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There are a number of common cooking lore techniques which revolve around the idea of "pores" in the surface of a pan. Two of the ones I've heard most often:

(1) When heating a pan where sticking is a concern, one should wait until the pan is hot before adding oil/fat. Supposedly the "pores" will close as the pan heats, leaving a flatter surface for the oil and less places where the food can get caught. This is often cited for cooking on stainless steel, but sometimes for other materials as well. (Some of the answers to this question, for example, cite this as a rationale for preheating pans before adding oil.)

(2) When seasoning cast iron (and sometimes other metals), one should be sure to heat the pan before applying oil. Supposedly, the "pores" open up as the pan heats, allowing the oil/fat to penetrate the surface better and create a better seasoning. (This has been mentioned a number of times here too, and it also is referenced on the Wikipedia page on seasoning pans, where preheating will "open the 'pores' of the pan.")

One obvious problem here is the contradictory logic of these claims: in the first case, one heats the pan to "close" the "pores," but in the second case, one heats the pan to "open" the "pores." Which one is it?

I've heard these claims about "pores" and their very existence disputed. At best, many commentators who seem to know something about properties of metals will say this is a strange shorthand term for the uneven surface of pans at the microscopic level. (The end of this post and the discussion in comments, for example, contains some speculation along those lines.)

For years, I dismissed a lot of this discussion of "pores" as some sort of weird cooking lore myth. For example, it's good to heat cast iron before seasoning to ensure it's thoroughly dry, regardless of the status of the "pores" in the metal. And some people have done experiments claiming it doesn't matter whether you preheat a pan before adding oil to avoid sticking.

But I recently came upon a reference to these "pores" in Shirley Corriher's Cookwise, where she refers to the first claim I listed above: heating a pan to "close the pores" before adding oil/fat. Alton Brown cites Corriher's claim too in a couple places, including in his Gear for Your Kitchen, where he refers to it as something you had to worry about to stop food from sticking in those ancient times before Teflon existed. Corriher and Brown have been known to be wrong occasionally, but they generally are careful before citing random cooking lore.

So, are these "pores" real? Do they expand or contract when heated? Are their supposed culinary effects real? Or is there some other mechanism or surface feature of metals that is being referenced here?

EDIT: Just to be clear, I'm hoping someone may be able to point to a more reliable discussion of what's going on from a reputable food science (or science in general) source. I have my own thoughts about what may be going on here based on my knowledge of thermodynamics, metallurgical crystalline structures, and general materials science. But in cursory searches, I haven't been able to find any solid discussions of these supposed "pores" based on actual scientific evidence.

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    There are pores, I've seen them in a micrograph. They aren't normal biological pores though, it's a misnomer. Your description of "uneven surface" fits better. And to the best of my knowledge, they close when heated, because the metal expands, reducing the gaps. I've never heard the "open the pores" claim. If I can find the micrograph, I'll post a full answer. – rumtscho Dec 8 '14 at 10:19
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I work for a carbon steel cookware producer in China and just like Athanasius, I too have become interested in the question of "Do pan “pores” exist, what are they, and what are their effects?" I have also watched the RouxBe video about making a stainless steel pan more non-stick through pre-heating. To summarize the main point, it says to heat the pan until the peppercorns (the pores of the steel) stop opening and closing. The right time to add the oil is when the pan is hot enough and the peppercorns have closed, thus creating a flat steel surface on which to add the ingredients for cooking. Although I think RouxBe is without doubt an excellent resource for learning about cooking, I was a bit doubtful about the peppercorns theory, because just like Athanesius, it appears to contradict the whole concept behind seasoning a pan.

I have briefly discussed this with my boss, somebody who is very knowledgable about carbon steel (he has run a factory in it for nearly 25 years). He says that the first theory is wrong, and that the pores do not close, they in fact open up (as pointed out in the second theory for seasoning). But actually it is in fact these 'pores' opening up at a hotter temperature which does make the pan more non-stick. When the 'pores' of the steel open up they create more space for the cooking oil to seep into for a better non-stick. In this way the food particulces cannot get stuck into the irregulaties (pores) of the steel and the high heat dries the surface of the steel and cooks the proteins quickly, thus reducing the chance of them 'setting' themselves on and into the steel. This, along with the Leidenfrost effect, whereby any moisture on the ingredients will turn to water vapour and create a barrier between the proteins and the steel, is why a hotter pan has a better non-stick effect.

