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So the backstory is that I purchased a reusable drinking straw that is copper coloured, but is advertised to be stainless steel. That got me thinking about whether I could be sure it was one or the other without having access to a laboratory.

I saw this answer that mentions that the conductivity of steel is much lower, but I don't think my voltmeter could really measure something this small. The other effect I know of would be the Hall effect (nice because it's a tube, so it's easy to demonstrate), but I wasn't able to find what the predicted behaviour is for a steel tube.

My question is then: are there any at-home/readily available ways to differentiate copper vs. stainless steel.

Qmechanic
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Kolichikov
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    You might want to ask this on Chemistry. They might give you a more definitive test relying on chemical rather than physical principles. – WaterMolecule Jan 22 '19 at 18:35
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  • So, was it copper or steel? I am curious to hear the answer. – Vanity Slug - codidact.com Feb 15 '19 at 18:20

10 Answers10

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Take advantage of the large difference in thermal conductivity between copper and stainless steel (approximately $400$ and $16$ $\mathrm{Wm^{-1}K^{-1}}$ respectively). If you put one end of a metal rod into contact with something held at a constant high or low temperature $T_C$, you would expect the other end to asymptotically approach that temperature like:

$$ T(t) \sim T_C + A e^{-\lambda t} $$

where

$$ \lambda = \frac{k}{\rho c_p L^2} $$

where $k$ is the thermal heat conductivity, $\rho$ is the mass density, $c_p$ is the specific heat capacity and $L$ is the length of the metal rod.

Assuming the straw is approximately $0.1$ m long, you should get $\lambda$ values of approximately $0.011$ $\mathrm{s^{-1}}$ for copper and approximately $0.0004$ $\mathrm{s^{-1}}$ for stainless steel.

A simple experiment would consist of putting one end of the your straw into contact with a container of ice water, or maybe a pot of boiling water, while you carefully measure and record the temperature at the other end as a function of time. (If the straw is longer than $0.1$ m, immerse the extra length of the straw into the water.) The best thing would be if you had some kind of digital thermometer that allows you to log data, but it could probably also be done with an analog thermometer, a clock, and a notebook. After taking the measurements it should be relatively easy to determine weather the temperature difference decreases with a half-life of one minute or half an hour.

There are many potential error sources in a simple experiment like this, but since the difference between copper and stainless steel is more than an order of magnitude, it should be relatively easy to tell them apart despite these errors. The experiment could also be carried out for other rods that are known to be made of copper or stainless steel (or of some other metal), to validate that the experiment gives approximately the expected result for them.

jkej
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    I think this is a great idea. It's not possible to hand-hold a copper item when it is placed in a flame a few inches away from the fingers, but it is not a problem with a stainless steel item. Now in order to be non-destructive one may want to reduce the heat to, say, boiling oil, but that should suffice. Best would be to have a similar comparison item of either steel or copper; a known steel straw should be possible to come by. – Peter - Reinstate Monica Jan 22 '19 at 14:05
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    @PeterA.Schneider I agree. There are many possible experiments along these lines that could be made. And the comparison item(s) doesn't really have to very similar in shape or size. The time scale of heat conduction only depends on material constants and the distance. – jkej Jan 22 '19 at 18:24
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    I think my concern with this suggestion is the coating. This is either copper or stainless steel with an unknown coating. I suspect that the coating could easily alter the thermal conductivity. In particular, it probably makes it slower, which might make it more closely resemble copper. You could end up with a false positive. – conman Jan 22 '19 at 20:02
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    @conman I would think that the influence of the coating is negligible, unless it's very thick, which seems unlikely. – jkej Jan 22 '19 at 20:26
  • I think that depends on how it is actually colored, and unfortunately we don't know the answer to that question. This answer to another question seems relevant to me: https://electronics.stackexchange.com/a/333824. I'm not a materials scientist, so I can't say anything definitively, but unless the results are absolutely definitive for stainless steel, I would consider there to be too many unknowns for the results to be conclusive. – conman Jan 22 '19 at 21:13
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    @conman all plausible coating/surface finish processes will affect only a thin (w.r.t the likely wall thickness) surface layer, so the thermal conductivity difference should be clear. Whether you could tell copper from aluminium that way is another matter, but density should do that (I've seen anodised aluminium utensils so wouldn't rule them out given the possibility of a misleading seller) – Chris H Jan 23 '19 at 16:32
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Why not density? At least for a quick check and as for the title question. You are dealing with about < 8 and 9 g per cubic cm, respectively for steel and copper.

