Honest Question: What causes these deep flow lines? (UNC 1976 P Ike T2)

With some exceptions, like buffalo nickels and UHR Saints, right?

 

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Definitely "DIE Deterioration". Later stage die strikes will show this cause and effect thing, as it is so called. So, it's nothing new.

 

There are concave and convex varieties of the 1948 Canada 50 cent specimen coins, and my profile image is 1947 ML SP, which I'm certain is concave (obverse side). I've wondered how they did that, since to have two separate sets of 1948 dies for such a low mintage seems odd. Perhaps they were experimenting as well. The reverse is often weakly struck (on the circulation coins) because there just wasn't enough metal to fill the design.

Very informative thread.

 

I disagree.
With Morgan Dollar dies, "basining" which was basically die face polishing, results in a concave die face, not convex.



https://www.ngccoin.com/news/articl...nst the face,during the coin striking process.

And I quote from the link by David W. Lange, "The new, sculpted designs submitted by outside artists beginning in 1907 gradually rendered both radius plates and basining obsolete. The models as submitted already included the desired curvature, though the Mint's own staff sometimes had to modify this radius in the hub reduction stage."

Dies haven't been flat since prior to the 1830s. the convex radius is in the design since 1907.

Can't prove anything by measuring the fields of a coin, the only way to prove this is to measure die faces at different points for the die radius, the blanks move and flex during the strikes. Checking a coin with a Micrometer will tell you nothing except the thickness of the coin at various locations that are checked.

 

Thank you very much for a very helpful thread.

Please don't confuse the surface marks called "flow lines" with the actual flow of matter. This is like looking at the surface of the ocean near a beach and calling the lines of wave crests "flow lines". Those obvious lines certainly do not describe the flow of water in the waves.

So are these surface marks caused by die wear or by ripples of fluid metal flow during striking, much like the lines of wave crests on a beach?

If it is all die wear then the characteristic "cartwheel" chatoyance of mint state coins is die wear! My predjudice is that this is nonsense and that any groves or ridges visible under ten power are certainly not surface ripples frozen from the time of striking.

Chatoyance is a diffraction and interference effect. Diffraction is the ability of waves to bend around edges. (Newton knew that light was a particle because the edges of shadows were sharp and sound was a wave because you could hear around corners.) So whether you have a ridge or a groove light can bend and reflect from where it shouldn't. If you have a lot of parallel groves or ridges then the reflected diffracted rays can constructively interfere producing the bright diffraction streaks of chatoyancy that move around as you play the coin in the light. It is important to realize that these diffraction streaks are perpendicular to the edges that produced them.

Currently I'm trying to understand MS (Mint State), PL (Proof Like or Proof Lustre), and SP (Specimen Proof) (Thanks John) Canadian silver dollars. The MS were supposed to be one off commercial stikes. The PL were supposedly double struck at possibly higher pressures and the SP were multiply stuck at even higher pressure. Does any one have any references on this? The delicious liquid suface of SP is obvious. The chatoyancy of any MS coin makes that obvious too. The PL have no chatoyancy but sadly neither do some MS coins and the relative frostyness is supposed to sort the PL from the diffractionless MS coins. So why are some MS coins not chatoyant? If the microscopic flow lines were frozen ripple marks in the metal, shouldn't they be in all MS coins produced the same way? A simple explanation would be that all flow lines of any magnetude are the result of die wear!

 

But the flow lines on coins do mark the actual flow of matter across the die. That makes ocean waves a poor analogy. A better one is land being eroded or deposited by flowing water -- although that's not great, either, as the die is eroded by repeated short movement, rather than continuous flow.

 

How do the deep grooves that started this thread and called "flow lines" mark the actual flow of matter? What direction was the matter flowing when it produced these grooves? Was the matter moving along the groove or perpendicular to the groove?

 

Coin metal moves in the direction of the grooves. Any grit on the surface of the planchet gets dragged along the face of the die in the same direction. Over thousands of strikes, these disturbances accumulate across the surface of the die.

At least, that's my mental model. I don't have anything to validate it, except what we see on coin surfaces.

 

by no means a professional, just an avid reader, anyways......

yes in a sense, "flow lines" are actually the cause of die wear or "die erosion". Terms like cartwheel, starburst are descriptive of the die wear pattern on the coin. strike after strike of the blanks, under pressure of the strike, the metal flows outward radially. over time, the "flow lines" of this movement cause die wear/die erosion and it's transferred back to the die steadily strike after strike, friction.....

true first strikes, don't have much in the way of these flow lines, and considered "prooflike". A die set needs to be broken in to produce really lusterful flow lines considered as "cartwheel luster" in appearance, if they aren't present, it's more like the smooth or wavy mirror of a proof coin until the die pair is broken inand starts wearing.

As the die wears on strike after strike, flowlines etch into the die faces and begin to wear it down. The transferred flow lines get stronger and deeper over time, and people then would describe the appearance as "starburst" and the dies are getting towards the end of the die pairs useful life. a whole host of things can happen during this period from cracks to chips or breaks to warping of the die face and even spread of the die face if they weren't re-annealing the dies and trying to maintain them to extend the pairs life.


if they are referring to "SP" as "specimen proof" then yes, it would be a more polished dies, and stuck under higher pressures, or more times, to achieve the desired effect, and even then, there would be die erorsion which would make them decide to correct the die, or retire it if it could not be repaired.

