Featured Have you ever wondered what luster looks like?

Great explanation. Here is another detail showing an extra ridge along the circumference and flattening at the letter tops. These both support the analysis of misaligned dies.

I did not know how to interpret this evidence.

 

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@RonSanderson

There is something we call micro whizzing that leaves a coin with cartwheel luster as it does not destroy the radials yet it does change the appearance of the "non-original anymore" mint luster. With this treatment the design does not acquire the characteristic "lip" necessary to be whizzing. Unfortunately, I have never been able to capture this micro-effect in an image. If I recall, this alteration first appeared on Indian and Lincoln cents in the 1980's. The grading services (except where I worked ) slabbed these coins as red Unc's until they finally caught on after several months or more.

I just saw a micro-whizzed obverse Morgan dollar today. The reverse was 100% original. The obverse was altered and then attractively toned. The collector thinks his coin is an MS-67+!

You may have one of these pieces.

 

physics-fan3.14, posted: "What we term "mint luster" is actually more accurately related to "chatoyancy." That would clear up the confusion with two meanings for one word. But I don't see that term catching on in numismatics, as we are far too traditional."

Oh my, I must disagree again. IMHO, there is NOTHING in numismatics that relates to "Chatoyancy" or produces that effect on a coin. Perhaps a 1/4" wide and 1" bright wheel mark on a Morgan dollar could be described as "chatoyant." Just not by me. Oops, I'm wrong! If I hold a lustrous BU Morgan dollar Rock Steady and tipped just right under a 100 Watt bulb in a dark room so that the reflection from the "Cartwheel" ray remains FIXED in one position fixed across the entire coin it does resemble a "Cats Eye" gem.

 

RonSanderson, posted: "Great explanation. Here is another detail showing an extra ridge along the circumference and flattening at the letter tops. These both support the analysis of misaligned dies. I did not know how to interpret this evidence.

View attachment 883491

I think you are showing a mark from "die polish."

Also, if we start to call coins that are struck with a tiny difference in rim width "Misaligned," Then most coins I see are! Heck, I just found two dimes, a quarter and three cents in my change dish that are that way. The quarter is more "off" than either of the posted images above.

 

So, I've read through this whole thread (with no regrets!), waiting for someone to call out the key difference between "luster" (from tiny parallel or radial ridges on a surface) and "shine" (from any smooth surface).

That difference is that luster depends on the light's angle.

Or, to be more technical and precise: luster is anisotropic reflection, different from specular reflection (light reflects at the same angle it hits, as from a mirror, no matter the angle) or diffuse reflection (light goes off in a random direction, as from a sheet of paper, no matter the angle).

On a lustrous surface, light hitting those grooves and ridges spreads out in a direction perpendicular to those ridges, but stays together in a direction parallel to those ridges. If the ridges spread out radially from the coin's center to its edges, you get the "wheel spokes" of "cartwheel luster" -- along the spokes, light is hitting the ridges perpendicularly, and a lot of it is bouncing into your eyes; along the dark areas, light is hitting the ridges in parallel, and nearly all of it is missing your eyes. (Or vice-versa, depending on the angle between the light, the coin, and your eyes.)

Does this make sense?

 

I've used these three types of reflection (but not by name) in my Morgan dollar class to discuss how dies go from DMPL (high specular reflection) to lustrous, and then how coins go from DMPL or lustrous to not having luster (diffuse reflection) either by overdipping or wear. Maybe I'll add the names to the class material.

 

Please help me understand something.
What causes this rougher surface on his dime?

Are they;
A) "flow lines" from a weak striking process that would have been eliminated if it was fully struck?

B) a result of metal flowing from the striking process eventually wearing into the die surface and therefore transferring the texture to coins being struck? Die Wear that can be corrected with polishing?

C) Neither (please explain?)

 

Ooh! Ooh! (raises hand)

If I've been paying attention correctly in other threads, it's B... right?

 

Yes, it is a result of metal flow and thus die wear. It cannot really be "corrected" with polishing, however (I assume you mean die polishing performed at the mint). Die polish can "reset" the surface - a properly polished die will remove all of these grooves and return it to a mirrored surface (hence many of the prooflike coins of this era). Polishing (as in cleaning the coin) will remove these ridges and leave a lusterless, shiny ugly coin.

Doug, I think what you are interpreting in that image may be an artifact from the imaging and not actually a misaligned die. @RonSanderson , are either of these struck from misaligned dies?

I think the difference in pressure may explain some of the difference in luster, but it has been theorized that San Francisco used a lower striking pressure to prolong the life of their dies. There is a difference between the mints, and a San Fran coin usually cannot be directly compared to a Philly coin.

