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Thursday, July 1, 2010

Worthless or Gold? ID of an odd bean-shaped coin: probability theory, measurement, and density/specific gravity.

I mean that literally--wothlesss junk coin, or is it an electrum (EL) "Hekte" (1/6th Stater, the standard denomination in the Greek world, though it varied by city-state/island)? These were some of the earliest coins, and often looked like little more than little beans, just made of a silver-gold alloy.

To set the context, for anyone unfamiliar with these, check out THIS PAGE (http://www.wildwinds.com/coins/greece/ionia/uncertain/t.html) to see how crude these coins appear and how one might end up looking like the coin in the slide show (especially considering that it might be a fourée coin--ancient forgeries, collectibles in themselves--and had been test-cut repeatedly because it "didn't feel right.") Notice that those are uncertain, minimally documented, mysteries of ancient coins. Who made them? Too crude to be sure.
Slide show (run your mouse over the image to see the reverse and side views):




If you don't have Flashplayer, see the exact same images HERE and HERE. Also try my public gallery HERE.)
"Why is it important to learn statistics," students used to ask me back when I taught. Answer: Because whatever your educational background, formal or informal, your tools of experise come in handy in ways and places you cannot predict--even in coin collecting. (I wish I understood this when I took "hard sciences" and engineering classes.) Fortunately, my statistical training at Wisconsin, Arizona, and UCLA came in handy here...
That, plus my experience as a youth haging out in Tucson's (now closed) brick-and-mortar Glass Shop Coins, the location of a dazzling private/commercial collection of ancient coins.

Opinion has been split on this coin. Some people think it's a fourée (a fake ancient coin meant to imitate the real thing) filled with lead (heavy enough to feel almost like Electrum) and covered in a thin layer of gold or EL, others think it's an Electrum 1/6th Stater, and some think it's a shiny piece of brass. I doubt the last, but I don't think it's identifiable to enough certainty to call it one way or the other. I lean toward EL, but if my measurements were off, it could be a fourée (still, a highly collectible coin).

For this one, since it's hard to tell by looking, and the size is consistent with such a coin (13-14mm, 2.8g), I had to go into more depth: measuring specific gravity, more commonly known as density.

I don't have the scientific equipment to measure specific gravity(called a "hydrometer" I think), so I did a DIY test: I filled a 1/8 teaspoon with as much water as it took to spill over (like 1.7cc or something like that). Next I emptied and added the coin. I then put the coin at the bottom, filled an oral cat/baby syringe with 1.7cc of water, and added until it spilled over. I took repeated measures (10 total; range = ~0.1cc - ~0.35cc -- extremely wide!) and got an average of exactly 0.2 cubic centimeters. That means the density, if measured accurately, was 14.0 grams per cubic centimer.

The reason for repeated measures is based on probability theory: the astronomer (and general scientist, including social science) Adolphe Quetelet recognized in 1835 that no matter what you measure, if you do it over and over again, you find that the measures vary (this is true even in the most precise modern statistics). Therefore, if you do it enough times, you expect that you get closer to the "correct" figure.

On specific gravity:
Less dense, less valuable materials:
Brass and Bronze are about 8.5, depending on alloy (min 7.4 – max 8.9). Copper about 8.9.

Denser, more valuable:
Silver 10.5. Gold 19.3.
Electrum 13-16 grams/cubic centimeter

So it would seem this coin is electrum, right?
Please give me your opinions, in favor or against my EL hypothesis.

1 comment:

  1. I hope he doesn't mind me quoting, but I got a fairly authoritative response from a serious internet expert:

    Doug Smith, on Cointalk.com
    "I see the lion head right on the top two and left on the bottom two. If all are the same item with different orientation and lighting this just proves we see what we want to see. I have never seen one that looks this bad and would have expected electrum to have weathered differently but have no experience to back up that thought. Unless you specific gravity test is off, it hard can be anything else but I'd stop short of saying the item is identifiable as is. A serious reservation is how the coin could be overweight and look that bad. Were it mine, I'd keep it as an educational item but would not be able to sell it as what I thought/hoped it was. One serious student I know calls a coin like this a 'ghost.'"


    Thanks Doug Smith!!!

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