Why do metals smell?

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Chris Bradley
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Why do metals smell?

Post by Chris Bradley » Fri May 21, 2010 4:39 pm

I tend to relate vacuum suitability with how much a material smells, on the basis that if you can smell it then it is emitting volatile substances even at room temp/pressure.

So why is it that some metals seem to smell more than some polymers? I can't smell anything from HDPE, but some metals like copper, nickle, zinc and iron have distinct smells. If I can smell copper at room temperature, then why does it make a suitable UHV gasket?

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Doug Coulter
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Re: Why do metals smell?

Post by Doug Coulter » Fri May 21, 2010 6:56 pm

As a machinist (hobby) I've noticed too and wondered the same thing. There may be a decent explanation, which I will hazard here with absolutely no backing.

I often have to get metals hyper clean for electroplating, and that's a level of clean most people can't imagine -- ten seconds of air exposure ruins it again. I use mechanical and chemical methods to get this done, and usually even allow slight amounts of the chemicals to get into the plating baths, as overlong rinsing in distilled water allows enough atmospheric contact (and gas dissolved in the water) and re corrosion to ruin the results. Plating is just plain picky about that if you want it to last more than a month or so.

You can *really* smell the metals at that stage, quite strong, and yes, each is pretty distinct, though they are all in a class of "smells like a metal" as well. Interestingly, this isn't as noticeable in most grinding or say lathe cutting operations -- it's there, but much less, even when you are inhaling enough metal to make you sick (don't grind or weld zinc for example without some good ventilation).

http://en.wikipedia.org/wiki/Metal_fume_fever
This is like having the worst flu ever, all compressed into a couple hours. Not fun, every welder has it happen (at least) once, and after that you get to be much more careful and use HCL to strip off even thin galvanizing before welding or even hard grinding.


What I think might be happening during that cleaning process that makes you notice is that you have microscopic "whiskers" and protrusions because the action of the cleaner takes out inter-grain stuff and leaves these behind. At that cleanliness level the stuff is very reactive (which might be why it's required to do good electroplating). Since oxygen and most metals combine exothermically, and oxygen is moving pretty quick at room temperature, I'm guessing that this combined with the exothem can knock atoms right off the surface as sub-oxides, and that's what you smell. It's certainly something everyone around here notices the first time they witness serious metal cleaning in action. I don't notice it near as much in lathe work, as I think the rubbing effect of the tool past the cut smoothes over these microscopic protrusions. It's noticeable but not as much in microfine sandblasting too, depending on the blasting media.

On the other hand -- for things real low vapor pressure -- I have never detected the slightest odor coming from (good) vacuum pump oil....Or diff pump oil, even when it is warm. Ditto vacuum grade beeswax, hard shellac, apeizon wax and so forth. Well cured epoxy as well, though that "well-cured" is a serious caveat, and learning how to really mix the stuff well enough for vacuum work is an earned skill to say the very least. Brings a whole new meaning to the word "thorough".

Another observation is that things contaminated with something very stinky sometimes turn out fine after only a little vacuum exposure -- if it comes off that easy, it all comes off quick, and is now gone.
Acetone might be a good example there (except in viton where it can soak in real deeply).
But I've also used it to rinse out mech pumps I was rebuilding, filled them with new oil, and in an hour they are fine -- all the acetone just got gone quick. Toluene can stick around a lot longer.

One last observation from out of the blue -- try sandblasting with silicon carbide in the dark sometime. It lights up bright orange....why is left to the student -- and why not with any other media.
Why guess when you can know? Measure!

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Steven Sesselmann
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Re: Why do metals smell?

Post by Steven Sesselmann » Fri May 21, 2010 10:46 pm

Chris,

This is a good question..., so I googled it, and faound this article.

http://www.physorg.com/news82229855.html

It says that metal has no smell, becayse there are no metal atoms in the odeur. the smell apparently results from chemicals in your skin coming into contact with the metal and reacting.

Another suggestion I would like to add, is that metal being a conductor might, when held, create some kind of conducting loop between your hand and your nose, and that a slight amount of natural ionization might be responsible for the apparent smell. In my opinion the smell of metal is similar to the taste you experience, when you put your tounge against a 9 volt battery, indicating to me that a current is involved.

So if a clean piece of metal has an odeor without being touched, then it appears that we don't have a clue

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG

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Chris Bradley
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Re: Why do metals smell?

Post by Chris Bradley » Fri May 21, 2010 11:04 pm

Steven Sesselmann wrote:
> Another suggestion I would like to add, is that metal being a conductor might, when held, create some kind of conducting loop between your hand and your nose, and that a slight amount of natural ionization might be responsible for the apparent smell.
I love your thinking, Steven. I can't tell if it is genius or madness on occasions, but it's left-field and that's why I love it!


> So if a clean piece of metal has an odeor without being touched, then it appears that we don't have a clue
Well, that's the thing.... I'm sure I can smell different smells from different metals when I am cutting or machining them. So I was wondering whether I am smelling oxides, or some other type reaction.