I think another large part of the difference in luster here is the die state - coin B is from a very different die state than coin A, and is thus expected to have a different character of luster (I often describe A as "creamy" luster, while B is more satiny as you describe it.) The type of luster will change over the life of the die, as the metal continues to wear against the die.

 

For those reading this thread who are unfamiliar with this term, please read the brief summary on wikipedia here: https://en.wikipedia.org/wiki/Chatoyancy

A tiger's eye is one of the best examples of chatoyancy in gems. The key quote from the summary on wikipedia is this:

The difference between the gem and a coin is that the fibres in a gem which reflect the light are all arranged in a parallel fashion. The ridges that produce cartwheel luster are arranged radially around the coin, converging on the center (usually, depending on the design). Thus, when you rotate the gem, you don't see a cartwheel - you see the chatoyancy as unidirectional. However, when you rotate the coin, you are changing which set of parallel lines you are reflecting light off of, according to which direction your light is angled.

The principle which is creating the effect, that of light bouncing off microscopic disturbances, however, is exactly the same.

A wheel mark, or hairlines from polishing, are absolutely going to be chatoyant as you mention. However, their stark unidirectional reflective pattern is the fundamental difference between that and natural luster - they don't have that cartwheel, and just as the gem, may be nearly invisible at the wrong angles. "Natural mint luster" should be radial, not unidirectional.

Based on this explanation, and my extrapolation of the term to numismatic, I stand by my assertion that this word may have some application in this discussion.

 

@Insider so how does one detect this "micro whizzing"? This is the first I've heard of it and I would love to learn more.

 

I’m going to have to learn more about die misalignment before I can answer your question.

I do have one very obvious thing left to mention. When you examine the closeups of the two dimes the surfaces are quite different.

The satiny 1941-S exhibits only very fine-grained radial flow lines. There are no lines that look like die polishing that I can find.

The reflective 1939 (surface details in post #23601) has die scratches that run in many directions. Because they are scratched into the die they end up raised above the surface level.

I expect the cross-section shape of the lines to be quite different, too, since one is created by erosion and the other by scratching.

There are two quite different surfaces and very distinct lusters.

I also have a question. When a new die starts out Proof-like, the mirror smooth die immediately starts to erode. I would think an early stage of wear would be just light radial flow lines. After a touch-up there would be polish lines in many directions. Do all new dies go through a brief stage where they produce satiny finishes?

 

More pictures!

Here is a proof Principality of Sealand half dollar. This picture was taken over the frosted parts of the devices. I see very granular surfaces which are consistent with a sand-blast or acid-etc treatment on the dies prior to striking.



Here is a picture from the mirror field. The diagonal lines are latent die polish marks since almost all of the lines I saw were in the same direction and went clearly under the devices. The specks may be dust, impurities in the metal, or imperfections in the die.



This was the most interesting image of the three. The polish lines are still going in the same diagonal direction. But lookie here! There are some lines which are perpendicular. I only found these near the edge of the coin, which makes me believe they are early metal flow etch lines. They all have a consistent shape: a head pointed toward the center of the coin with a tail that gradually gets thinner. Logic and a very limited knowledge of metallurgy suggests that as the metal accelerates during striking, the metal etches deeper into the die. If that is the case, then this is clear evidence that metal flows from the rims inward, as @GDJMSP says happens.



Now this coin is a Perth Mint 1 oz Star Trek silver round. The interesting thing about this coin is that it is a reverse proof. The following picture was taken in the field, which shows the granularity seen on the devices of the Sealand half dollar above.



When I placed my microscope over the brilliant devices, however, I saw a very cool image. With no flow lines or die polish to distract the camera, you can see the crystalline pattern of the metal.



While taking the pictures, I accidentally touched the surface of the brilliant area. To my horror, I saw that I had really messed up the surface. This goes to show just how delicate proof surfaces are.

 

Sealand silver dollar



Star Trek 1 oz

 

Excellent definition.
chatoyancy:
(of a gem, especially when cut en cabochon) showing a band of bright reflected light caused by aligned inclusions in the stone.
late 18th century: French, present participle of chatoyer ‘to shimmer’.

 

Did you actually mess up the surface, or are these streaks the oils left from your hand? I strongly doubt you actually changed the metal flow lines, but rather what you are seeing is the residue (fingerprint) that you left behind.

 

That’s entirely possible, but the “touch” was more like a small rub.

 

Amazing images @TypeCoin971793 !