[pct-l] Carbon dating

[yumitori@montana.com (Ron Martino)]

> Wait a sec -- are there any radiochronologists among
> us who can clear up this matter?  It's true that 1500
> years is "peanuts" on a geological scale,  but carbon
> dating (I assume that's C-14 dating--is there any
> other kind?) can only date things back to 50,000 years
> or so, and is supposed to be accurate within centuries
> if not decades, to the best of my knowledge.

	Okay, let's talk about how carbon-14 dating works. A certain percentage
of the carbon in the body of every living thing is carbon-14 (as opposed
to 'normal' carbon-12). Carbon-14 is radioactive, and has a half-life of
5700 years (meaning that after 5700 years, half the current amount of
carbon-14 has decayed into carbon-12. So, since the amount of carbon-14
in organisms is constant, we can date when something died by comparing
the difference between the amount of carbon-14 in a specimen with the
amount in living things.

	So the accuracy of carbon-14 dating is limited by the sensitivity of
the instruments used to measure the amount present. To get within a
couple of hundred years, you need to able to get within a percentage or
so of the correct amount of carbon-14 present.

 
> I can't say anything about the Civil War button.  If
> the story isn't apocryphal, there could only have been
> something wrong with the sample or how the test was
> performed.  However, it's by studying just such an
> obvious error as this that scientists learn to
> fine-tune their methods.

	So, now to our button. How could we be carbon-14 dating it? Well, first
it has made of organic material. Wood is a likely possibility. A metal
or ceramic button cannot be dated by this method. But the date will only
be accurate if the button was made from green wood, cut shortly before
being manufactured. If it were made from a long dead piece of wood, say
an old piece of furniture being recycled into buttons, then carbon-14
dating wouldn't measure the date the button was made, but rather the era
the tree was cut down.

	And as I pointed out earlier, accuracy is also a major concern. If the
sensitivity of your instruments is only +/- 2000 years then dating the
button to 1500 years old is within the margin of error. If it's +/- 200,
then there's a problem. Assuming the problem is with the method itself
isn't necessarily the most logical deduction. Doing so ignores such
issues as contamination and human error, for example.
 
	Ron
-- 

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