Definition of "Skin"

[leeg]

I've seen several folks use this term as far as coins go. I've never seen, or found, a definition for it. My thoughts are that it's an "as minted" condition. Unmolested and undipped. I also would think it would need to be a highly graded example to have any "skin" left? Am I close? What are some other thoughts on this term. Thanks in advance. To me this PR Kennedy Half seems to have quite a bit of "skin" left:

 

 

 

 

 

Heres a post from ATS that has some good discussion about this in it:

 

SKIN

[Denver Dave]

 

To me Original Skin and Luster are one in the same. When a coin is stamped the pressure is so great the surface of the metal turns molten and fills the impressions of the die, leaving a fine luster that can not be brought back after it is lost...

 

A coin can have many bag marks and still have it's original skin, so the grade range can be lower in some cases...

[MikeKing]

 

a dipped coin that's aged for years and acquired a patina may be considered as having a (new) skin, but if the new skin is old enough, who would know?

[karpman9-migration]

I have always considered skin to be a thin layer of toning or patina that forms over time on gold, silver or copper coins. I often think of skin on classic coins as opposed to modern coins.

 

As an example, I like collecting mint state Saints and other gold type, and in doing so, I always look for Saints with original skin, no dip. The look between a MS65-66 Saint having original skin vs a MS65-66 Saint where the original skin has been dipped off is worlds apart.

 

Obviously the same applys to crusty old bust halves, Morgans or any other classic type. I'm sure some collectors of modern coinage will make the argument that their coins all have original skin too, and they would be correct if the coin has not been dipped. However the context in which *I* use the term skin is almost always in regard to coins minted pre-1965.

 

Best regards,

[Conder101]

When a coin is stamped the pressure is so great the surface of the metal turns molten and fills the impressions of the die

The surface of the metal does NOT melt, it cold flows. If it actually melted there would be no luster. The luster is formed by the microscopic scratches created on the surface of the metal as it is forced across the surface of the die. If the surface actually melted like a liquid it would simply flow to the new position and re-solidify. No scratches would form and no luster. It would be like the surface of a casting.

[JRocco]

To me--this is skin - I will throw this in a new post and ask opinions.

[MunkyMan95]

To me this is skin

 

[Amanda]

I think there is a difference between 'crusty' and 'skin'.

 

-Amanda

[WoodenJefferson]

Looks like the debate goes on...good topic since I don't think there is a definitive answer to this question, it will always be a compilation of ideas perceived from others and experience.

 

My thoughts:

 

Pertains to a coin that has not had the luster broken on high spots due to circulation, sliding back and forth in a coin case drawer, surface interruptions from clear plastic album sliders, dipped or cleaned to any extent...in other words, as is from the mint presses with or without bag marks, nicks, ticks but with no abrasion, mishandling or storage concerns.

 

Also, patina and toning are after mint products that are incorporated on the surface of the original skin, the question is, how can you tell if a toned or patinated coin started with and retained it's original skin?

[cpm9ball]

TomB wrote an excellent article on original & altered surfaces in WYNTK that may help answer some questions.

 

Chris

[TomB]

I wrote the following recently in a thread regarding what I consider original skin on circulated coinage-

In my opinion, the term original should be applied to those coins that, once removed from circulation, have not been exposed to intentional surface manipulation or alteration. Depending on their storage, these coins will change appearance at varying degrees with time. Additionally, after a period of time few will be identical in appearance with respect to how they looked immediately after being pulled from circulation. This happenstance-created skin will be original, in my opinion, and efforts to change or improve the look of the coin will result in stripping away of the original skin.

 

Others may or may not agree with my usage of the term, but it makes logical sense to me and has helped me over the years in my collection.

[TomB]

Additionally, as Chris has pointed out, I wrote a thread on original and altered surfaces. By the way, we have a "What You Need To Know" forum with substantial information that many of the newer members likely have not read.

 

Perhaps we should fold those threads into the regular US Coin forum?

