Thickness of final plating on U.S. Lincoln 2010 To Date Shield Cents

For the mathematically challenged:
+/-5 micron = +/- 0.0002 inch variance

For your future reference, here's how I arrived at that.
(a) 1/25.4 = 0.03937007874" (one millimeter in inches)
(b) 0.03937007874"/1000 = 0.00003937007" (one micron)
(c) 0.00003937007" x 5 = 0.0001968503" rounded = 0.0002"
(d) Final thickness = 0.0008"+/- 0.0002" , or 0.0006" - 0.0010"

Thanks Kirkuleez !

ToddB67

 

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If a current cent, weighing 2.5 grams is 2.5% copper by weight, then it contains .0625 grams of copper.

The surface area if a cent (19.05mm diameter, 1.52 mm thick) is 661 mm^2.

661 * density of copper (0.0027 g/mm^3) * thickness of copper (mm) should equal 0.0625 g. If you solve for the thickness, it comes out to 35 microns, assuming there is no copper in the zinc substrate.

 

Too bad I don't have one of these in the garage. Then there would be no more debates on plating thicknesses or composition errors.

http://www.labx.com/item/hitachi-s-4500-sem-with-edx/3998544

 

I don't think it's that simple...

The high level specifications are listed here:
https://www.usmint.gov/learn/coin-and-medal-programs/coin-specifications

That gives you 2.5% Cu, 97.5% Zn with weight of 2.5g, diameter and thickness (19.05mm and 1.52mm).

I've seen it stated that the thickness of the plating was increased after the first couple years to improve wear. The plating is a trivial portion of the 2.5% Cu.



FYI - the two companies that produce cent blanks were identified (1996), here, http://www.gao.gov/assets/110/106568.pdf as

Alltrista Zinc Products - now Jarden Zinc Products
LaSalle Rolling Mills which sort of seems to still be in business (in 1996, 90% of its work was cent blanks) (May now be known as Zinco)

 

At this time, Jarden is the sole provider of copper-plated zinc blanks for the US Mint.

 

Huh? The plating is almost ALL of the 2.5% Cu

 

You are correct...


Per the Mint's specifications, the cent is 19.05mm in diameter (9.525mm radius) and 1.52mm thick (height) at the rim.

It's never that simple

The alloy is 97.5% Zn and 2.5% Cu.

• Zinc has a density of 7.0861 g/cm^3
• Copper has a density of 8.9286 g/cm^3

An alloy of 97.5% Zn / 2.5% Cu (by mass) would have a density of 8.8808 g/cm^3. And a cylinder of 19.05mm x 1.52mm would weigh 3.847g.

BUT the mint specification is 3.11gm (and we know most of the coin does not reach the height of the rim or it would not stack).

Simple math gives us 3.11/3.847 is 80.83% or an AVERAGE (height) of 1.229mm.

Then applying the volume of the cylinder formula (v = Pi * r^2 * h) gives us the volume of 350.3 mm^3

The volume of the zinc core is that of a cylinder 20 microns smaller (9.505mm x 1.189mm - you subtract 20 microns from the radius, but 2x 20 microns from the height) gives 337.5 mm^3 and a weight of 3.00g.

The volume of the plating is the subtraction of the two: 350.3 - 337.5 = 12.8 mm^3 or 0.11g.

So the 20 micron thickness is basically proven.

 

That's the mass of the old cents. The Zincolns are 2.5 g. Fortunately, you switched around your alloy accidentally to be close to the old cents when you computed the density to allow you to arrive at the thickness. Unfortunately, your mass of the plating is too high. I come up with an average (i.e., pre-striking) thickness of 1.223 mm. In my post above, I put the wrong number in for density of copper (not sure where I got it from). If I fix it, and change the coin thickness from 1.52 to 1.223 (surface area becomes 643), then the plating thickness comes out to about 10.9 microns.

 

D'oh ...

19.05mm diameter, 1.23mm thickness... volume is 350.52 mm^3

Zinc mass is then 2.4375g or volume of 343.98mm^3

If we call the plating thickness p, height h and radius r (1/2 diameter d) we get the standard formula:

v = pi * (r-p)^2 * (h-2p)

from which I make it 10.18 microns of plating...

 

Converting microns to inches :

10.18 microns /25400 = 0.0004007874 inches

ToddB67

 

Actually, given the accuracy of the 10.18 value is accurate to just 2 significant digits (limit of the mint's specifications), you would have a hard time saying better than

0.0004"

10.185 / 25400 = 0.00040098425
10.175 / 25400 = 0.00040059055

 

Thanks Burton !

So I presume you are suggesting that the thickness might even be thinner than 0.0004".

Compare that to the average size of the finest hair (Flaxen), 0.0007" to 0.002". http://hypertextbook.com/facts/1999/BrianLey.shtml

Even with moderate mineralization, it's no wonder post '82 pennies come out of the ground "ate up" !

ToddB67

 

I am uncertain if it is acceptable to comment on an older thread.

Zinc Cent Plating thickness = 8µ, according to:

ALTERNATIVE METALS STUDY Contract Number: TM-HQ-11-C-0049 FINAL REPORT August 31, 2012 by Concurrent Technologies Corporation, Submitted to: United States Mint, Page 1, Section 1.1

 

Sure, Why not. But not all the members that were here in 2017 are still around. Some are.

Thanks for the information!

 

Thanks!

Rumor has it that Plating on proofs is thicker and that they were plated twice. I find no reliable source for that information and hope someone can provide a reference

 

It’s just amazing to me that the plating stands up to the metal flow of the striking process. At all. My brain says it should look like marbling after the strike. Obviously, it does not.

Rick L.

 

have you ever seen split plating and all the other anomalies ?
I've forgotten the striking pressure (I think it's 35 tons for the penny), but it prevents it from looking weird (until the die wears, or splits, or needs maintenance, etc).

 

I have been studying Copper Plating Dynamics. Here is a rough draft of what I have learned so far. (I have never tried to post a .pdf here so not sure if it will work!)

I would be interested to see any examples of cracks, splits or ripples anyone is willing to share!

 

The contractor that supplies “blanks” to the mint for cents delivers RTS “blanks” (RTS = Ready To Strike). This means that the contractor passes the “blanks” through an upsetting mill. I have noticed that the contractor does not seem to know the word “planchet” as applying to blanks after they have passed through upsetting, so understanding the process may be confusing!

“When the material arrives in planchet form, the supplier has handled all those previous steps, and all that remains for the Mint is to strike the planchets into coins (as is the case with the one-cent).” 2014 Biennial Report to the Congress, as required by The Coin Modernization, Oversight, and Continuity Act of 2010 (Public Law 111-302), United States Mint, Department of the Treasury , December 2014, Page 4