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Using Liquid Nitrogen to clean coins- experiences and comments.
joefro
Posts: 1,872 ✭✭
Hello fellow forumites,
This may be pointless to most of us, but I thought I would share the following experiences with you. At work, I use liquid nitrogen for a variety of cooling purposes and have basically unlimited access to it. Now I know that everyone is against "cleaning" coins because of the long term adverse effects associated with chemical residue and such. However, most people agree that soaking a coin in mineral water to remove "gunk" is an accepted method of non-abrasive cleaning. However, I have read and has personal experiences where some soakings can take over a month to remove dirt and other debris.
LN2 being inert, I conducted the following experiments at work the other day with some pretty nasty coins I had laying around. I soaked the coins in the LN2 bath for about 3 mintues or so until the liquid actually wet the surface of the coin (to assure a mimimum surface temperature had been reached). The coins were quickly transfered into a warm/hot water bath. The massive delta T caused a sudden thermal expansion of the gunk and debris stuck to the coin (significantly different from the thermal expansion of the coin's metal), causing much of it to crack and flake off! The great thing about this is that LN2 is completely inert so it does not chemically affect the coins whatsoever, it is purely a thermal expansion effect that is causing debris to come off the coin. Therefore this won't work at all for cleaning off natural patina (definitely a good thing) as LN2 has even less solvent properties than plain water. It is only good for removing gunk and green grime that is often found on old coins in what appears to be a completely non-abrasive and non-harmful method.
I am curious if anyone else around here has tried cleaning coins using a similar method. I sure have never read anything about this before and am curious if it has been explored. 3 minutes in LN2 sure beats a month in mineral water, if you have access.
Joe
This may be pointless to most of us, but I thought I would share the following experiences with you. At work, I use liquid nitrogen for a variety of cooling purposes and have basically unlimited access to it. Now I know that everyone is against "cleaning" coins because of the long term adverse effects associated with chemical residue and such. However, most people agree that soaking a coin in mineral water to remove "gunk" is an accepted method of non-abrasive cleaning. However, I have read and has personal experiences where some soakings can take over a month to remove dirt and other debris.
LN2 being inert, I conducted the following experiments at work the other day with some pretty nasty coins I had laying around. I soaked the coins in the LN2 bath for about 3 mintues or so until the liquid actually wet the surface of the coin (to assure a mimimum surface temperature had been reached). The coins were quickly transfered into a warm/hot water bath. The massive delta T caused a sudden thermal expansion of the gunk and debris stuck to the coin (significantly different from the thermal expansion of the coin's metal), causing much of it to crack and flake off! The great thing about this is that LN2 is completely inert so it does not chemically affect the coins whatsoever, it is purely a thermal expansion effect that is causing debris to come off the coin. Therefore this won't work at all for cleaning off natural patina (definitely a good thing) as LN2 has even less solvent properties than plain water. It is only good for removing gunk and green grime that is often found on old coins in what appears to be a completely non-abrasive and non-harmful method.
I am curious if anyone else around here has tried cleaning coins using a similar method. I sure have never read anything about this before and am curious if it has been explored. 3 minutes in LN2 sure beats a month in mineral water, if you have access.
Joe
Lincoln Cent & Libertad Collector
0
Comments
Tom
<< <i>A little off topic but I'm curious- If you place a dollar bill into some liquid nitrogen, will the bill shrink? >>
Nope, Ive tried it. It just turns a little brittle for a while much like cloth and clothing when you submerge them. Pretty much only gases will have a noticable thermal expansion/contraction even at temperature differences this extreme. Physical properties such as brittleness, ductility and electrical conductivity are all greatly affected by these temperature differences, but not what you are stating. One exception might be long pieces of wire which can actually shrink a significant amount (from an engineering standpoint) when exposed to such extreme temperatures.
<< <i>Another question: If you dip a silver quarter/dime/half in liquid nitrogen, will it break or shatter if you bend it quickly with plyers or hit it with a hammer? I know that the new Lincoln cents that are made of zinc will break. How about a copper cent?
Tom >>
While it might break at a slightly lower force, again I don't think it would be significant. All of the coins I pulled out were still very strong and I couldn't have broken them if I tried. Metals generally don't weaken as much as other materials when exposed to such low temps. In fact, certain objects become harder at these temperatures. Many people have seen the demonstration where a rose is dipped in LN2, then shattered into a million pieces. However, did you know that if you take a banana and dip it into LN2, you can hammer a nail into a board with it!? This is due primarily to the thread-like nature of the banana material. When frozen, the internal nature of the banana results in a material much stronger than it is at room temperature. Sorry to be going OT but I love this stuff... but to answer your question...