RCM says "Proof Coins with a frosted relief over a brilliant field. Proof coins possess the highest-quality finish for a numismatic coin, and are usually struck twice to reveal the smallest details of the coin's design.
Specimen A brilliant image relief is struck against a matte or lined background in order to achieve maximum visual impact. Specimen coins are struck up to two times on numismatic presses".

I would think, in the descriptions from the RCM:
MS is the business strike, they might take a little more care for collector versions. U.S. Mint does the same.
PL are also business strikes perhaps they are "first strikes" until the die pairs are broken in for the rest of the MS strikes.
Proof/PR Described to be like the U.S. regular proof strike in my opinion.
SP are the RCM equivalent to Enhanced Proof in the U.S. I think by the sound of it.

 

That thread has given me so much more knowledge about the minting process than I ever expected.

This is why I recommend this site to anyone who wants to know more about numismatics.

 

Thanks Jeff. That's my position too.

So your position is that flow lines are like the scratches left on granite by long gone glaciers. When I began writing today mine was that die wear was a red herring and true flow lines were ripple marks, not scratches, and required no debris or foreign matter to form. Now I'm not so sure. How do you explain both chatoyant and non chatoyant MS coins of the same issue? Higher die pressure for some? Longer strike time for some? (Since coins have been milled they are pressed and not "struck".) Clean dies is the simplest explanation but how to test it?

Your scratch theory suggests tests for the debris. Can debris size be correlated to scratch size? In glacial scratches you get the direction of motion from the abrupt end to the groove when the debris finally disintegrated. Can anything similar be seen on a coin? A possible weakness here is that the glaciers came by only once and we are looking at surfaces produced by dies used many times but only once on any commercial strike. It is not clear to me how the debris would wear the die. Each new planchet brings a little new debris adding to what has already accumulated on the die. Would the debris wear scratches in the die that would make ridges on the coin rather than grooves?

 

It's all getting too hypothetical to be of use, I guess, but:

More chatoyant ("lustrous") coins would come from dies with more flow lines. The first few strikes from a die would be "proof-like", with smooth fields that haven't had time to accumulate flow lines.

I don't think debris would accumulate on the die, necessarily; each individual bit would likely be carried away after one or a few strikes. But each new bit would tend to move along the same directions as the old ones, and maybe even be forced into the existing channels as it moves, reinforcing them.

The flow lines on the coin are ridges, I think, although if there are a bunch of them close together it's hard to distinguish parallel ridges from parallel channels.

 

Whose to say that at least some of the metal doesn't flow tangentially (90-degrees from radially)?

 

Sometimes it does, and the luster patterns follow. I'm thinking particularly of the Mercury Dime reverse, although I'm having trouble coming up with an image or animation to show what I mean.

 

On a more fundamental level: Is there any relative mtion between the metal and the die? Does the metal skid across the die to where it is needed? Or does the metal roll across the surface of the die to where it is needed once the metal contacts the die?

In a skid there is relative motion between the two materials at their interface. Here that's the die and planchet. In a skid an asperity attached to the die would dig a groove in the planchet and an asperity in the planchet's surface would try to dig a groove in the much harder die. This could account for one off grooves on the coin and, eventually with enough wear, parallel grooves in the die that would produce parallel ridges on the coins. A one off scratch on a coin won't produce chatoyancy, but the die will eventually wear with many parallel scratches producing the observed chatoyancy.

However we learn from fluid mechanics that in non turbulent flow the boundary layer of fluid around a static body is at rest with the body. It is hard to think of the short distance flow that fills the die as turbulent. So here the metal contacting the die stops still and the metal atoms away from the die surface then roll across one another to where they are needed. In this case the die would faithfully express its ridges as grooves and its grooves as ridges. This cannot account for the observed build up of parallel wear lines on the die that should produce riges on the coin giving it chatoyancy.

Maybe a metallurgist could tell us whether metal skids or rolls as it fills a die on striking?

 

Good question and images by the OP. And an interesting question.
Even though no one asked for it, here are my two cents.

IMHO, these die flow lines are caused by metal flow towards the rim. The longer a die is used the more pronounced the lines get. (Sort of like wrinkles on a bikers face from the wind as they get older.)

Since the planchet is smaller than the collar die, the metal must flow to the edges to fill the collar. The closer the metal is to the rim the more it is affected with movement. This radial movement towards the rim is evident by the depth of the die flow lines in the OP's images. They are deeper and wider towards the rim than the central part of the coin. The metal in the central part of the planchet flows up into the deeper central devices, but the metal in the field flows towards the rim to help fill the collar.

BTW- My understanding of the reason for the proto-rims on the planchet is to help form the rims and protect the details on the coins from circulation wear. I'm sure that helps reduce the metal flow towards the rim but it doesn't eliminate it.

 

I can't help thinking that you're working too hard here. Don't model the planchet as a flowing fluid. Think of it as a solid object being hit by a more-solid object, and deforming as a result. From the engineers I roomed with and dated and married, I got the impression that Deformables and Fluid Dynamics use different approaches.

Sure, it's hard to think of the short distance flow within the planchet as turbulent -- and it's not useful. Wrong model. What's the right model? You have the right idea in your last paragraph: ask a metallurgist.

 

Why does die age couse doubling?