Luster So, we all know that 'luster' is a highly desirable attribute on mint state business strike coins. The loss of this luster is evidence of a coins circulation or improper cleaning / preservation techniques and is important when a coin will be TPG slabbed and graded. I'm pretty confident that luster isn't an artifact of the dies, but seems to be more prevalent with new dies vs. worn dies. Therefore it stands to reason that the best looking lustrous coins depend on nice fresh dies, correct? Let's now talk about Proof Coins. Proof coins are struck with the best possible condition dies. Because the luster isn't a design artifact of the dies, why don't proof coins exhibit this desirable attribute, luster? Let's take this one step further. Cameo devices. Are the devices (raised areas of the dies) of proof coins sand blasted with a satin texture, or is this evidence of luster on proof coins? Is this why the cameo look fades as the proof dies wear? And finally, Reverse Cameo Proof coins: It has been my understanding that proof dies are 'highly polished' and are kept polished throughout their life span. If this is the case, how do the fields of reverse proof coins stay 'satin finished?' Maybe this is all US Mint trade secrets, but inquiring minds . . . dontchakno. Discuss this amongst yourselves and let me know the conclusions. Z
Partially correct. Luster is, first and foremost, created by the metal of the coin flowing into the recesses of the dies. That is why most luster appears to originate at the center of the coin and cartwheels outward as you rotate the coin. Now, each time a coin is struck, there is some wear on that die. The metal is going to tend to flow in the same pattern every time a new coin is struck. The wear on the die is going to change the surface of the die, and thus change how the luster appears. There will always be luster on a newly struck coin, but depending on the time in die life, it might be an intense, bright, cartwheel, or it may be a more subdued luster. Again, not necessarily. Proof coins are struck with specially prepared dies on special presses, but they aren't necessarily any better or worse condition. They all use the same steel, just the proof dies are treated differently. They do. It is just a very different characteristic. The polished mirror surfaces wear on the die much less, but they still do wear. If you take a proof coin under a strong light and almost look "into" the surface, you'll see the lines that cause luster. As a proof die ages, this effect gets stronger. It is especially apparent on classic era proofs (1950's - 1960's) where they used the dies much longer. Some classic proofs had dies that were sandblasted, but they haven't done that for a hundred years. Most proofs before the modern era were created by soaking the die in acid (pickling), which would etch the high points of the coin (the low points of the die). They could then polish the fields (the high points of the die) to create the mirrored finish. Because the pickling only created surface imperfections in the die, and it wasn't a thick layer, that cameo effect quickly wore off and matched the mirrored finish of the coin. In the modern era, the cameo effect is applied by computer controlled laser technology. If you look at it closely, it has a "snakeskin" effect where you can clearly see the laser etching. As mentioned for the previous question - a reverse proof is essentially the same as a regular proof, just they laser etch the fields instead of the devices. The laser etching is considerably thicker and more durable than the pickling of previous eras, so it can last much longer. It helps that the mintages of reverse proofs tend to be lower, so you don't need it to last as long. If you want to read more, check out a pair of threads I wrote a while back: https://www.cointalk.com/threads/luster-a-guide-for-beginners.58435/ https://www.cointalk.com/threads/di...ie-from-first-strike-to-terminal-state.59262/
Seems I read a book that covered that subject pretty well???.... @physics-fan3.14 what book am I trying to recall?
Well... why don't you give some of US a chance to sound smart? Yes, exactly everything you mentioned. I guess the burnishing of mirrored fields (frosted proofs) isn't done with diamond abrasive pastes and Dremel tools anymore then, right? Is that also done with computers and lasers nowadays?
It sounds like the process you are describing (laser etching) takes place after the coins in question leave the press? Would it be conceivable for the etching to become 'misaligned?' Even small degrees of rotation should be highly visible, right? Sounds like an expensive process. I guess this isn't considered PMD since the damage is intentional and takes place within the walls of the mint. If the Nawlin's or Dahlonega mints were still operational, that's how Bubba would do it. Z
Let's be clear - I love the concept of these. The whole reverse proof thing is cool. I hate the execution of them. The laser application of the frost just looks absolutely ridiculous. Judge for yourselves the quality of the cameo texture through the decades (photo courtesy of brg5658 and featured in my book):
