Why do metals smell?

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Chris Bradley
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Why do metals smell?

Post by Chris Bradley »

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 »

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.
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Re: Why do metals smell?

Post by Steven Sesselmann »

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 »

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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Re: Why do metals smell?

Post by Frank Sanns »

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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We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Re: Why do metals smell?

Post by John Futter »

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 »

> 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 »

Let's just say that idea wouldn't get through peer review in most journals.
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Re: Why do metals smell?

Post by Frank Sanns »

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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We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Re: Why do metals smell?

Post by Brett »

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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Re: Why do metals smell?

Post by JohnCuthbert »

Many, if not most, finely divided metals are spontaneously flammable in air. Clusters would be even more reactive.
Cleaving a bunch of atoms from the bulk would take more energy than taking a single atom. The vapour pressure is so low that we know the number of atoms coming off a metal is practically zero. The number of clusters would be even less.

Some metals like selenium, arsenic and osmium have strong smelling oxides that are fairly readily formed in air and would explain the odour associated with them.
There's a definite smell associated with working aluminium. It is due to things like arsine and phosphine formed by reduction of the metals present as impurities in the Al in the presence of atmospheric water.
The odour of iron and copper has been explained as due to decomposition products of skin oils formed in the presence of air and the finely divided metal.
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Re: Why do metals smell?

Post by Doug Coulter »

While Frank is correct that a lot of things are (mostly fortunately) odorless to us, that's far from the whole story, and in fact, smell can detect levels bordering on a single molecule. That is fact, well proved, in many sorts of animals (and non animals) that have some sensitivity of which humans are just one of the samples, and we're not the best at this by far in the living kingdom.

In the '70's, when I was an engineer for a beltway bandit (ENSCO), we worked on a project for the forerunner of DHS (FBI and the DOD security crew) that used rats to detect high explosives. Yes, really. At the time, the only thing that could match a rat's nose for sensitivity was a combination of GC and mass spectrometry, and it took best of breed of both together to rival a rat's nose. Neither portable or quick enough for the requirement.

You could train them to get excited when they smelled TNT etc by the usual methods every experimental psych student learns, but there turned out to be a glitch. They were so sensitive that you could only use the same cage, or even lab just once!

We were using EEGs to detect their excitement when they sniffed a TNT sample, and at first it worked pretty well. The Air Force (Rome labs) then contracted out for bulk rat training since the experiment/prototype had been so successful. Well, the next trainers weren't quite as careful, and couldn't get rats trained correctly, as they contaminated their own lab with ~ fraction of a milligram of TNT and from then on, the rats they tried to train *always* smelled TNT and so learned to cue off other things in the experimental procedure that were irrelevant in the field where the rat was in a dark suitcase with a little vent fan and a Z80 processor and analog amps. This was proved when we went back to the original, more careful lab for another try (they were using techniques similar to serious bio-hazard or chem warfare as their norm) and it all started working again.

Needless to say, something so hard to do and fairly impractical didn't make it into mainstream use, but it's a true story just the same. Rats (and some other more high maintenance animals) are so sensitive *and* selective (for some things) that it becomes the problem rather than the solution.

Feynman had some good words on rats everyone should look at that kind of show the problems.
I can provide text of the lecture to any interested.

Humans aren't as good, and we don't train ourselves (mostly) as well as a being who may depend on the skill for survival moment by moment more than we do, but that doesn't make the fact that you (and a lot of people) *can* smell metals and even tell which sort bunk -- the proof lies right here and in a lot of other places.

The fact that it tends to happen most when the metal has just been made really clean indicates that the well known fact that super-clean surfaces are highly reactive takes part in this. Remember, it only takes a few molecules, as in a single-digit number, if it is something you are sensitive to. So all the usual arguments about low vapor pressure, non-reactivity have to be viewed in that light. Could be that clean metal reacts with some rare air component (say a sulfur or chlorine atom) and then does have a higher vapor pressure, for example. Some people can smell the difference between clean and dirty copper, there must be a reason, because IT EXISTS. Calling it bunk does not make it go away.