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Frank Sanns
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Re: Why do metals smell?

Post by Frank Sanns » Sat May 22, 2010 1:53 am

Smell and taste are THE worst senses to rely on. If sight were as unreliable as smell and taste, there would be invisible items all around us that we would be bumping in to.

Something like natural gas (methane) has no odor. Dangerous concentrations could exist to the point of explosion or suffocation if it were not for a trace amount of the additive methyl mercaptan. Smelling an item and trying to predict its vapor pressure or its toxicty is futile at best.

Metals also present another interesting trait; they catalyse many chemical reactions. Different metals will cause different reactions. It is entirely possible that one person will handle iron and then touch aluminum and have two completly different smells especially as time passes. It will not be the smell of the metal itself though but rather surface chemicals. There are also oils that are very often used when items are produced and much of virgin metal smell are these oils that are used during the extrusion, cutting, or forming processes.

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Re: Why do metals smell?

Post by John Futter » Sat May 22, 2010 4:11 am

For a start I don't know!!!

but when I open our implanters @ work I can usually tell what metal was used in the ion source without looking at the book as to what has been done.

both selenium and Asenic smell like garlic with selenium having a metalic touch as well, Erbium smells like an unwashed rubbish bin,
potasium has a bite in the back of the throat but I cannot describe it further.
Chromium, Nickel, zinc oxide, ITO, copper erbium Hafnium, europium, dysprosium scandium, titanium, all are different in smell from each other and hard to describe.

remember all these have been under high vac until the ion source is removed. I presume that some of the ions become aerosols and of course oxidise, nitride, nitrate and the human tougne /nose pathetic as it is detects these products.

FWIW

edit
elements used so far
H D2 He Li B C12 C13 C14 N14 N15 O16 O18 Ne Na Mg Al Si P S Ar K Cu Fe Co SM SC Er Eu Yt Yb Zn Ds Au Ag Ni Hf Gd Ga Ge Cd Ca Va Nb Xe Rb Pb plus a few I've forgotten

The horrid group of F Cl I etc will have to wait until our negative ion source comes on line we have tried with the positive ion source but not an ion emerges!!

Radioactive elements have not been used so far due to the difficulty of of cleaning the implanter

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Re: Why do metals smell?

Post by bpaddock » Sat May 22, 2010 7:34 pm

> So why is it that some metals seem to smell more than some polymers?

The late Dr. Riejo Makela had an interesting take on smell and other items:

"Smell: Actually it is assumed that smell is perceived when the
molecules, aromatic substances having a definite size and
corresponding to the shade of smell, get into the appropriate holes of
the mucous membrane of the nose. Wrong. The writer has shown that an
odor is the electromagnetic radiation of the molecules and atoms,
which will change the electromagnetic state of the receiving mucous
membrane. This change in electromagnetic activity will be transmitted
to the corresponding center in the brain. Some dogs can "smell" the
infrasounds which can be measured in seawater before a storm. - "Odor
analyzers" can be used as an early diagnosis of many illnesses."

See "Living Cells Are Electromagnetic Units" http://www.earthpulse.com/src/subcatego ... subcatid=4 for the full text.

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Re: Why do metals smell?

Post by JohnCuthbert » Sat May 22, 2010 8:01 pm

Let's just say that idea wouldn't get through peer review in most journals.

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Frank Sanns
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Re: Why do metals smell?

Post by Frank Sanns » Sun May 23, 2010 1:01 am

Good input from your experiences.

My conclusions are the same though. A metal does not have sufficient vapor pressure to be in the air at any concentration that would suggest detection by smell. There just are not sufficient atoms based on anything else that we smell. The vapor pressures are orders of magnitude too low. Can you smell mercury as it has a super high room temprature vapor pressure (orders of magnitude higher) compared to most of the other metals?

What I think is happening are small particles or even single atoms of the metals that you are working with are still present when your chamber is opened. Sure they are diluted greatly with nitrogen or air but they were present in ultra high numbers during your implantation process. I could imagine a very small number or even a single atom of a metal could produce some electrolytic or chemical reaction at an olifactory center. I just can not imagine many of them coming off a solid chunck of tungsten just sitting on a bench at room temperature.

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Re: Why do metals smell?

Post by Brett » Sun May 23, 2010 1:25 am

No kidding; The functioning of the sense of smell is pretty well explored, and is very closely related to the way the immune system generates a huge variety of anti-bodies, which is why the sense of smell can identify so many different chemicals. That "electromagnetic theory of smell" is quackery.

I, too, have noticed distinctive odors around freshly machined metal in the shop. While the vapor pressure of most metals is vanishingly small, I suspect that freshly machined metals may be shedding extremely small clusters of atoms by a process unrelated to evaporation. Such tiny clusters are known to have powerful catalytic properties; Maybe what we're smelling isn't the metal itself, but some reaction products in the mucous that the metal clusters are catalyzing the production of.

Single atoms of metal, on the other hand, lack such catalytic properties, which may be why mercury hasn't any odor.

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