I thought I'd share this neat documentary on US Mint coins. This may have already been posted in the past, but I think it's nice to bring up again, especially for those who haven't seen it yet. Right from the beginning, it takes you through a journey of how proof coins are struck at the SF Mint. Most of you probably already know the process, but for those of you who don't, this'll be an interesting watch.
Actually had never seen this, but initially i was put off how one of the people said that the pressure of striking caused the surface to turn "liquid" because of the heat.
If it flows, is it not liquid, at least at that time? Remember, melting point, like boiling point, is stated for a particular pressure. Water boils at room temperature if the air pressure is reduced enough. Does not a severe pressure similarly change a melting point? I mean, if they can describe what happens to filled land in an earthquake "liquefaction", why not metal under severe nearly instantaneous pressure?
Perhaps the pressure is high enough to lower the melting point of the metal, but because the pressure is only that high for a short duration is there actually enough time for the material to undergo the phase transformations? A pure guess is that the peak pressure is sustained for just a few milliseconds. I don't know the answer but I am just voicing my internal bullshit detector's voice. BTW, liquefaction is a loss of shear strength due to shaking, not necessarily a large increase in pressure.
Yeah, deformation because of malleability isn't melting. Someone who is a better materials guy here once did a calculation that kind of called this BS.
I have no idea how such a thing could be done, but it sure would be neat to see a severe slo-mo of what goes on at the moment of striking. I've seen all the diagrams, but something more photographic sure would be neat.
Actually the melting point of silver goes up with pressure. See: Melting of Copper, Silver, and Gold at High Pressures, Lewis H. Cohen, William Klement, Jr., and George C. Kennedy, Phys. Rev. 145, 519, May 1966. However, solids can flow without becoming liquids, i.e. they do not undergo a phase transition. Press are piece of rubber, and it flows without becoming a liquid. Press metal hard enough, same thing happens. In a liquid, the molecules (atoms in the case of purified metals and many alloys) can move about relatively freely. In a solid flowing under pressure, it is a stepwise process of molecules or atoms being wrenched away from their current neighbors, reforming new strong bonds a short distance away, having these bonds broken to establish new strong bonds in the direction in which the pressure is moving them, etc. Cal
In simple-minded chemistry (my favorite kind) we talk about metals (or alloys) using a "sea of electrons" model where the electrons on the metal atoms are held so loosely that the metal atoms can slide about relatively freely without being tethered to any particular electrons.
Thanks for putting this up! I actually saw it for the first time a couple weeks ago and was thinking about posting it here. I'm a visual learner so it's a lot easier to understand the minting process than by reading about it in a textbook
