I want to attempt an explanation of how "Original Mint Luster" and "Mirrors" are formed, on the surface of a coin, during the minting process. Using a scientific explanation in layman's terms, my hope is that whomever reads this can achieve a good understanding of the processes that create both without too much difficulty. At times you may wonder, "where is he going with this", but I promise, if you hang with me, I'll tie it all together. First let's explain luster, or what causes the reflection that we call luster. Luster is nothing more than the reflection of light. Most all solid substances reflect light.That is how we see color, remember the color wheel in grade school, white light is made up of all those colors. When light is shown on gold, all colors of the spectrum are absorbed, except those that are reflected back, that give the gold color. Absorption of color is a little too complicated for this discussion, and probably doesn't seem relevant, but Iet me explain. Lead is typically considered dull grey with no reflection. Grey is a color, but at times it too can have luster. If you have ever experimented with pouring your own fishing weights you will have observed it. When melted down, the liquid lead has luster as does freshly poured lead that has hardened. Why is that and what on earth does it have anything to do with coins? The coin part will come as I promised, but for now let's discuss luster. How liquid lead reflects light so well has to do with what we call surface tension. We have all heard of surface tension, but what actually is it? To explain it could get complicated but it really is easy to understand. Tiny molecules of the same substance are attracted to each other and pull themselves together like a magnet. The outer most layer of molecules do not have anything pulling at them on one side, so their attraction to the inner molecules is much stronger. Kinda like being pulled by your arms and legs, in opposite directions, until someone lets go. This added attraction, or pull in one direction, makes them very compacted. So compacted in fact, that very little light can be absorbed. This in turn, gives us a reflection or luster. Raindrops have surface tension. Ever watched as two beaded raindrops attract one another and become one? . That same surface tension and luster remain when the lead cools and hardens but the luster is confined to the surface and remains very thin. Another thing I believe should be explained here is how a cool ( there is heat from friction in the process) solid metal can flow like a liquid. When a planchet is stuck in the coining press, the metal of the blank planchet flows to the recesses in the die to form the coin. How is that? To explain how cold metals can flow, I want to use water and mercury. We all know that water becomes a solid or ice at 32ºF and normal atmospheric pressure. But when ice is placed under higher pressures it will return to a liquid at that same 32º. Crazy huh? Case in point; when you scrape the ice or frost off of your windshield, ever noticed ice forming right behind your scaper? Have to scrape it again? This is because the pressure you put on the ice with the scraper caused it to return to a liquid state that refrose when it retured to normal pressure. Cool huh? Even cooler is that mercury is a liquid at normal atmospheric pressure, but will become a soild in a vacuum or a vapor at high pressure. Surface tension is what makes liquid mercury look like a mirrored ball. Now to tie all this rambling together. When a coin is struck in a press, it is put under tremendous pressure. So much so, that the metal surface of the planchet becomes liquid and begins to flow to fill the die. Since the metal is now liquid it takes on the properties of surface tension and becomes lustrous. When the pressure is released, or obsorbed by the anvil, the metal will again freeze in its current form and all those lustrous flowlines remain. Being that these are actually tiny thin mirrors in a way, they will reflect light in crossing patterns that tend to give the reflection a texture. Like a hologram effect. Oh and the cartwheels. Most of the flow of the metal will be from the outside of the planchet to the devices or designs engraved in the center of the die. As the metal flow gets closer to the center, the flow lines become concentrated because they have less room to flow. This is what makes the flow lines appear to be spokes that radiate and get wider from the center. When the coin is rotated in light, the luster of those lines appear to go around, just like the spokes in a cartwheel. Mirrors? To understand this one is fairly easy. It implements all the above explanation of luster less the flow lines. When a proof coin is first struck, it too will have lustrous flowlines, but proofs are struck at least twice. Since most of the flow of metal has filled the devices in the die on the first strike the second strike causes these lines to melt and flow together. The result is a flat surface that resembles a mirror. Kinda like the surface tension of a clear lake on a still morning. I sincerely hope this, as rough as it is, actually makes sense.
It's a great write up, but I feel that you have written about the proof and business strike techniques as one in the same. There is really no molten state of business strike coins, and I would argue (against the grain) that there is really no molten surface state of proofs either. Why would die polish lines be evident on proofs if this were the case. Every proof ever struck would be UC.
@Kirkuleez Die polish lines are viods In the die the same as the letters and devices. Metal flow will actually follow these lines because it is a path of least resistance. In doing so, the friction produced will erode the die along those lines, making them more pronounced on later struck coins. Thus the need for repolishing. Everyone has seen a late die state coin in which these polish lines are almost exaggerated. In some cases, it is my believe, that alot of these lines aren't actually polish lines, but lines left from the hubbing process that weren't completely removed in the polishing process. Edit to add: Ever put a coin on a train track and after the train hit it picked it up? There is a tremendous amount of concentrated energy in the coining process that is converted to heat
Metal from the planchets doesn't flow because it's molten, so much as because it's being pushed against the anvil die by the hammer die with enough force that some of the metal has to move into the voids in the dies. In other words, it has to go somewhere, and it's easier to go where there's air than where there's already metal in the die. This is why the metals used for dies have to be harder than the metals being coined. Otherwise, the dies would not work well and would wear out after just a few strikes, leading to weak strikes and lots of cracked dies. (Which, BTW, we do see in a lot of early nickel coinage -- nickel is much harder than gold, silver, and copper; the steel used to make dies wasn't quite up to snuff for a while.)
Exactly, I believe that the molten state of the surface of the coin has always been told but is misrepresented. There is not nearly enough friction in the process for this really to be true. I do like the story though, it adds to the mystique of early (and modern) proof coinage.
I beg to differ. Metal does flow. One of the reasons I used mercury and lead as examples. The die has to be made of a sustance that requires greater heat and pressures to be transformed in to a liquid state to be resistant to erosion. i.e.Higher melting point.
Yes, there's such a thing as metal flow; I'm saying it's not because part of the metal becomes molten. If I have a block of lead and hit it moderately hard with a hammer, I'm going to leave a dent, but you can't tell me that's because I've somehow heated the lead under the head of my hammer to its melting point of 327.5 C. The reason metal flows is because it's malleable. What exactly causes metal to be malleable, I don't know. The best I can do is something like "*mumble, mumble* quantum mechanics ...*blah blah*.
Not all metals are malleable. I won't break out my periodic table to explain which are and which aren't and the reasons for there positions on that chart. This was supposed to be simple, not a lesson in quantum phyics.
@Paul M. You are totally missing the point that the melting point of lead is 327.5º at one atmosphere of pressure. Put it under 60 tons psi and see what the melting point is.
All of the metals used in making coinage are very malleable. The pressure from the dies is not enough to liquify the surface metal of the planchet. Though I would love to hear your explanation of malleability of the elements, coins are typically made from alloys which can drastically change the physical properties from those of the elements used in the alloy. For example, both strontium and aluminum are very soft metals, but when combined in an alloy, aluminum-strontium becomes very hard and brittle depending upon the concentration of strontium added. The industrial application of aluminum-strontium is for aluminum alloy car wheels.
Geez, Chucky, if some people think I've made a big deal about "there & their" or "your & you're" in the past, you've gone through the roof all the way to Mars. I think there is an easier way to explain luster. Simply said, the luster on a coin is created from the flow of metal during the striking process. In this flow, peaks & valleys are formed as the metal flows outward from the center. We see luster as the light that bounces off these peaks & valleys. As an aside, when a coin is improperly cleaned (over-dipped) these peaks & valleys are eroded to the point that the light, rather than bouncing off in a specific direction, bounces in all directions off a rounded surface. Chris
Actually Chris, the cleaning, or appearance of cleaning, was going to be the second part of this thread. I now feel that would be a waist of my time.
@Lehigh96 I will be more than glad to discuss whatever you like, but not on this forum, I do not feel it is appropriate. Might I refer you here. http://www.thephysicsforum.com/forum.php
Yes! That is a good example, and if you were to magnify a section of the coin, the flow lines would be more understandable to the untrained eye. Chris
Sorry! I can't read minds, but I don't see how you can discuss luster without explaining how improper cleaning can affect it. Chris