I can detect fusion even if only a tiny fraction of the D's in there get involved, too -- with the right gear, and that gear is mostly less sensitive than the nose is in terms of input particles per successful detection. See many of Carl's posts for detector sensitivities that work out to big negative exponents that still work fine for us. What's so different here? Nothing at all. We are simply debating the mechanism, not the existence of the effect.
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Re: Why do metals smell?

Post by JohnCuthbert »

Can you cite evidence for the "single digit" molecules for odour detection in humans?

Also, since it's documented that, for example, dogs have at least 10 fold better sense of smell, does this mean that they can detect a few tenths of a molecule?
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Re: Why do metals smell?

Post by Chris Bradley »

Maybe there's a little hearsay in this thread but, hey, when did dogs ever smelt lead tin and copper? Maybe it's an evolutionary thing - if you could sniff a prehistoric smelting site and head towards it, then you were destined to evolve into modern man, whereas the neanderthals who did not smell it died out. Maybe it's just that the human nose has evolved to smell metals, as they have been so important to us? Just a thought!
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Re: Why do metals smell?

Post by Frank Sanns »

John,

The way I see it, a single atom could produce an reaction at an olifactory center if the atom finds its way to the exact proper location. If it falls near but not near enough then no productive reaction so statistic are in full play here.

Even if the atom or cluster of atoms do find a single smell detecting nerve, I see no reason why we would detect the "smell". Nerves experience noise just like any detectors that we may use. The body learns to ignore the background nerve firing so to get a meaningful stimulus, many neurons need to be involved which means many atoms or molecules or clusters.

I do not have a reference but I think the reason dogs can smell better is simply that they have a much bigger olifactory surface area (bigger detector) than humans. Bigger detector usually means more counts for a given concentration and better signal to noise ratio. I see no reason why physics should change for a biological detector over a non biologic.


Frank Sanns
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Re: Why do metals smell?

Post by JohnCuthbert »

It's perfectly possible that a single molecule or atom of a material landing on a receptor will cause a signal. However that's not the real question.
The question is how much stuff does it take before a person perceives the presence of a smell? (in fact, we are asking for a more difficult job- the identification of a particular smell iron vs. copper vs. Aluminium...).

A bit of Googling indicates that the lowest odour thresholds are about 0.01 ng/litre and these compounds have molecular masses of about 200.
That means we can detect ( given a good healthy sniff of a litre or so) about
30,000,000,000 molecules assuming I have kept track of the zeroes correctly
If anyone really thinks that's "single digit" they should be in advertising.

It's also possible that there are molecules out there that are smellier, but we would know about them.
There's no doubt that metals have a smell associated with them.
That smell is not the metal per se.
Incidentally, TNT has a vapour pressure of about 10 ppb so it's fairly well into the detectable range of odours- it just doesn't smell very strong to us.
Rats, like many animals, have a better sense of smell than ours.

Mankind has not been smelting metals for long enough to affect evolution so the lack of rats that smelt metals is a red herring.
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Chris Bradley
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Re: Why do metals smell?

Post by Chris Bradley »

John Cuthbert wrote:
> Mankind has not been smelting metals for long enough to affect evolution so the lack of rats that smelt metals is a red herring.
I don't dispute the possible irrelevance of my observation, but evolution can take place not necessarily through genetic changes but also through selection of existing traits. If some select group of a coastal species senses an earthquake is coming and makes for high ground, whereas the rest do not and drown in a tsunami, then that species will have selected for the complex attribute of 'sensing earthquakes and going for high ground' within a single day.

There is no greater evolutionary event than "not dying"!

A similar thing has been found with lactose intolerance, so I seem to recall. Before domesticated cow milk, lactose intolerance was widespread, but mostly died out, as a general genetic trait, within just a couple of generations for communities where cows were domesticated - all the babies fed on the stuff died! (which was probably nothing too unexpected anyway for folks in those days, just a slightly higher rate than an already high rate of infant mortality, I guess)

I was just speculating that, just perhaps, the size of one's organ doesn't necessarily dictate its efficiency at its particular task! Maybe the human nose is particularly sensitive to metal smells and needs be no bigger? A sensitivity in mass/volume is clearly just an approximation and will differ for different substances. My posit may be wrong, but I do not feel it is *so obviously* incorrect without some substantive evaluation.