  • Agree! Pores get bigger when heated, at least in normal metals. Think about any hole in a piece of metal. If an object is stuck in the hole and you heat it up to free it. If the pores close upon heating, that would crush the object rather than free it. Look up coefficient of thermal expansion and 3D expansion. You can get exotic materials that behave differently, but cast iron is not one of them. Surface pores are just a casting product. – user110084 Jun 10 '17 at 7:42
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This is the microstructure of SAE 304, a steel type commonly used in pans:

enter image description here

At this magnification, its "pores" look like cracks. Now see it at other magnifications (still a SAE 304, other types of steel look completely different, especially if you look at martenistic steels):

enter image description here

enter image description here

It gets even more complicated than that, because steel structure differs between the surface and below the surface:

enter image description here

As you can see, the holes are nowhere "pore" shaped. But they exist; steel is not even at the microscopic level.

I cannot point you to more cooking-relevant sources, but this at least confirms that holes exist in the type of steel used for pans. Somebody else will have to tell us what happens to them when heated.

Note that there is a version of the "pore" theory which insists that the "pores" are still moving at certain temperatures and static when the pan is properly preheated. I'm not sure if it's true. On the one hand, it's presented in a rouxbe video, which is generally a very good source, and I can imagine steel doing funny things on the crystal level. On the other hand, the use of the "pores" term and the lack of explanation of the underlying mechanism make me doubt it.

  • Thanks for the research; I'm confused on a few points. (1) On a macroscopic level, holes generally don't get smaller heated: if you heat a hollow cylinder, the entire thing will enlarge slightly, including the hole in the middle. Are things different at these scales for some reason? (2) The link you gave (which, you will note, was already in my question) explains that the non-stick effect is due to the Leidenfrost effect, which has nothing to do with surface unevenness. And comments there imply that sticking may be caused by "pores" contracting/closing when cold food cools the surface. – Athanasius Dec 18 '14 at 23:54
  • No,the hole in the middle of the cylinder will get smaller. It will expand both to the outside and to the inside at once. To see what I'm meaning, open an image manipulation program and enter the letter o, this is your cylinder cross section. If you apply a "resize image" effect, the hole will get larger - I think this is what you are imagining is happening to the pan, but this is the wrong way to think about it. Imagine typing a bold o instead, this is what expansion looks like on both the micro and macro level. – rumtscho Dec 18 '14 at 23:59
  • As for the article we linked, just watch the second video, the part with the peppercorns. It talks about pores independently of Leidenfrost. I actually forgot that it was in your question too, because I didn't reread it now, my mistake. – rumtscho Dec 19 '14 at 0:00
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    (1) No, sorry, you're mistaken, at least at the macro level. (2) Yes, I watched the videos before I posted the question; in fact, they caused me to post the question. I'm looking for some actual scientific sources (not hearsay) that might, for example, show the actual behavior of these holes/grooves/pores at higher temperatures and perhaps demonstrate that their changes at high temperatures are actually meaningful in a culinary sense. – Athanasius Dec 19 '14 at 0:04
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    J. Wulff obviously is not a metallurgist. Stainless steel grade 18/8 is completely austenitic, a ferrite (iron) layer does not exist. Deformation is caused by cold working. If the stainless is overheated the carbon may diffuse into the grain boundaries, this can cause galvanic corrosion. This results in stainless that can rust unless the stainless steel is solution heat treated. – user45215 Apr 21 '16 at 12:18
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tldr The point of oiling a stainless steel pan is to lubricate the (already mostly smooth) surface, and the point of seasoning cast iron is to fill the irregularities with a layer of non-stick polymer that results from burning off the oil.

  1. Are the pores real?
    This depends on the material the pan is made of. Cast iron is not porous in the way sponges and unglazed ceramics are. Neither is stainless steel. However, both surfaces are covered with irregularities, most of which are too small to be seen. There are no "pores" to open and close like pores in your skin--but the irregularities do change shape & size when they're heated.
  2. Do the they open or close when heated?
    Because the sizes and shapes of the irregularities vary, the effects of different temperatures on shape of the metal will vary as well. Very small divets may close at low temperatures, and slightly larger ones may not significantly change the shape of the surface until they're heated. It's not a linear relationship, since we're dealing with many different divet-sizes & shapes.
  3. Stainless steel vs. cast iron
    This might sound a little circular, so be warned. Cast iron needs to be seasoned to prevent two things from occurring: rust and sticking. Contrast stainless steel, which doesn't rust, and usually isn't seasoned. The answer to your question is in the size of the irregularities, which change shape non-linearly as the pan is heated.
    Stainless steel has a much finer grain. Therefore, it is best to heat it to a comparatively low temperature and allow the oil to completely coat it (getting into all the little grooves and so on) before cooking. When seasoning cast iron, the point is to burn off the oil until even the larger indentations are filled with the residue. It requires a much higher temperature--and the reason it needs to be seasoned in the first place is because it's so irregular.
  • Thanks for the thoughts. Does your #2 imply that the stainless steel thing is a myth, then? If only "very small divets" will close, would this really have any effect on sticking? After all, most people seem to agree that visibly scratched stainless shouldn't have a huge effect on sticking, so do these microscopic changes actually have a practical impact? Also, do you have any sources for this information? I know enough about materials science that I could speculate on what's going on too, but I'm looking for a reliable discussion of this stuff. – Athanasius Dec 7 '14 at 21:07
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I am retired but for this purpose I'm putting my machinist's hat on temporarily. I have machined a lot of cast iron in my day and when it is machined the cuttings that come off of the part break up into small pieces as compared to how steel cuts giving off cuttings of a wiry or curled nature. The nature of cast iron and the way it cuts in the machine shop leads me to believe that it is porous throughout.

I use my cast iron frying pan a lot and keep it seasoned. When I clean it I simply rinse it out, fill it with hot water and a light scrubbing with a copper scrubbing pad and a final rinse...no soap. The next time I use it I heat it up and put either margarine or olive oil in it and spread it around the bottom of the pan with a paper towel. That brings back my non-stick surface.

Back to the porosity, it is my contention that the porosity allows a certain amount of the oil to completely penetrate the frying pan from the inside to the outside. Evidenced is the black crust buildup on the outside of the pan. I am very careful about spillage when I'm cooking and when I'm cleaning the pan I'm careful about the runoff from the pan so I don't believe that the crust is accumulating on the outside from something that I'm doing. It seems to me that the oil is going through the pan by way of the porous nature of the cast iron.

Inside of the pan

Outside of the pan showing black crust buildup

  • Your right it does over the years soak threw. Hot oil is forced out as pan heats but some is forced down into the pan. My grandfather was a blacksmith. As a young man I did some help at the forge. & help casting. In heat treating cast iron carbon soaks in at low heat. Steel were pours are hammered close high heat & much more time to soak in. – J Bergen Nov 7 '17 at 23:08
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Not sure if this applies... When my friend does Powder Coating (it's similar to paint but more durable), he washes the steel first with alcohol. Then he heats the steel, and an oily film appears. He says that he has to do several cycles of heating, re-washing with alcohol before the steel stops oozing a surface film when heated. Only then, is the surface ready to receive the powder (powder is applied to a heated surface).

I've asked him about where the film comes from. He says steel is "porous".