[MrSpud-migration]

I believe that by "skin" one is referring to the thin oxide layer that forms on the surface of the metal of a coin. The layer may be composed of sulfides or various other chemical constituents, but basically it is whatever forms when fresh metal is exposed to the atmosphere. Fresh metal is highly reactive and, depending on what it is exposed to, will react on its surface and produce a layer of metal compounds that are more stable and less reactive than the virgin metal (unless the virgin metal reacts with something that is aggressively corrosive).

[Shamika-migration]

"Skin" is simply the surface of the coin whether circulated or mint state. "Original Skin" is thus a term used to describe a coin that hasn't been altered in an unnatural way.

 

And for the record, I consider a quick dip unnatural.

[rjf810-migration]

[Denver Dave]

When a coin is stamped the pressure is so great the surface of the metal turns molten and fills the impressions of the die

The surface of the metal does NOT melt, it cold flows. If it actually melted there would be no luster. The luster is formed by the microscopic scratches created on the surface of the metal as it is forced across the surface of the die. If the surface actually melted like a liquid it would simply flow to the new position and re-solidify. No scratches would form and no luster. It would be like the surface of a casting.

 

Conder101...

 

I'm not an expert on the minting process but I did sleep at a Comfort Inn last night..."-)

 

I have to respecfuly disagree, the metal does become molten.

 

Here's a Quote from The Coin Site, The Coin DOC's Answers, Archive #24

 

http://www.coinsite.com/content/cdanswers/cdarchive24.asp

 

When a coin blank is struck by the dies, the metal becomes molten and flows into the recesses of the die and outward from the center to the rim. The metal flow lines are responsible for what we call luster. These lines sit directly on the surface of the coin and are easily removed. Any disturbance of the flow lines by cleaning is immediately detectable by an expert.

[cpm9ball]

I agree with Conder. In Alan Herbert's Price Guide to Mint Errors the striking process is explained beginning on p.29.

 

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 metal 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 strictly through the pressure applied, and not by heating or melting the metal to get it to move. The ability to flow or move under pressure is one of the characteristics of copper which makes it such a good coinage metal.

 

In most case coins are struck on planchets at room temperature. In special instances coins, or especially large medals, may be struck on planchets that have been heated red hot and then placed in the coining chamber, but such a pracice would be virtually impossible in the high speed striking of circulation coins. I have seen this heating method used at the Portugese Mint in striking very large medals.

 

Perhaps you may be one of the many people who assumed that coins were made with molten metal. Because of the secrecy surrounding the minting process - as a defense against the public learning the process and using it to counterfeit coins - this and many other guesses and assumptions became "fact," and a number of them are still around.

 

Chris

[JRocco]

Conder is right--coins do not go thru a hot molten state in the coining process (contrary to what most believe).

[MikeInFL]

When a coin is stamped the pressure is so great the surface of the metal turns molten and fills the impressions of the die

The surface of the metal does NOT melt, it cold flows. If it actually melted there would be no luster. The luster is formed by the microscopic scratches created on the surface of the metal as it is forced across the surface of the die. If the surface actually melted like a liquid it would simply flow to the new position and re-solidify. No scratches would form and no luster. It would be like the surface of a casting.

 

Conder101...

 

I'm not an expert on the minting process but I did sleep at a Comfort Inn last night..."-)

 

I have to respecfuly disagree, the metal does become molten.

 

Here's a Quote from The Coin Site, The Coin DOC's Answers, Archive #24

 

http://www.coinsite.com/content/cdanswers/cdarchive24.asp

 

When a coin blank is struck by the dies, the metal becomes molten and flows into the recesses of the die and outward from the center to the rim. The metal flow lines are responsible for what we call luster. These lines sit directly on the surface of the coin and are easily removed. Any disturbance of the flow lines by cleaning is immediately detectable by an expert.

 

Conder is correct on this topic.

 

The above author misuses the word "molten". As was pointed out by several folks already, the planchets are not heated before being struck, and the word "molten" implies the use of heat to induce flow. The author would bave been more correct if he had instead said:

 

When a coin blank is struck by the dies, the striking pressure causes the malleable metal to flow into the recesses of the die and outward from the center to the rim through a process called cold forming.

 

That being said, if the memory of thermodynamics, a college co-op job, and childhood experimentation serves me correctly, I believe that the planchets do become warmer when they are struck, but it is not the heat itself that causes the metal to flow but rather the heat is merely a byproduct of the pressure/striking.

 

Hope this helps and respectfully submitted...Mike

[MikeInFL]

Back on topic....

 

To me, skin is something that is akin to toning but it doesn't pertain to the coloration. I use the word patina as a synonym to skin.

 

Fresh new coins don't have skin. Skin is aquired over time. Sometimes it is "hazy", other times it looks "powdery", and often is is a general darkening of the coloration.

 

Different metals get a different type of "skin". Silver takes on many different types of skin, including hazy, powdery, a general darkening often with color. Gold tends to get powerdy or satiny. Copper has a patina entirely different that I struggle to describe verbally.

 

Often times skin can be "secondary", i.e. the coin was dipped/wiped some time ago and has since grown a "second skin".

 

The most prefered is "original" skin -- the coin which hasn't been altered since it was removed from circulation or received from the mint.

 

Attached below is the only coin I have ever seen that I can be sure has a completely original skin. This coin lived in a small metal box the basement of my grandparents house in Philadelphia from the day it was removed from circulation in 1930 or 1931 until it came into my possession in 1978 and it has been in my possession since then.

 

 

In hand, this coin has a very distinctive powdery or satiny appearance that sort of comes through in the pictures. This is a good example of original skin on an early 20th century coin.

 

Have fun...Mike

[D.D.]

Skin to me is any Walker that Supertooth owns.

 

 

[MrSpud-migration]

Here is an excerpt of an article from http://www.key-to-nonferrous.com/default.aspx?ID=CheckArticle&NM=142 that talks about the "oxide skin" on the surface of metal, in this case aluminum.

 

"When aluminum surfaces are exposed to the atmosphere, a thin invisible oxide skin forms immediately, which protects the metal from further oxidation. This self-protecting characteristic gives aluminum its high resistance to corrosion. Unless exposed to some substance or condition that destroys this protective oxide coating, the metal remains fully protected against corrosion. Aluminum is highly resistant to weathering, even in industrial atmospheres that often corrode other metals. It is also corrosion resistant to many acids. Alkalis are among the few sub stances that attack the oxide skin and therefore are corrosive to aluminum."

 

This example is about an oxide skin forming on aluminum, however the same is true to more or less of an extent for other metals including copper and silver (only the different metals have different sensitivities to acids and alkalis than aluminum, for example coin dips that strip the skin off of copper and silver tend to be acidic). Also, the term "oxide" is used loosely, the skin can and usually a combination of oxides and sulfides and other chemical constituents such as chlorides and stuff. Whenever a coin is dipped in Jewel Luster or similar dips, the "oxide skin" is stripped off and unreacted virgin metal is exposed. Likewise, when planchets are made and minted, the coin starts out with unreacted virgin metal exposed. The unreacted metal is inherently unstable and begins to react with whatever it is exposed to in an attempt to become more stable. The metal can react either in a good way and form a skin (that can either be transparent, or show colors due to thin film interference if it forms in a certain way at the right trhickness) or in a bad way and cause corrosion or splotchy discoloration if too much of certain chemicals are present either in the atmosphere or accidently get on the coin (like from a sweaty acidic fingerprint) or if the humidity is too high (water tends to speed up chemical reactions).

 

When a nice layer forms on the surface of the coin as the metal reacts this is called the "skin". Because the metal has formed more stable compounds than the original unreacted surface, the skin becomes protective and aggressive corrosion is inhibited. However, if a stable skin hasn't formed yet and/or it is dipped off and the coin is exposed to a corrosive atmosphere then the coin can become damaged.

 

Here is a paragraph from the Intercept Shield patent (U.S. Patent 6,593,007) which gives a good explanation of the normal things that react with a coins surface. These same substances are involved in both forming a "skin" and in aggressive corrosion.

 

"Metals in nature always assume their most stable state, which in most cases are sulfides, chlorides, oxides, or other salts. As a result, metals begin corroding as soon as they are exposed to the environment. In addition, the most sensitive time for a metal is when it is pristine. In storage, shipment, or manufacture, metals are pristine, and are targets for corrosive gas attack. This corrosion or tarnishing is a particular problem for consumers purchasing fine silver, valuable coins, and so forth."

[leeg]

Some super thoughts expressed so far. I appreciate everyones contributions. Mike that Peace $1 is exactly my thinking on what skin is. You can almost "feel" the originality of the piece.

[MrSpud-migration]

Here is a more simple metallurgical definition of skin from Principal Metals Online http://www.principalmetals.com/glossary/sdoc.htm

 

"Skin - A thin surface layer that is different from the main mass of a metal object, in composition, structure or other characteristics."

[Denver Dave]

 

Again I must respectfully disagree...

 

The Mint uses a process called Coining, not Cold Forming...

 

The surface metal does become molten for a split second under the great pressure applied...

 

The word Molten is used by Mint Officials to describe the process...

 

I never said they use heat in the process, please read my prior post...

 

From Wikipedia, the free encyclopedia

 

Coining is a form of precision stamping. It differs from simple stamping in that enough pressure is used to cause plastic flow of the surface of the material. A beneficial feature is that in some materials, such as phosphor bronze, the plastic flow breaks up the surface grains, work hardening the surface, while the material deeper in the part retains its strength, toughness, ductility and flexibility.

 

Coining sometimes uses several dies to produce near-net shapes.

 

Mints use coining to produce money (coins), medals, police and fire fighter's badges, precision-energy springs and precision parts with small or polished surface features.

 

http://en.wikipedia.org/wiki/Coining_%28machining%29

 

Cold forming (cold heading): Cold forming is similar to extruding in that it squeezes the blank material but cold forming uses the punch and the die to create the desired form, extruding does not.

 

http://en.wikipedia.org/wiki/Die_%28manufacturing%29

 

We have yet to hear from Conder or Mark on the subject...

 

Thanks for the lively decision...

[realone]

I just have to thank you guys on this unbelieveably informative thread, for a newbe I lways wondered about the coin's exterior and how does one know what is original or not, and how it was formed and what it is defined as. This was excellent, thanks from the otherside of the universe.

[truthteller]

I believe that by "skin" one is referring to the thin oxide layer that forms on the surface of the metal of a coin. The layer may be composed of sulfides or various other chemical constituents, but basically it is whatever forms when fresh metal is exposed to the atmosphere. Fresh metal is highly reactive and, depending on what it is exposed to, will react on its surface and produce a layer of metal compounds that are more stable and less reactive than the virgin metal (unless the virgin metal reacts with something that is aggressively corrosive).

 

 

Good defintions.

 

Essentiallly Skin = Patina. On virgin new silver, the skin is a thin layer of silver oxide which forms a powdery white surface. On circ old silver, it forms a more greylike patina. The nice colors on silver form from oxides associated with high sulfur content paper holders and the like. Skin can be put back on silver coins after dipping with rapid heat application, thus creating an accelerated silver oxide formation. Skin is absolutely imperative for original, non artificially toned coins.

 

 

TRUTH

[JRocco]

Hey Dave,

I know I am not Conder or Mark, but I have a small handle on numusmatics

There is no melting of the metal in the coining process of a coin. The metal fills the dies strictly by the pressure exerted by the press.

[MikeInFL]

We have yet to hear from Conder or Mark on the subject...

 

Thanks for the lively decision...

 

 

Paging TobB, Conder, or Mark... Paging TomB, Conder, or Mark...

 

I realize you didn't use the word heat, but you did use the word molten -- which implies heat.

 

mol·ten

 

adj.

1. Made liquid by heat; melted: molten lead.

2. Made by melting and casting in a mold.

3. Brilliantly glowing, from or as if from intense heat: "A huge red bed of coals blazed and quivered with molten fury" (Richard Wright).

 

from American Heritage Dictionary

 

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.

 

Neither sleeping in a Holiday Inn Express, quoting Wikipedia, or prodding me with a stick will make you any less wrong about this.

 

Take care...Mike

 

p.s. back on topic, here's another coin from the "basement hoard" exhibiting a thick skin:

 

[MikeKing]

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)?