[metallurgy filter] Copper, tin, and smearing
January 11, 2019 11:51 AM   Subscribe

I've noticed that the tin lining smears more easily on some of my copper pots than others, and I'm hoping a metallurgist/chemist/physicist will support or debunk my idea about why that is.

This question pertains to vintage copper cookware that has a hand-applied tin lining.

Disclaimer: I'm not a metallurgist, chemist, physicist, or really any good at anything, but I'm curious, and I would really appreciate help to understand this.

When a tin-lined copper pan approaches 450 degrees F, tin's melting point, the tin lining can soften and melt such that you can inadvertently wipe some of it off onto your food or spatula or whatever. In the world of vintage copper, this is called smearing because you leave smear marks in the tin that persist when the pan cools off. (Smearing is a cosmetic issue and not a serious problem unless you were to really dig down and scrape the tin off to expose copper.)

What I have noticed is that certain pans of mine are more likely to smear while others don't smear at all. The ones that are most likely to smear have a thicker, newer lining, while those that don't smear have a thinner lining that feels harder, almost like the surface of an eggshell.

My conclusion is that the thicker the tin lining is, the more likely it is to smear. Assuming this is correct, why?

My hypothesis is this: Tin atoms in contact with copper is tightly bonded and are least likely to smear. (Coppersmiths often have to grind off a base layer of a tin lining because it just won't let go.) Tin atoms in proximity to those tightly held tin atoms are stabilized by that bonded layer and are less available to smear even when melted. Tin atoms REALLY far away (relatively speaking) from the bonded layer are least stabilized and are most available to smear when melted.

Ergo, the thicker the tin lining, the more tin atoms are far away from the stabilizing layer of immobile tin atoms. When the tin hits melting point, these are the first atoms that'll let go, while the atoms further down -- closer to and stabilized by the bonded layer -- hang on better.

It would also follow that there is an ideal thickness for a tin lining, where there's a good layer of stabilized tin and a minimal layer of "loose" tin that could smear; perhaps my non-smearing pans have achieved that ideal thickness by having all their loose tin already taken off.

So my question for you guys: Does any of this make sense? If this is a totally nutball idea (seems likely), is there some other possible explanation for why some tin linings smear while others don't?

Happy to post photos of smeared pans if you want to see what it looks like. Thank you in advance for any input you may have.
posted by woot to Science & Nature (11 answers total) 1 user marked this as a favorite
I think there might be a simpler explanation. The pots that don't smear are tinned with pure tin. The pots that do smear are tinned with an alloy (technically a soft solder).

There are solders that contain no lead, so there's no need to panic, but a silver-bearing solder (my fave, SilvaBrite 100) will melt at around 450 degrees F.
posted by the Real Dan at 12:15 PM on January 11 [1 favorite]

That's a good thought, but in this case I know for certain it is pure food-grade tin and not solder. All these pans are either new Mauviel or retinned by one of two well-known tinners I send work to. I had one set of pots retinned with solder so I know what that looks like (and had them redone with pure tin).
posted by woot at 2:43 PM on January 11

Hello! I am a metallurgist.* I haven't had a chance to look into this (yet), but what you are suggesting has some pretty sound basis in science.

When tin is coated on copper, an intermediate layer is created called an intermetallic that is sort of the "glue" between the two metals, and is really a different material all together. The intermetallic is harder and more brittle, and would not be easily removed. I suspect this is what you are calling the stabilized tin, as it is a silvery color and would just look like tin to the naked eye. Tin metal itself is pretty soft, and so could easily smear when being scraped, especially at higher temperatures.

I would be interested in seeing some images, in case this is a more unusual phenomenon than what I am picturing in my head.

* although IANYM ;)
posted by blurker at 3:14 PM on January 11 [3 favorites]

Similar to what blurker is saying (as near as I can tell), the tin is not a layer like paint which can peel off. The inner surface starts as more or less pure tin for some distance, and then begins to have copper atoms mixed in, then becomes a tin rich bronze, then becomes copper rich bronze, and finally more or less pure copper.

And a typical melting point for bronze is like 1700 F.
posted by jamjam at 5:33 PM on January 11 [1 favorite]

jamjam is correct that there will be a higher percentage of copper near the copper layer and a higher percentage of tin near the tin layer. However, this manifests as a variation in the stoichiometry of the intermetallic rather than developing into a copper-tin alloy such as bronze. The intermetallic nearest the copper is Cu3Sn, and nearest the tin is Cu3Sn5. (Stop me if I'm being pedantic.) :)
posted by blurker at 7:08 PM on January 11

You guys are awesome. Here are some photos:

Smeared tin: Example 1, example 2
Old pan that never smears: Photo, closeup 1, closeup 2

You can see on the "old pan that never smears" that there is sort of an under-layer of what I call "eggshell tin" that is nice and hard. Even though this pan looks awful it cooks like a dream. Food just slides off.

