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RE Mixtures of Metals
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To Keith P. Walsh,
First, insults will get you nowhere.
Second, this is a voluntary list. I myself travel a bit and am not always
available to answer questions. Sometimes it takes me a while to reply,
even when I know the answer. Which, as often as not, I don't.
Third I can't speak for the International Thermoelectric Society, but as
for myself I admit to knowing next to nothing about dentistry.
Fourth, since you didn't mention the composition of dental amalgam I did a
quick Web search and found
http//www.sonic.net/~nexus/amalgam.html
"Amalgam, or what dentist sometimes call "silver fillings," is made from
fifty percent mercury, thirty-five percent silver and fifteen percent tin,
or tin mixed with copper, and a trace of zinc."
I've never seen data on the Seebeck coefficient of amalgam, but from this
composition I'd expect a value less than say +-10 microVolts/K.
Conceivably, you might get 30 oC from eating ice cream or other frozen
foods. That would give you a maximum voltage of about 0.3 millivolts.
Mind you, I doubt very much that the actual thermal emf generated is
anywhere near that high.
I'm also not a physiologist, so I have no idea if any receptors in the
mouth could sense 0.3 millivolts.
I would expect much larger effects from 1) the thermal conductance of the
amalgam and 2) chemical reactions. In the first instance heat goes right
down the amalgam to the root of the tooth, which I suppose might hurt. In
the second instance, various food stuffs and organics in the mouth can be
expected to react (however slowly) with amalgam, which might have bad
consequences.
OK, thats about all I can think of. I'm just a thermoelectrician. It
occurs to me that dentists put in millions of amalgams each year. Why
don't they already know such properties of these things?
Hope that helps.
ZU,
Cronin
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>-----Original Message-----
>From 10065 [dot] 2574 [at] compuserve [dot] com [mailto10065 [dot] 2574 [at] compuserve [dot] com]
>Sent Saturday, October 23, 1999 902 AM
>To news_group [at] seebeck [dot] zts [dot] com
>Subject Mixtures of Metals
>
>
>
>It is manifestly evident that the International Thermoelectric
>Society knows
>a great deal about the thermoelectric properties of mixtures of metals.
>
>Nevertheless at the same time it appears that the
>International Thermoelectric
>Society knows nothing at all about the thermoelectric
>properties of the mixtures
>of metals used in dentistry.
>
>Is there a rational explanation for this apparent scientific anomaly?
>
>Keith P Walsh
>
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Cronin, I'd like to ask a
Cronin,
I'd like to ask a couple of further questions about the comments below which you made some time ago regarding the possible thermoelectric behavior of dental amalgams:
<< Fourth, since you didn't mention the composition of dental amalgam I did a quick Web search and found http//www.sonic.net/~nexus/amalgam.html "Amalgam, or what dentist sometimes call "silver fillings," is made from fifty percent mercury, thirty-five percent silver and fifteen percent tin, or tin mixed with copper, and a trace of zinc."
I've never seen data on the Seebeck coefficient of amalgam, but from this composition I'd expect a value less than say +-10 microVolts/K. Conceivably, you might get 30 oC from eating ice cream or other frozen foods. That would give you a maximum voltage of about 0.3 millivolts. Mind you, I doubt very much that the actual thermal emf generated is anywhere near that high. >>
I appreciate that when you made these comments you were only guessing anyway, but I wonder if in your assessment you have failed to take into consideration the inhomogeneous nature of the internal structure of dental amalgams.
The thermoelectric behavior of a material is dependent not only the respective percentages of the component elements which make up its composition, but also (and perhaps even more significantly) on how these component elements are arranged.
As a result of the process by which they are formed (i.e., mixing grains of a solid alloy together with liquid mercury at room temperature and allowing the mixture to harden), amalgams have a much greater degree of material inhomogeneity than other types of mixtures of metals such as alloys (where all of the component metals are mixed thoroughly together in their molten states before being allowed to solidify by cooling at a controlled rate).
That's the reason why amalgams are called amalgams and not alloys.
You can see a graphical representation of the internal microstructure of a typical dental amalgam at:
http://book.boot.users.btopenworld.com/setting.htm
Clearly, although the overall mercury content of the material may be 50%, a significant proportion of its volume, as represented by the "unreacted cores" of the solid grains of alloy used in the mixture, contains no mercury at all.
Were you taking this feature of the material into account when you made your initial assessment of its thermoelectric behavior?
And, if not, do you think that it would make any significant difference? For example, would you expect the application of a thermal gradient accross the material to produce thermoelectric eddy currents to circulate around the "unreacted cores"?
Regards,
Keith P Walsh
RE Mixtures of Metals
"Cronin B. Vining" wrote
>It occurs to me that dentists put in millions of amalgams each >year. Why
>don't they already know such properties of these >things?
>
Cronin,
This might be because no-one has ever bothered to find out what the thermoelectric
properties of the mixtures of metals used in dentistry are.
The only other possible explanation that I can think of is that someone has
found out and decided that dentists need not be told.
Dentists sometimes screw metal alloy retaining pins into the root sockets
of a patient's tooth and then encase the heads of the pins in dental amalgam
(Ref.1), which is itself an inhomogeneous mixture of dissimilar metals (Ref.2).
I believe that the dental profession should therefore have carried out experimental
investigations in order to verify that the thermoelectric voltages generated
by the mixtures of metals used in dentistry are not able to dissipate electrical
energy through the nerves in people's heads.
Do you agree?
Regards,
Keith P Walsh
Ref.1 Newsgroup correspondence, sci.med.dentistry, 14 Oct 1999
Ref.2 Introduction to dental materials, R VAN NOORT,Mosby, 1994