/AgBr with UV-light leads to the reduction and formation of molecular silver. So in the worst case your white spots will turn into black spots. If it was that easy NCS would have come up with a solution.

I suppose an electrochemical option probably has the best chances to reduce the impact of white spots but prevention is better than cure. I wonder if we got closer to the problem causing the whole issue in the first place. 

[sovereignsteve]

12 minutes ago, Roy said:

Urine?! 

still ammonia

 

28 minutes ago, augur said:

Ammonia dissolves AgCl but again the surface is altered as you would dissolve a piece of the coin. 

worth trying, I have some somewhere, just need to find a milked coin now.

even if it left an area that wasn't perfect but was shiny silver rather than milk, that would be an improvement surely?

[Roy]

Love science.

@augur, all my coins would be .999 pure and 70% proof.

I could provide a COA too! 

[tbone]

1 hour ago, UraaniBarbaari said:

Absolutely, gold is better in many ways imo.

But why you rarely see spotting on Perth Mint coins? Something in their process obviously that produces better quality.

Yes, Perth mint is great.  I don't know for sure why milk spots happen, I've heard some say its the cleaning solution they use for the dies (or was it the planchets?).  Maybe some impure elements to the solution?  As you can tell, I really don't know, but would love see if anyone has more info.

[augur]

Can professional conservation from NCS remove white spots?

Numismatic Conservation Services (NCS), an independent affiliate of NGC, has conducted extensive research but has been unable to determine a conservation technique that can prevent or remove white spots without damaging the coin. In some cases white spots can be minimized, but they cannot be removed.

 

With that in mind, it would be interesting to test varying methods, in particular now that we possibly have a different mechanism in mind than the "contaminated planchet" theory.

Anybody with coins at scrap value volunteering their coins for the benefit of science and mankind?

[tbone]

PCGS will apparently try to remove them for you (not sure the fee), by dipping them .  I guess sulfuric acid solution of some degree?  Would be easier and cheaper to try it myself.  Anyway:

Quote

If you would like us to try to remove spots from your coins by "dipping" them, we may be able to do that. It is our experience that spots on proof Silver Eagles can be removed in about 80% of the cases, but spots on Mint State Silver Eagles will only come off approximately 10% to 25% of the time.

From: https://www.pcgs.com/silver-coins-spot-policy

 

[tbone]

1 hour ago, shortstack68 said:

And how do you think NGC conserve? Its all done by cleaning, only, when you do it, it's naughty, when they do it, its conservation

True, although I don't doubt they are much more experienced.  I've messed around with carefully (properly?) cleaning coins, and messed up and ruined several coins (dipping too long, too strong a solution, adding hairline scratches, etc).  I don't trust myself on any coins of real value But a milk spotted modern coin?  I would either try dipping it myself or sell/trade it (for gold)

[tbone]

14 hours ago, shortstack68 said:

If you're using silver dip, you can't leave it for any long than 10 seconds max and the coin must be washed under cold running water so the solution has been washed off

From my experiments, more like 1-2 seconds, and really only for BU or proof coins.  It will remove luster quickly!  In most cases its a bad idea, but I'm too stubborn to listen to reason