In response to last week's question about the use of silver plugs in silver coins, Tom DeLorey writes:
I was one of the group of authenticators that attributed the first "silver plug" Bust dollar.
My understanding of the process was that the underweight planchet being so treated was pierced in the center with a sharp instrument, such as an awl, that pushed metal aside without removing any metal, or removing very little.
The plug was then placed inside the hole so created and tapped with a hammer, mushrooming it out on either side (similar to a rivet) so that it would not fall out of the hole as it was being placed in the coin press. The now-heavier planchet was re-weighed and adjusted back downwards if necessary.
When the planchet was then struck by the dies the excess metal in the center of the planchet would spread outwards, and any gaps between the planchet and the plug would be filled in.
As to the question about the silver plug in the 1794 SP66 dollar, Chris Pilliod just wrote about the research he did on a 1795 dollar in the most recent issue (August 2013) of Longacre's Ledger, the Fly-In club journal. He proved what we all suspected. The plug hole is very small and the piece used for the plug is longer than the hole (it was cut to the proper length, per the weight needed) and was smashed down before the coin was struck. Thus, the amount of silver added is more than that removed, bringing the planchet to proper weight.
I have spent many hours with the "special" 1794 silver dollar, of which much has been written. There is debate as to why the 1794 dollar contains both a silver plug AND adjustment lines. Was it lightweight and a plug was added increasing its weight to over the required amount, requiring filing to bring it down to proper weight? Was it too heavy and they filed it too much and had to add a silver plug to bring it back to standard?
I have published another theory on several occasions. Both the above scenarios seem unlikely to me, so I believe this dollar was a "regulated" silver coin, just as Brasher and others regulated gold coins. I believe that the Mint was saying: we are going to produce coins of correct weight and fineness.
There is debate on this point, of course, and I have no proof (pun intended) that this is the case.
However, those who have examined this coin realize how special it is – as noted by me and others in The E-Sylum and elsewhere, I believe this is David Rittenhouse's personal specimen and is the first silver dollar struck (right after the copper die trial). There is no proof (yep, very punny or is that, punnie?) of this, but this is the only SP 1794 dollar, as well as the only pre-clash example other than the copper die trial. Martin Logies and others have looked for other State a (State I, per Logies) examples and have found none.
Why would they select this planchet to polish and strike a special coin? I believe the only logical answer is the one noted above.