I’m baking gold coins of different pureness in my kitchen oven, watching (with an IR camera) as they cool, and recording the length of time each coin takes to cool down.
I believe I’ve figured out a way to not only determine the karat (pureness) of gold coins but also a non-invasive, non-damaging way to test precious coins and detect fake coins without scraping them or using chemicals.
Obviously I’m in the wrong business as I only have some pocket change (see attached) to work with! You are on to one of the most challenging concepts any thermographer faces, Nick: thermal capacitance or the amount of energy required to heat a material (or the amount it gives off as it cools).
Interestingly, the “Midas touch” is only so good! Plain old water actually has a greater capacitance than your gold or my pocket change. Amazingly this is true whether based on volume or weight.
As buildings are heating up or cooling down—and they nearly always are—we must understand the capacitance issues to make sense of the thermal signatures. They can either help us or make our lives miserable. A 4" wooden framing member, for instance, can take up to 4 or 5 hours to come to equilibrium when a wall changes temperature.
By the way, the coins in my visual image are a set we work with to demonstrate the challenge of measuring temperature or emissivity of any “bright” metal.
As for your experiment, it is a good start but I would urge you to bake more cookies and try more coin experiments as I think you may have some other “unknowns” beside the karat weight!
ASNT NDT Thermal/Infrared Level III #48166
The Chinese regularly pump out fake U.S. silver dollars. They look just like the real thing but are not made of all silver. I have a bunch (yes, I got snookered). I’ve already convinced myself that I can detect these counterfeits with an IR camera and a kitchen oven.
I don’t think I read the whole thread correctly. I just went to the oven to check on the Silver Certificates I put in about thirty minutes ago and I am sure they are Chinese copies, as all that are left is ashes.