Please Post Your Coins Which Have A Hemispherical Depression

Thanks for the questions, cwb, I wish I had answers!

Here is an image of a blanking strip. (I've had it for years and don't know what date it was taken)

The depressions on your coin are virtually identical to mine! Amazing! What year is your coin? Are both photos the same coin? Is there any bump present on the obverse?
Thanks for posting!

I see no bump on the obverse of my coins. Here they are:

The cent is 1980-D although I believe they are PMD as a bump does show on the opposite side for each depression.
Seeings yours has no bump, I wonder if its possible debris from the roller itself, as the 2 rollers meet they provide even pressure on opposite sides, not leaving a bump... Just a guess.



bump in Lincolns collar

Then the question is, where is the Rockwell Tester located or do they have more than one? If there is only one and it is located in Philly, that would disqualify your coins as they are from Denver and Jarden Zinc would ship planchets directly to each mint. However, given the softness of a zinc planchet, I am skeptical that a test mark would survive post-strike.

Yes, Jarden Zinc is the sole supplier to the US Mint. Jarden also supplied the RCM with copper-plated zinc planchets before the switch to plated steel.

Rockwell Hardness Test with Zinc Planchets


I am listing here a summary of my findings on this subject.

Here is one explanation of hardness testing and Rockwell hardness values: "Unlike other measurements that directly relate to a material's physical properties (e.g., weight is directly related to mass), hardness is a comparative measurement. It's typically defined as the resistance of a test specimen to permanent indentation. Using standard testing methods, operators compare the results of a test specimen to a known specimen -- i.e., a hardness test block -- whose hardness they'd like to match.

How hard can this be? Actually, not hard at all if done properly. Unfortunately, hardness tests and testers often are misused, leading to erroneous results. The problem may lie in the user's assumption that the tester is measuring an absolute physical characteristic such as weight or length. While it's always true that a piece of brass that is 1.0023" long is the same length as a piece of steel 1.0023" long, it's not always true that a piece of brass which gives a particular Rockwell hardness value has the same mechanical properties as a piece of steel with the same Rockwell hardness.
An even bigger mistake is assuming that converting from one hardness scale to another is the same as converting from inches to millimeters. Incorrectly specified parts, bad procedures, conversion charts and unfamiliarity with the nature of hardness testing leads to incorrect measurements and, ultimately, to product failure." (1)

With a Standard Rockwell Hardness Tester, ball indenters may be used instead of diamond tipped indenters. It appears that for products of the U S Mint, ball indenters are used - as can be seen in an image which is an example of a Rockwell Test mark. A Rockwell test Mark leaves a depression. The depression should have no pressure ridge around it and have a smooth floor. The circular design might be distorted into a slight oval. The basic shape of the depression approximates a hemisphere. (2)

The U S Mint uses the Rockwell 15T measurement protocol. (3)

A hardness between 62 and 72 Rockwell 15T are considered nominal for RTS Zinc planchets at the United States Mint. (RTS= Ready to Strike). (4) The U S Mint obtains one cent planchets RTS. (3) (5) This hardness range is slightly higher than that for Cupronickel (the Incumbent Material for 5-cent coins) and even higher than that for Cupronickel-Clad C110 (the Incumbent Material for Quarter Dollar Coins). (6)

There are 4 major Zinc Alloys. They are identified by numbers: Alloy 190, Alloy 304, Alloy 720, and Alloy 750. Only Alloy 750 has a Rockwell 15T within the standards of the U S Mint. (7)

Zinc planchets need no annealing. (8)

The US Mint does own a Rockwell Tester and used it in a follow up study on alternative metals. (9)

A Rockwell Test Mark on a Zinc Planchet will still be visible after a coin is struck on that planchet. It will probably be altered somewhat, and it is impossible to say exactly how because too many other factors are involved. (10)

The company which supplies zinc planchets to the U S Mint does Rockwell Hardness Tests on their Rolled Zinc Strip (11) and on blank planchets before shipping where customer specifications require that it be done. (12)

I have attempted, unsuccessfully so far (no surprise there), to communicate with the U S Mint about this question. However, I simply cannot imagine that the Mint would not test these planchets against all their specifications (including hardness) or require certification from the commercial producer that they meet specifications - or both. It would be quite a costly mistake to go into mintage with planchets which are substandard.

I see no way to prove, conclusively, that the marks on these coins are Rockwell Test Marks. However, with what I have learned, I think that the odds that they are Rockwell test Marks have increased substantially.

Please feel free to ask more questions or give additional thoughts about this. They have certainly been a help to me so far in pursuing this research.

Thank you all.

Footnotes
(1) http://www.qualitydigest.com/april9...ardness.html
(2) http://www.error-ref.com/?s=rockwell
(3) Page 40, Alternative Metals Study, Final Report, August 31, 2012 by Concurrent Technologies Corporation, http://www.usmint.gov/about_the_min...12-08-31.pdf
(4) Page 42. Ibid.
(5) http://www.treasury.gov/about/educa...cturing.aspx
(6) Page 44 Alternative Metals Study, op.cit.
(7) "Technical Brief: Physical and Mechanical Properties of Solid Zinc Strip - Comparison to Other Metals" by Jarden Zinc Products (Suppliers of Zinc Planchets to the U S Mint) http://jardenzinc.com/Coinage.aspx
(8) http://jardenzinc.com/Coinage.aspx
(9) The U S Mint 2014 Report to Congress, along with a photo of the tester. (Paragraph 3.2.6)
(10) Personal Communication with an individual involved with the Alternative Materials Study for the U S Mint performed by Concurrent Technologies Corporation.
(11) http://www.jardenzinc.com/techdata/...olidZinc.pdf
(12) Personal Communication with a Sales Representative from Jarden Zinc.