You definitely dont need to worry about your coins breaking from this method.
<< <i>I'm not so sure your process doesn't damage the coin. I seem to recall some materials processes where cooling of metals changes their properties. I sure am interested in this though - thanks for sharing...Mike >>
You may be absolutely right. Here is an example of what you are saying. If you take an average rod of steel and austentize it (heat is up red hot) and then wave it around in the air until it becomes cool, you can still bend the metal back and forth like you could before. However if you take the same piece of metal and austentize it again, but this time quickly quench it in a beaker of room temperature water, the metal will become extremely brittle and snap in your fingers rather than bending. Rate of cooling greatly affects the crystaline structure of the metal and therefore its material properties such as strenth as well. You could be right and you are actually weakening the coins, but I can say that they are still strong enough that I cannot break them even with a lot of force. I guess when I said it did not damage the coin, I meant from an aesthetic/numismatic point of view. The coin definitely doesn't look any different as there are NO chemical reactions going on here. Think a nice slabbed Morgan could be worth less if had a different Brinell Hardness number???
Editted to add: Gotta actually get back to work now, I'll check back here to see people's thoughts later as this is very interesting to me.
Two very important words!... original surfaces!!!
<< <i>Two very important words!... original surfaces!!!
I really don't think this would alter the original surfaces of the coin at all. In fact LN2 is probably the least reactive cleaning agent you can use. Even water has solvent properties (it is the universal solvent after all) and cleans by acting as a solvent. This is different, the cleaning is purely thermal. Nitrogen will DEFINITELY not alter the surfaces of a coin as the air we breathe is composed of over 80% nitrogen. I guess you would definitely want to use pure lab-grade LN2, although I would imagine this is the easiest/only kind accessible anyway. Some people will say that a coins surface can become altered by heating to high temperatures (toning silver in the oven), but even then it is not the heat causing the discoloration, it is a chemical reaction that is being catalyzed by the heat that may not have occured without the heat or occured at a dreadfully slow rate. Any coloring/toning that occurs is most likely trace H2S found in the air, but the toning process has been expedited by heat.
Please let us know when and if you do! Take care...Mike
I would be curious on the effects to the luster. I understand your point about it not "cleaning" the coin, but what is it doing to the crystaline structure at the surface? I also wonder if the dipping into hot water promotes oxidation at the surface after the cold. The only thing I can think of to compare is if the cold openned up the pores(reaction sites) allowing oxygen to react when it hits the hot water (which has plenty of oxygen in it).
I would assume this only does anything if the coin has a build up of gunk. I would think just a small amount of gunk would be able to flex with the coin.
Sounds very interesting! Any chance of taking before and after scans the next time you do it? And, have you tried this on a variety of different coin types (metals)?
Great idea -- I like it! Maybe you need a control experiment: on the kinds of heavy gunk that LN2 works on, would soap and water (plus ultrasonic bath) also be as effective?
My suggestion for your next experiment is to try a copper cent with carbon spots. Will the spots pop off or be reduced in size? Inquiring minds want to know!
On a slightly related topic, I always wondered how supercritical fluids would work on coins, but perhaps they'd be too effective.... Got any at work? I don't.
Good luck,
John
I trust that Joe used gloves, aprons, and eye protection appropriate for cryogenic temperatures.
<< <i>Minus 270 Kelvin is pretty darn cold!
Very true, since 0 Kelvin is Absolute Zero!
Is imaginary temperature like imaginary numbers?
<< <i>Since I have had a great deal of experience with liquid Nitrogen and Liquid Hydrogen while working 35 years in a Petrochemical plant, I would say that you are very lucky to not have lost some of your fingers fiddling around with these products. We used these Elements in our processes, so I have no idea what they would or would not do to a coin, I know that at Minus 270 degrees Kelvin, they are extremely dangerous to handle.
Don't worry, I've taken all the appropriate safety courses regarding the handling of cryogens. Let me assure you I am not fiddling around here. It's actually pretty hard to lose a finger working around liquid nitrogen. NOT THAT ANYONE SHOULD TRY THIS BUT... did you know you can actually dip your bare finger in liquid nitrogen for a short amount of time with no adverse affects? This is due to the fact that due to the extremely low boiling point of LN2, it boils off before it comes in contact with your skin! There is essentially a nitrogen vapor bubble surrounding your finger that grows smaller and smaller as the air cools and the actual liquid comes closer to the object. Once the LN2 finally makes contact with the object (your finger) there is a short violent "eruption" as there is a rapid heat transfer and the actual liquid finally makes contact (~77kelvin, not -270)). I have actually seen a guy put liquid nitrogen in his mouth, swish it around and spit it out although I WOULD NEVER personally do it. If you didn't swish it around enough and let it sit in one place youre asking for it... even more so if you accidentally swallow it as the LN2 expands to over 600 times its volume as it goes to the gaseous state. (Why you NEVER want to use LN2 in a closed atmosphere). Basically all I'm saying is it's not that easy to lose a finger. You would have to do more than spill some on your hand, you would have to dunk it. As a matter of fact let me make one thing clear now..