Not overly laborious and especially non destructive at all.

To measure the volume you can submerge the object in the narrow container of your kitchen. If isn't graduated you just mark the displacement. The same with a graduated one for better precision. Collect that volume of water and move to a kitchen scale. As for you can weight the object as well, the experimental part is over.

Also note that as it is a straw it could also be possible to measure the thickness of the wall. So you calculate the volume. A caliper might be uncommon at home (incidentally I have one) but it remains a possibility. Inner diameter then is measured by passing/non passing wires. This might be tedious.

Indeed a straw complicates the procedure as its volume brings things down to ten grams or so.

See also the comment of Andrew Morton below. Having a dietary scale and using his trick he did indeed identified the nature of a straw.

Alchimista
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What about Eddy currents? If you've got a pair of strong neodymium magnets (doesn't everyone?) - move the straw in between two magnets with their poles opposing. Copper/Aluminum will have a strong interaction and you'll feel some kind of resistance or tugging (not unlike moving it through a thick fluid). Stainless steel won't produce any such effect.

Disclaimer: I've run this experiment with solid slugs/pucks of copper, aluminium, nickel, chrome-plated steel, and #316 stainless in our shop. I haven't tested with hollow cylinders like a straw.

user3772748
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To see if the straw is stainless steel with a copper-colored coating, you can carefully sand or file off a bit of material from the end of the straw and see whether or not it is copper-colored throughout its thickness.

To see if the straw really is copper, dip one end of it in a boiling solution of 1/4 cup white vinegar and 1 teaspoon of salt. This mixture is commonly used to remove tarnish from copper objects and will quickly make the end of the straw very bright and shiny. Stainless steel will be unaffected (i.e., not brightened) by this.

niels nielsen
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    Note that the second test would not distinguish between a copper straw and a copper-plated steel straw. – Michael Seifert Jan 22 '19 at 17:50
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    This completely ignores the range of possible treatments available to color stainless. For instance, PVD stainless coloring uses a titanium/nitrogen process - how will such a layer react to vinegar/salt? If this process is amenable, there are lots of others. – WhatRoughBeast Jan 22 '19 at 17:55
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You can measure the voltage when immersing the sample as an electrode into lemon juice or salty water and use known steel or known copper as the other electrode of a galvanic cell. Better use pure copper as the other electrode as different steels have different electrochemical properties. Some uncertainty, whether you don't just have steel with the same electrode potential as copper, may remain.

Vladimir F Героям слава
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    Since the straw is "copper-colored", the reaction will be with the surface coating. Unless you know what the coating is (and it's entirely possible that the straw is copper-plated) you can't interpret the results. – WhatRoughBeast Jan 22 '19 at 17:59
  • And who has a tester at home? ;) – Alchimista Jan 22 '19 at 18:51
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    This won't work if the copper is coated in a protective layer, eg of lacquer or clear enamel. And if it isn't coated, it will have a distinctive taste, and it'll tarnish pretty quickly. – PM 2Ring Jan 22 '19 at 19:51
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    @WhatRoughBeast I missed that bit, it is quite obvious that this method works with the surface. Anyway, I used this method with a great success at high school when we were mailed a small piece of metal a metal wire and were to determine the composition with as many methods as possible. It was silver. Some other answers on this page require removing the coating as well. – Vladimir F Героям слава Jan 22 '19 at 20:05
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    @Alchimista almost anyone? We always used to have one, it's indispensable when something breaks. I also remember buying one to my friends as a requested a wedding present, it is a must have home item. – Vladimir F Героям слава Jan 22 '19 at 20:11
  • It might be culture and location dependent. I am not interested to launch a poll and especially not here on Physics SE :) we could just ask OP. – Alchimista Jan 22 '19 at 20:48
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Here's an idea (this is not my area however): Consider using the difference in resonant frequency. This depends on length ($L$), Young's Modulus ($E$), mass per length ($M/L$, requires mass of straw, likely found from manufacturer or else use e.g. a food scale) and second moment of inertia ($I$, area moment, unique to the geometry, a tube of known inner and outer diameter can be calculated). Copper has $E = 36\times 10^6$ PSI, stainless steel (according to Engineering Toolbox) has $E = 26 \times 10^6$ PSI.

For a tube of inner diameter ID and outer diameter OD, the second moment is $$ I=\frac{\pi (OD^4 - ID^4)}{64} $$ Note that those are fourth powers of the inner and outer diameter.