@blurker: Please be pedantic. In fact, be MORE pedantic. :)
posted by woot at 7:45 PM on January 11

One more -- this one is a skillet, smeared and bubbled: Example 3

This is also a skillet, pretty much the "before" version of the "old pan that never smears".
posted by woot at 7:53 PM on January 11

Yeah, I'm going to go with the intermetallic theory, which is darned close to your original thought. That last image really clinched it for me.

Over time the intermetallic will grow, consuming any free tin, so the smearing would happen less often as the free tin becomes thinner, eventually leaving only the intermetallic layer. Intermetallic growth is accelerated with temperature, but will also occur due to diffusion even at low temperatures.
posted by blurker at 8:57 PM on January 11

blurker and jamjam, THANK YOU. This is brilliant and exactly the kind of info I was hoping for.

So about this intermetallic layer: I want to make sure I understand it. It's not bronze, per se, but a bunch of Cu and Sn that have bonded but not in the same way that would make bronze, yes? The tin is bonded to the copper with the help of flux -- people use different substances but it's all called "flux" -- does that play into the formation of an intermetallic versus actual bronze?

It's fascinating to me that this intermetallic layer will actually grow, sucking in more free tin. Users of tinned copper are used to thinking of the tin layer as diminishing over time, not hardening up.

Lastly, is it possible then that the intermetallic layer is the actual "tin lining" -- that the free tin is waste tin? Tin as a cookware lining is quite non-stick (well, low-stick) but could it actually be the intermetallic that's the non-stick surface?
posted by woot at 4:24 AM on January 12

The atomic bonding of intermetallic materials include both metallic and ionic bonds, thus they are not actually metallic alloys like bronze. This makes them hard and brittle, as opposed to bronze which is quite ductile.
Additionally, their stoichiometry is very specific (Cu3Sn and Cu6Sn5)*, where you can have a range of mixtures in bronze (mostly copper and tin, but other metals are added to bronze in order to gain specific properties of the resulting alloy).

Flux is basically a way to clean off the surface where you are attempting to create a metal to metal bond. It usually consists of an acidic element, which concentrates and activates with heat. Since most metals react with the atmosphere, typically forming an oxide, the flux is needed to eat away the oxide so that a true metal to metal bond can occur, which creates the intermetallic. You would not be able to create a bronze alloy without melting down and mixing the constituents, so this really isn't a way to keep from "forming bronze," it's just the way this type of metal joining works.

The tin layer *is* diminishing over time, but some (a lot?) of that is due to development of the intermetallic. It's not that the tin is hardening, it's that it is being used as a constituent of the intermetallic, which is harder than the metal. In the electronics industry we refer to that as the copper intermetallic "consuming" the tin, but I'm not sure if that's just our own jargon.

Whether the metallic tin or the copper/tin intermetallics are really the non-stick surface is a great question. I'm not sure how to find out other than taking a really good non-stick piece and cross-section it to look at the microstructure of the layers. I'm not sure anyone would want that to happen to a really good pot! :)

I'm actually surprised that these pots getting anywhere near the melting point of tin, but I guess I never checked to see how hot my stove top gets.

*I see I mistyped that previously. How embarrassing!
posted by blurker at 8:15 AM on January 15 [1 favorite]

blurker, thank you so much. I went down the rabbit hole with intermetallic copper-tin and learned a lot about it. Most of the info has to do with research on lead-free soldering for circuit boards and all that and I'm having a hard time relating what I'm reading to the fairly simple situation with tinned copper -- just Cu and Sn. May I ask you a few more questions?

-- Would Cu6Sn5 actually "stabilize" pure Sn in any way? Like, does melted Sn cling to Cu6Sn5 more than it would to itself?

-- One of my pans has some chips in the tin lining. What's going on here? The lining is pretty old -- a decade or more, likely. Is this intermetallic copper-tin that's gotten brittle and has been struck and cracked, like an eggshell?
Example 1: https://imgur.com/EYnRuzy
Example 2: https://imgur.com/QevSEwi

-- Along those same lines, on those two photos, if you look within the chipped area there's very dark gray tin surrounded by lighter tin. I see these same dark, almost powdery patches on otherwise shiny tin. They don't wipe or wash off. I suspect it's beta/alpha transition ("tin pest"). What do you think?

-- What causes the vertical cracks around the rim of this pan? It's very old -- early to mid 1800s. The pan is sort of funnel shaped, so that the rim was hammered a lot to thin and flare it out.

I am so grateful for any time you can give to these questions. I've spent hours online and it's hard to find straightforward information. You have been incredibly helpful and I really appreciate it.
posted by woot at 5:44 PM on January 15

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