Definition of "Skin"

[JRocco]

Hi Mike.

There is heat generated in the coining process, but no where near enough heat to melt any metal. Kind of like when you rub your hands together you generate heat, but you never put your hands in danger of melting or changing shape due to melting.

Here is a quote from Alan Herbert in his book "Minting Varieties and Errors":

 

"THE STRIKING

Once the planchets have been prepared and the dies made, it is time to strike the coin. This is done with a machine called a coin press. It is basically a heavy duty press which can apply the required tonnage to a moving die to cold form the planchet into a coin.

Cold forming means to force the planchet metal to flow into the design cavities in the die and against the collar to form the design; this is done strictly through the pressure applied and not by heating or melting the metal to get it to move. This ability to flow or move under pressure is one of the characteristics of copper that makes is such a good coinage metal."

[MikeInFL]

If you can show me a definition of molten that does not include the application of heat, I'll change my tune.

 

As for the stamping versus cold forming/forging. Let's just agree to disagree. IMHO, the only stamping process in coining is in the manufacture of the planchet from rolled metal. The "striking up" of the coin, while techincally a type of stamping, is more accurately described a form of cold forging.

 

Regardless of what you call it, in no case is the metal ever molten, as all of these processes cause metal flow by plastic deformation rather than heat. Again, heat is a byproduct and not the reason for the metal flow, and I don't care what the ill-educated mint officials call it.

 

I see your point, and hate to add fuel to the fire (no pun intended), and I am not a metallurgist or physicist, so my apologies if I confuse my ignorance with knowlege, but in 'cold forging', when the die strikes the metal, wouldn't the energy of the strike translate into heat which mobilizes (melts) the metal such that it becomes platicized and can form fit into the recesses of the working die? And would it be the wearing of the working die that causes flow lines as the mobilized metal (liquification) traverses the surfaces of the worn die.

 

The heat being a byproduct of the strike that by the law of the conservation of energy would 'plasticize' or melt the metal such that it can flow? And that once having molded into the recesses of the working die, enough energy is absorbed by the die steel that the metal of the planchet has cooled enough to harden it once again. I am not saying that there is enough heat generated by the strike to melt the whole planchet, rather, that there is just enough localized heat generated to cause metal flow.

 

edited to add:

 

I'm also thinking, if the planchet metal wasn't meltable enough by the heat generated by the strike, wouldn't it then crack? Such as, the die steel being so hard, that it is not so affected by the heat generated, and therefore, subject to cracking as a result of the stress of repeated strikes (i.e., the energy of the strike couldn't translate into softening of the die, but the energy had to go somewhere and went beyond the tensile stregnth of the die steel causing it to crack)?

 

Mike,

 

Nice thought process, but I believe you are misguided.

 

First off your application of the conservation of energy principal would work for an ideal gas (you may recall the ideal gas law; PV=nRT), but solids don't work that way.

 

What I believe you are overlooking is that coinage metals below their melting point are malleable (i.e. bendable or formable or plastic) -- this is the characteristic that allows them to be struck up by cold forging. While metals become more malleable if heated, that's not really what's going on when a coin is struck during the minting process because the pressures used don't heat the metal up enough for this to happen. What's happening with coinage is just like if you pressed a coin into a piece of chocolate -- the force moves the chocolate through plastic deformation into an impression of a coin, and not becuase of any heating and subsequent resulting flow.

 

Using the same conservation of energy principal (along with a bit of thermodynamics), had the surface of the coin really melted, all that heat would still be present in the coin after striking. Remember that the metals used for coinage all melt above 1600 degrees farenheit and they are superb conductors of heat, so if they truly melted during the striking even only on the surface, they would still be red-hot after striking, and I do not believe this is the case. While I've never held a just-struck coin, I have held a just-struck rivet formed by cold forging and while they were warm, they were nowhere near hot enough to melt the metal (aluminum in this case).

 

Furthermore, had the dies "absorbed" all this heat, they too would melt and deform. Again, this is not the case as they crack (and deform) from the force/pressure rather than deform from melting/heating.

 

While I do not consider myself an expert by any means, I base my statements from my college education in physics, chemistry, thermodynamics, and material sciences and my on-the-job experience with cold forging of fasteners used in the aviation industry...Mike

 

p.s. here are two pieces of gold with nice thick skins:

 

 

 

[JJSingleton]

I'll leave the science to the experts and just take a page from Justice Potter Stewart and say:

 

"I shall not today attempt further to define the kinds of material … but I know it when I see it".

 

The easiest way to quantify the skin is to take a coin and dip it. What you no longer see on the coin was the skin. But doing so would upset me greatly.

[TomB]

The easiest way to quantify the skin is to take a coin and dip it. What you no longer see on the coin was the skin. But doing so would upset me greatly.

 

To illustrate FatMan's point with an actual coin, below is an oxidized, original skinned Maine commem that I decided to dip (for illustration purposes on this thread) and the resulting white finish with incomplete oxidation removal is imaged below it. Too bad I dipped it, I think it was more attractive before.

[MunkyMan95]

You dipped a great coin for illustration purposes? AAH!

 

[JJSingleton]

OMG...Say it ain't so!

 

I'm really hoping you're pulling our legs here. I don't see the reverse die polish on the toned coin so I'm hoping they are different coins.

[PerryHall]

They aren't the same coin. The first one has a small diagonal cut below the 9 of the date which the second one doesn't have. I think he's pulling our leg.

[MrSpud-migration]

[MikeKing]

They aren't the same coin. The first one has a small diagonal cut below the 9 of the date which the second one doesn't have. I think he's pulling our leg.

 

Tomb...you're a LIAR LIAR!!!

 

 

 

MikeinFl, thanks for clearing things up for me. What I would like to know is what kind of chocolate were you using? Hersheys' chocolate? Godiva? or Lindt?

[TomB]

I imagined that everyone would know by now that I would never dip a coin like that, but with all the strange goings on in the internet coin world I guess folks just aren't certain about anything these days...

 

The top coin is mine and it is a very nice Maine that is currently graded PCGS MS64, though I believe the coin would just as easily be at home in an MS65 holder. The bottom coin is not mine and I have never seen it in-hand, the image was simply lifted from Heritage and then cropped and resized to match the image of my coin. The white coin has been dipped in the past, I believe, and is currently in a holder graded PCGS MS66. I would much rather own my coin, and at prevailing market prices, than the dipped piece that is graded two points higher.

[JJSingleton]

TomB, thanks for the confession. But it's too late. I just dipped mine to see if I would get the same results. My Maine apparently had more skin than I thought. I simply cannot believe what happened.

 

Before:

 

 

 

After:

 

[MunkyMan95]

[Amanda]

 

-Amanda

[leeg]

I love how this has turned into a Minting/Coining Process and Maine Commem picture opportunity! Here is my Maine Commem. Ok class, hows the "skin" look on this Maine?

 

 

 

[MikeKing]

TomB, thanks for the confession. But it's too late. I just dipped mine to see if I would get the same results. My Maine apparently had more skin than I thought. I simply cannot believe what happened.

 

Before:

 

 

 

After: