Thermal conductivity in identifying metals

[Dougie Webb]

Earlier today, I found some chunks of metal that looks like slag (I find a LOT of slag!). However, it has an extremely high specific gravity - around 10.5 (which is real close to silver), BUT the SG of lead is about 11.5 - and it seems WAY more likely that somebody let a couple of chunks of lead fall off the wagon rather than silver. My scale is pretty precise - but still...silver?! I don't know...

I read that silver has a pretty high thermal conductivity and that it melts an ice cube faster than most metals. Its conductivity is about 420 at room temperature, compared to about 35 for lead. I didn't have a piece of lead to test against, but I do have iron (75) and aluminum (240). The hunk of metal I found consistently melted the same size ice cube faster than both the Fe and Al. Not surprisingly, it melted way, way faster than the iron.

Now here's the thing. I can't *imagine* it's actually silver. But if it's lead (which is the only other thing that's even near as dense), then how the heck did it melt ice so much faster than iron, which has better thermal conductivity (not to mention aluminum)?

Any scientists out there? I guess my main question is this: Am I understanding thermal conductivity correctly?

Thanks in advance!

PS - The chunk of metal polished up real nice, but tarnished in bleach.

 

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[TheGreenBoy]

Try schraching it. If it is soft it's probably lead or some lead-tin alloy. Perhaps you should measutre thermal conductivity with more accurate experiment, usually it involves dedicated instrument. Alloy composition is best assest with athomic spectrometer, the instrument is usually present in some methalurgical labs in plants where metal castings are performed.

 

[Almy]

Thermal conductivity is how quickly heat moves through a material under a temperature gradient. Therefore something with higher TC would cool or heat faster that one with lower. If there were a heat source on one side of it, ice on the other would melt faster that for a lower TC material. However, your test seems like it is more a test of specific heat, the amount of heat a material can store per unit of mass. A metal with higher specific heat at a given temperature would be more capable of melting ice than one with a lower specific heat at the same temperature. Maybe that's more the property your melting test is estimating. You can check specific heat of metals tables to determine how iron, aluminum, silver and lead compare.

 

[Dougie Webb]

Thank you both.....and now my head is hurting worse than ever!

 

[ToddsPoint]

They make a machine to ID metals. Coin shops usually have one. Gary

 

[Dougie Webb]

They make a machine to ID metals. Coin shops usually have one. Gary

Yeah - maybe I should stop with my amateur geologist stuff and consult the pros!

 

[Jim in Idaho]

Yes...the ice cube method is a way to test for silver, and it works, but it won't differentiate between various alloys of silver. Watching the speed of melt when an icecube is placed on a silver plate, or coin is really interesting. The speed of melt is almost instant. Silver has the highest heat conductivity of all metals. Diamond has the highest of all gems, and is called "ice" because of it. Hold either against your lips on any day below 98.6 and it will feel cold.
Jim

 

[Dougie Webb]

Thanks Jim. One of the things I keep forgetting is that there's a high likelihood that whatever it is, it's probably alloyed with something else.

I got my soldering iron up over 800 and stuck it on there...it took a good minute or so, but it did start to penetrate the metal. It looks like the melting point of lead is 621 and that of silver is 1763...so that kind of confirms my suspicion that it's something much closer to lead.