Please Post Your Coins Which Have A Hemispherical Depression

Fascinating confirmation, Pete. This immediately pushes me into contemplating metal flow during the instant of strike. Some of these marks are located so close to devices that one has to imagine the area should have contributed to the formation of the device. You'd think the round marks would elongate in the direction of metal flow, and the only way to fill devices is if pretty much everything flows.

I think after today's financial carnage we are facing a hemispherical depression

This is really exciting. I have been waiting for an opportunity to do some more buying! I remember very well the crash of '87! The Dow was somewhere around 2600 then I think. We used to have an analyst in our firm who said 10,000 by 2000. That was very hard to believe!

I thought that I would address the notion that it may be possible for the hemispherical depressions seen on some Lincoln Cents to have been caused by a BB strike. I have done a few calculations to try to see what a BB strike would need to look like. The calculations are easier if the imagery is reduced to a circle with a chord, representing the surface of the coin, cutting through it (rather than a sphere with a plane cutting through it).
The formula to calculate a chord length = 2 x the square root of (the radius squared - the distance of the chord from the center of the circle squared). (It is a lot easier, even, to use a Chord Calculation site such as: https://www.easycalculation.com/are...h-circle.php

I invite you to check out my math.
Imagine a sphere which makes an indentation on a blank. If it indents ½ way (a full hemisphere), then it leaves a mark with a diameter = the diameter of the sphere. If it indents ¼ (a ½ hemisphere) then it leaves a mark of approximately 2/3 the diameter of the sphere.

The caliber of a normal BB gun is 4.5 mm. The normal diameter of a BB is 4.4 mm.

For a Rockwell Ball indenter, "B" Scale, the diameter is = 1.58 mm

The diameter of many of the marks we are seeing on these Lincoln cents is in the neighborhood of ½ to 1 mm.

The depth of the mark is what translates into the hardness number and also what translates into the diameter of the mark. The strike of the die will also adjust the diameter a bit and may also give it some elongation.

For a "B" Scale Rockwell indenter, a 1 mm diameter will represent an indentation of approximately ½ of the hemisphere. This is enough to leave rather steep sides to the mark.

For a BB to leave a 1 mm diameter indentation, the depth of the indentation must be only about 1/20 of the diameter of the BB - which would leave an extremely shallow slope to the sides of the mark.

This means that most of the marks we have been thinking are BB strikes have sides which are too steep to be made by a BB. They are, in other words, too deep to be made by a BB and also leave only an approximate 1 mm diameter depression. The 1 mm diameter depression left by a BB strike would be almost flat! A smaller diameter mark would be even flatter and very shallow! In addition, a BB would most likely leave a pressure ridge around the circumference of the depression. It may also leave a raised area on the opposite side of the coin.

I have made a crude drawing in an attempt to illustrate this. It is not exactly to scale, but I think that it is close enough so that the concept can be understood.

I have been working on planning and devising a test which can show, with a higher degree of certainty that a specific indentation originated with a Rockwell Test. I realized, after making my drawings for a previous post, that the shape of the indentation is critical in establishing the possibility of it being a Rockwell Test Mark. It should conform to the shape of the sphere of the Rockwell Ball Indenter! By placing such a sphere beside and into an indentation, I think that we should be able to say either that the indentation does not conform to the shape of the sphere and thus could not have been made by a Rockwell Ball Indenter OR that it does conform to the shape of the sphere and thus the odds that the indentation was made by a Rockwell Ball Indenter increase exponentially.

I would appreciate receiving any questions, comments or reasons for doubts. They can be quite helpful for doing follow up research.

Here are 2 photos of the mark on one of the coins, beside which is placed a 1/16" steel ball - the same size as a Rockwell Indenter. I am led to conclude that these indentations were not made by a Rockwell Ball Indenter. The sides of the indentation appear as if they do not conform to the shape of the ball. Please look at the curvature of the steel ball and tell me if you think that it does or does not match the curvature of the hemispherical depression. I am thinking that this depression was made by something like this steel ball, but with a much smaller diameter.

{See my following post which suggests that my judgement here is faulty. I should have run that little test first! It seems to suggest that these indentations can, indeed be made by a 1/16" steel ball!}

Seems to me that it should! How does one factor in for that? The more that I try to think about this, the more confused I become!

But I may have the next best thing. I am preparing another post where my pendulum is swinging back the other way ---> to think there is an argument that these are Rockwell Test Marks. Stand by.

You can clearly see, even in the photograph, that there is a raised area around the mark.
It does look close to the same size though.