Need info on mint's annealing furnaces

Interesting. Did it harm the coin? And when you say "ammonia," what exactly did you use on it?

 

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IIRC I used the common ordinary 28% Mallinckrodt material. I used it both right out of the bottle and diluted with water. Tried it at room temp and with heat. No removal of the silver chloride residue and no apparent effect on the coin from what I could tell. I also tried using dilute sulfuric acid with no luck. Adding hydrogen peroxide to the acid solution was ineffective. Made up some Caro acid aka piranha solution from the H2O2 and conc sulfuric acid. Diluted that with water and still no luck taking the milk spots off.

 

I do not believe that those conditions are sufficient to produce ammonia. Look up Haber process. Usually significant pressure is required.

 

It would appear from the articles that this is not true then. The only place that chemicals are introduced is in the burnishing step and the planchets don't appear to get washed after that.

 

From http://www.coinnews.net/2013/09/20/how-the-philadelphia-mint-makes-coins-for-circulation/

First, There is the quenching bath after annealing. "Quench tanks are filled with water and a chemical "slippery water" that helps transport blanks through a whirlaway and into a washer and dryer"


and then the mentioned washer "The washer dryer machines at the Philadelphia Mint, with one of them shown below, clean the blanks to a shine and then dry them. Chemicals are also applied at the washing step for brightness and anti-tarnishing."

Notice that the term chemical is plural for the washing step.

 

That's a little different than the process described for the silver blanks at the SF mint. The implication is that a quenching bath is not used. Of course they could use both a quench and no quench method at West Point.

 

That seems somewhat dated and not sure why it would be necessary here. As a practicing chemist, I used to make Copper(II)Hydroxide from Cupric Sulfate and sodium hydroxide. It was very slow to filter and equally slow to settle out. Adding a little bit of a flocculant called Separan made the water "slippery" and the Cu(OH)2 settled out much faster. It was essential since the copper compound had to be made free of sulfate ion. Not sure where slippery water fits into the planchet/blank preparation process.

 

I think the idea might be that since the "slipperiness" additive effectively decreases the viscosity of the water, they can pump more efficiently or at higher pressures than the equipment might otherwise handle.

 

At any rate, we don't know that the mint uses the same process of planchet/blank preparation for all of the various silver eagles.

 

I think I said that.

 

Indeed you did.

 

Looks like I spoke too soon. There is a manual washing step as well as a drying step, but not a lot of detail is given about them.

 

http://forums.silverstackers.com/message-755770.html

The electron microscope images provided there are really intriguing. That's the first time I've seen milk spot at that resolution. Nice.

A lot of talk in that forum towards the end about "heavy metals" in the water supply being the culprit.

 

That should be easy enough to prove or disprove.