I AM NOT CONDONING OR ENCOURAGING ANYONE WITHOUT EXPERIENCE TO GO EXPERIMENTING WITH CRYOGENS. Hell, I don't even like the thought of inexperienced people using acetone.
Everyone: I will try more experiments tomorrow and later in the week regarding luster and various grime removed/not removed from coins with LN2. I tried a brittleness experiment with a couple different coins and found the following results: A quarter and a dime after being heated very hot AND THEN quenched in LN2, I was still able to bend a 90 angle into the coin applying the normal amount of pressure that would be required to do so. However a 2005 cent broke clean in half when subjected to the same extremes and forces. BTW, I had never seen the inside of a copper coin, I was suprised to see that it had a glittery silver core! Neat!
Barry: You cannot go below absolute zero (no negative kelvin temps). Temperature is actually an indirect measurement of molecular motion within a substance. The hotter you get something, the more the molecules move (why you go from solid to liquid to gas as you heat something up) Zero degrees Kelvin is actually defined as the temperature at which all molecular motion ceases, therefore you cannot physically get something colder than that. In fact, absolute zero is only a theoretical temperature which supposedly cannot actually be reached (Kind of like an object with mass can theoretically not travel at or faster than the speed of light)
Unfortunately, I am still very new when it comes to coin photography. My pictures would not come out adequate enough to show changes in luster, although I can at least attempt to let you all know what I'm seeing. I could always treat someone elses coin (prephotographed) and send it back to them to be photographed again. They do only have to be cheap 2005s from circulation after all.
Im working for NASA this summer on a return to flight issue. We are trying to mitigate ice formation on the External Tank during Liquid Oxygen (LOX) fueling. During the 8 hour fueling process ice builds up from condensed moisture in Florida's air and builds up a thick layer of ice around the Feedline Bellow's bracket. At my job, we are trying to identify a coating we can apply to the metal which will either stop ice from forming at these temperatures (not likely) or weaken the ice-metal bond so that the ice falls off much sooner. Right now it falls off from Mach4-Mach5 and poses a debris hazard. Basically all I'm doing these days is coating metal samples, putting water between them, and freezing them using good ole' LN2. Then we tear them apart. The coating which results in the lowest adhesion force is the winner! Well, enough messing around... I gotta finish up the day. I'll check again later at home. Thanks for the great conversation everyone!
Joe - I'm aware of that. I was being facetious, based on comment from beartracks42.
Btw, I have a solution to your icing problem. Move the launch site to Arizona.
Anyone know why they picked FL in the first place? Seems kinda dumb considering it's the lightning and hurricane capital.
New collectors, please educate yourself before spending money on coins; there are people who believe that using numismatic knowledge to rip the naïve is what this hobby is all about.
Be just my luck to spill some of that liquid nitrogen on my crotch.
I have my family all made and everything, so it ain't like I wouldn't have any children, since I already have em, but you see I still like to stand when I pee.
Ray
Actually that's not quite true; -500 degrees kelvin is the temperature at which hell freezes over and the Braves win the world series.
Travels fastest = Has to go a greater distance (larger circle) than the poles in the same 24 hour revolution.
Don't mean to make anyone feel stupid. I was just in this exact conversation last week and many people didn't understand why it was going faster.
<< <i>I ain't trying it.
Be just my luck to spill some of that liquid nitrogen on my crotch.
I have my family all made and everything, so it ain't like I wouldn't have any children, since I already have em, but you see I still like to stand when I pee. >>
<< <i>FL because it is part of the US that travels the fastest relative to space therefore requiring less effort to get it into space.
Travels fastest = Has to go a greater distance (larger circle) than the poles in the same 24 hour revolution.
Don't mean to make anyone feel stupid. I was just in this exact conversation last week and many people didn't understand why it was going faster. >>
You don't make me feel stupid. You make perfect sense. In a way.
The Earth spins faster in Florida?
Maybe. But since the world is solid, something has to give somewhere.
Now, explain if this is where you think earthquakes come from.
Ray
<< <i>Be just my luck to spill some of that liquid nitrogen on my crotch. >>
Yah but you would save the 10 bux by not needing a Viagra pill at least for that night.
I also have access to a large number of solvents and chemicals and have often thought of "conserving" or trying to tone up some coins (for educational purposes only).