For $E$ in PSI (pound-force per square inch), $L$ in inches ($\text{in}$), $I$ in $\text{in}^4$, and $M$ in $\text{lbm}$ (pound-mass), for a simply supported straw (note: length is length from support to support), we can calculate the resonant frequency by (source): $$ f = \frac{\pi}{2 L^2}\sqrt{\frac{EI}{M/L}} $$ Where $f$ is the frequency in $\text{Hz}$.

The challenge might be supporting the tube properly, maybe fix the ends internally, or with some steel wire (or paperclips). The resonant frequency can (hopefully) be measured by your phone, with an audio spectrum analyzer.

For an example, consider this straw from Amazon. $OD = 0.3125\text{ in}$, assume $ID = 0.3\text{ in}$ for sake of example, $L = 8.5\text{ in}$, and let $M = 0.01\text{ lbm}$ (product net wt, not sure what packaging it has). It's made of food-grade steel (18/10, 303 grade) with $E = 27-29\times 10^6\text{ PSI}$. The second moment is calculated to be $I = 0.00017\text{ in}^4$. Assuming the straw is ideally supported, the stainless steel straw would have a resonant frequency around $43-44\text{ Hz}$, while a copper straw would have a resonant frequency of around $49\text{ Hz}$, assuming I haven't made a mistake in my calculations.

Sam Gallagher
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Copper is much more malleable and ductile than stainless steel, so a very simple but possibly destructive test would be to try bending the straw with your fingers.

A copper-plated steel straw will still have the strength of steel. A solid copper straw would be much softer and easily bent.

For comparison, try bending a paperclip, then try bending a piece of copper wire the same thickness as the paperclip. The difference is very noticeable.

barbecue
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I'm surprised no one has mentioned hardness, since this is what metallurgists and machinists would use. Basically, you can buy sets of files made out of different grades of known metal hardness. If the file scuffs the metal, the metal is softer than the file. If the file doesn't scuff the metal, instead the metal smooths down the file, the metal is harder than the file. You work down from the hardest file in the set down to the softest, until you don't see any more scuffing, and bingo, that gives you the hardness of the metal, with some degree of accuracy.

Having said that, steel is typically so much harder than copper, that it should be incredibly obvious from using ANY steel file, whether you are struggling to cut through steel, or chew right through copper. Copper is so soft you can usually bite it and leave a mark, although this isn't recommended. Use a file to do the biting for you.

J.J
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Touch it with a magnet. Steel is magnetic. Copper is not.

Crisp
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    Stainless steel often isn't magnetic at all, so nope. If it is magnetic, it's likely some kind of steel, but if it's not, it doesn't tell you anything. – Luaan Jan 22 '19 at 11:54
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    You are correct. Apparently the most common stainless steels are 'austenitic' - these have a higher chromium content and nickel is also added. – Crisp Jan 22 '19 at 12:06
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    Good stainless steel is not magnetic. However, sometimes stainless steels can be a little magnetic, specially when they have been worked, https://www.bssa.org.uk/faq.php?id=24 . Although non being magnetic isn't reason enough to rule out stainless steel, being a little magnetic can be a reason to rule out copper. – Pere Jan 22 '19 at 14:34
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    Despite the fact that it may not work with all possible steels, if you perceive a force than there should definitely be ferritic steel and not only copper. In this case this test is the simplest of all those presented here. – lcv Jan 23 '19 at 00:54
  • There are two main types of stainless, ferritic, cheaper, less stainless, slightly ferromagnetic, and austenitic, more expensive, more stainless, essentially non-magnetic. – Neil_UK Jan 24 '19 at 10:03
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    400 series SS is magnetic, but to different degrees depending on how it's alloyed. 300 series is non-magnetic. – Mike Waters Jan 24 '19 at 22:00
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Taste it, copper and stainless steel taste nothing a like.

Michael H.
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    Agreed, but if it is copper, it probably has a coating to reduce chemical reactions. – PM 2Ring Jan 23 '19 at 00:28
  • Yeah but you can scratch that off on a small spot with your thumb nail and just taste it, and that would be a good way to see if they did put a coating on the stainless steel as well. – Michael H. Jan 23 '19 at 15:20
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    @MichaelH. Agreed. Also, copper has a unique odor. Ask any experienced machinist or toolmaker. My freshman year at Macomber VTHS machine shop, the teacher taught us boys how different materials smell. – Mike Waters Jan 24 '19 at 22:04