I guess a simple initial test question would be - Steven S.; having worked with gold and platinum, d'you think you could smell which one was which in a blacked out room? I presume there would be no chemical intermediaries with noble metals?
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Re: Why do metals smell?

Post by JohnCuthbert »

When did an individual or an entire community last die out due to the inability to smell iron?

Babies are not lactose intolerant (where the definition of baby is someone young enough to subsist largely or solely on milk) no matter what their genetic make-up unless they have a very unfortunate lethal mutation.
Lactose intolerance only troubles those old enough to have been weaned.
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Chris Bradley
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Re: Why do metals smell?

Post by Chris Bradley »

John Cuthbert wrote:
> When did an individual or an entire community last die out due to the inability to smell iron?
I could invent scenarios, but they would be inventions. I'm not trying to convince anyone here, it was just a comment!

Seems like there are a load of guesses coming up as to what people actually smell when they perceive a 'metal' smell, so if others can make guesses then I feel at liberty to make some aswell!

Wikipedia says for iron, the smell is aldehydes released on contact with skin secretions. Well, that may be so but it doesn't smell like any aldehydes I know. If these metal smells are released chemicals, then it should be easy enough to prove by getting hold of, or reproducing a proposed chemical, and seeing if it smells like the metal.



> Babies are not lactose intolerant (where the definition of baby is someone young enough to subsist largely or solely on milk) no matter what their genetic make-up unless they have a very unfortunate lethal mutation.
Yeah!!... not any more...that's my point! I'll look out for a citation on this research. It was one of these studies looking at distributions of genes in the population, and by whatever means they have developed to analyse changes in gene patterns, they identified the lactose intolerance died out essentially instantaneously around the time of cattle domestication. The cause would seem pretty obvious!!
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Re: Why do metals smell?

Post by JohnCuthbert »

I don't mind guesses, but silly ideas like people being able to smell single molecules are just a waste of bandwidth.
Also, no baby mammal is lactose intolerant. All milk has lactose in so any baby born with this condition would die.
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Re: Why do metals smell?

Post by Chris Bradley »

Well, maybe it's not lactose, the memory begins to fail, but there's something about cow's milk that makes it fatally different to human milk in a [now] tiny number of cases. I'll be looking out for the citation...

But back to the topic; you've discussed the single-molecule thing more than anyone, so if it is so silly as to be unworth of mentioning, then you can drop it, if you like.

The other side to this isn't about the nature of the vapour pressure of metals, but that the nose detects smells [as I understand it] from the shape of the molecules it detects. Seeing as it can detect huge polymer molecules, what is the objection that the nose can detect ultra-fine metallic dust, as was mentioned before about having chunks of atoms coming off. In such a case, I would expect such small particles to have a uniform crystal structure and, so, would each have a distinct smell according to the crystal shape most likely to come off the metal as an aerosol.
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Re: Why do metals smell?

Post by Richard Hull »

This thread seems to have been milked to death.

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Re: Why do metals smell?

Post by Charles Snyder »

But the thread isn't really complete until we realize...

...no one will ever smell metal in this lifetime and possibly never because mother nature does not give up her smells easily unless the "lucky donkey" just happens to smell it one day.

I couldn't help myself after reading so many threads where Mr. Hull offers the final cripling blow to all of my fusion enthusiasm
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Re: Why do metals smell?

Post by JohnCuthbert »

Indeed, but it would be a shame to close it without giving Doug a chance to respond.
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Re: Why do metals smell?

Post by Carl Willis »

Indeed. What a thread.

Anyone ever "feel lucky" on teh Google (or even look at the goddamn thing anymore)?

1. Type "why do metals smell?" into Google. 2. Click "I'm Feeling Lucky." 3. An authoritative answer fills your screen. 4. About thirty posts of contention, low-information, non-information, and mis-information on a specialty hobby forum is thereby avoided.

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