Also, I don't want anyone to get the idea this grime and crap on these coins is shrinking away in the cold or anything. I realize now that all that mumbo about thermal expansion differences and such was a bit much. It's just that there are slight changes in size in the coin and everything attached to it. Most substances on Earth (with the exception of water) shrink ever so slightly as they freeze. Different substances shrink at different rates. As the coin and it's gunk shrink at slightly different rates, the actual bond sites between the gunk and coin are broken from resulting shear and compressive forces (very tiny ones, no doubt!). It's just enough to have a lot of the gunk wash right off once it hit's that hot water bath and they expand at slightly different rates as well. Granted, I'm not 100% sure this is actually what is going on but it's my best and only guess so far.
Kranky: Of course! I'll look around tonight for the shiniest copper and clad coin I can find and half dip 'em both tomorrow. There will be some violent bubbling as the LN2 boils off the coin so there wont be a visible solid line, if anything, but I should be able to keep half out for the most part.
Barry: I would like that actually, being from New Mexico I could actually go watch the launch! I think they're looking for a slightly cheaper option though if possible.
The reasons for the FL launch site have already been named. Yes the closer you are to the equator the faster you are moving. Take for instance that someone theoretically at the north or south pole would just be spinning around once every 24 hours, but for the most part staying in one spot compared to someone at the equator who is covering a much greater distance in 24 hours to get back where they started. The faster you can start at, the less fuel you need. They chose East coast because the Earth spins toward the East so that way you are using as much of the Earth's natural velocity and not going against it (west). In addition there is a debris hazard so it is best to launch over the ocean.
Aaron
<< <i>This sounds like an excellent way to conserve shipwreck coins. Dunk them in liquid nitrogen and let all of the crust just break off.
Aaron >>
Wow, I'm really glad I posted this. Some of you guys have great ideas and applications I would have never thought of. Just as important as the liquid nitrogen dunking is the immediate dip into a warm/hot water bath. This will send the crud flying! Then again, there are the issues brought up earlier in the post which still need to be addressed. For instance, would anyone care if they had a shipwrecked coin that looked great but would snap in half before bending?
Joefro, thanks for sharing. This is something you don't often get to read about!
Fascinating - every bit of it.
Thanks.
<< <i>This definitely couldn't change the weight of the coin, just some of it's other physical properties. >>
Well most of the changes might be temporary if that. This sort of reverse annealing/ stress relieving will result in a change in hardness of the coin but since no one will be touching it there will be little to fret about.
He then asked if anyone would like to donate their hand for a similar experiment, the opportunity of which I could not pass up (hey, I've got two hands, if anything goes wrong I have one left). He then went and poured liquid nitrogen onto my hand and it felt pretty neat. The feeling is hard to describe and probably harder to replicate. Probably not the smartest thing to do, but oh well.
Er, sorry stray off topic, I just wanted to share in your enthusiasm for liquid nitrogen.
<< <i>FL because it is part of the US that travels the fastest relative to space therefore requiring less effort to get it into space.
Travels fastest = Has to go a greater distance (larger circle) than the poles in the same 24 hour revolution. >>
I don't think that's quite right. It doesn't take any less effort to get into space because gravity is the same regardless of the spin of the earth. It takes just as much effort to get into space from the north pole as it does the equator, neglecting altitude differences and the equatorial bulge. However, launches that take place closer to the equator take less effort to get to stay in orbit because the orbiting object needs to circle the Earth in order not to come crashing back down. Thus because Florida is spinning fastest this extra angular velocity can be used to the spaceship's advantage, whereas the spaceship launching from a point farther to the north would have to burn more fuel to get to the stable orbit.
At least that's how I remember it from college...
Have fun...Mike
<< <i>Using Liquid Nitrogen to clean coins- >>
Rule#1 - Don't drop the coin after cleaning - they might shatter!
“It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so.” Mark Twain
Newmismatist
<< <i>This way of cleaning has been around for decades. Show some before and after pics. >>
Thats great, could you send some link(s) or do you know where any articles have been published. Any info could save me a lot of time from horsing around at work. Before and after pics won't work because my photography skills are still horrible and lighting differences alone would stop you from being able to tell what happened to the luster. I like the idea earlier of half dipping a coin. I may be able to get a good enough pick of a half-dipped coin if there is any difference in luster.
<< <i> It takes just as much effort to get into space from the north pole as it does the equator, >>
This is simply not true. The closer you are to the equator the less additional velocity it takes to reach escape velocity. (At long as you launch East with the spin of the Earth, and you have to launch at an angle, not straight up. If you launched straight up then you would be right, the north pole would be the same, but the Shuttle curves out to the East upon launching.