  • This is more of a comment offering anecdotes rather than an answer – user110084 May 29 '17 at 21:44
  • A question, When you take a old cast iron skillet. One 50 years old. Treated with care. Never been washed with soap. You dry wipe it with a cloth. Till it is dry on the inside. No oil. no carbon comes up. You then heat it. Were does the light oil film come from if not out of the iron in the skillet when you rewipe it with a clean white cotton cloth. That oil has to come out of the iron. To do that it needs soak in. If the pours are not closing Then is it from the oil expanding? Also how do you explain the oil that develops on the bottom of cast iron skillets after long years of use. – J Bergen Nov 8 '17 at 10:22
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    This answer is correct even if worded like a comment. Anyone who actually works these metals such as welding or super heating knows that oil is often impregnated throughout the iron and not just on surface. I've TIG welded engine parts such as cam and crank gears (that spent years in oil) after cleaning them with acetone and huge amounts of oil weeps out of the metal as it heats and it will catch fire as the metal becomes molten if you don't heat cycle it first. – codenheim Dec 15 '17 at 14:24
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The pores on the surface of a pan are microscopic. Oil covers them completely. The surface of the oil is smooth. It isn't perfectly flat. It follows the shape of the pan, including the larger irregularities of the surface. But it is smoother than the microscopic irregularities.

When you season a pan, you heat the oil. Oil is made of long molecular chains. When heated enough, they cross link. That is, they form bonds that join them together into a solid mass. When this happens, the oil changes quite a bit. It no longer dissolves in soap. It forms a hard, smooth, slippery surface.

So I don't see how it matters whether microscopic pores open or close. Either way they will be buried.

  • Actually, oil is made of a mixture of long (heavier) and short (lighter) molecules. They do not only just cross link when heated. They also "crack", break into smaller molecules. At smoke point, you see the vapours of the "crackates" along with some of the normal smaller molecules escaping. The reduction in lighter molecule concentration (and thus increase in heavier ones by default) causes thickening and gumminess in addition to just polymerisation. – user110084 Jun 10 '17 at 14:11
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A fact of metals is that no metals have pores, cast iron included. It is very hard to find good information on the subject however. Cast iron can have "porosity" which is not the same as "pores".

For the poster who thinks that the cooking oils travel THROUGH the iron to build up on the outside of the pan I can say to them to break open a cast iron skillet and all you will see on the inside is clean, pure grey iron with no black from having oils getting "into" the iron itself.

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    Your answer would be substantially better if you explained what "porosity" means, and how it differs from having "pores". – Cascabel Nov 7 '17 at 19:25
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    Incorrect assumption based on your limited experience. Try this. go to salvage yard and buy a cam gear from any old engine. take home and clean with your choice of method, solvent, acid, etc. hit it with an acetylene torch or tig torch. watch the oil weep out. – codenheim Dec 15 '17 at 14:30
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If you heat a cast iron pan to 400f, the pores, if they exist, will expand 1/5 of 1%. To get an idea of how much that is, if you had a 1" hole in your pan it would expand by the thickness of 1 hair. Yep, 1 hair. Do you think this 1 hair makes a difference in a 1" hole. 1/5 of 1%.

  • if you look at the other answers, maybe 1 hair doesn't make any difference but because a pan surface is full with these "hairs" then it does. – Luciano Mar 15 '17 at 15:25
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    I agree with @Luciano. A single hair makes no difference at all. But many thousands of them add up. Trying to make a comparison with observations that, even if correct, do not accurately reflect the reality of the question, is neither helpful nor contributive; not to the OP, and not to others who may be interested, or who have added their input. – Paul Beverage Mar 15 '17 at 15:45
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    Do you have a source for this? – pyro Mar 15 '17 at 16:07
  • @pyro see the other answers – Luciano Mar 15 '17 at 16:54
  • The "pores", if they exist, would be the grain boundaries of the material microstructure. A thermal expansion of 0.005 is certainly enough to change the thickness of those boundaries. – Mark May 20 '17 at 1:08
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Cast iron has many pours. Oil will sink in. As you heat it the pours close. cool they open. Steel the pours are hammered or mashed together reducing there size. To carbon harden cast iron you carbon at about 350f. To carbon steel at around 900f. This is in a black smith shop. Hammer & forge. This is why you heat steel. Close what pours are left threw heat expansion then oil. So the oil floats on top. Cast iron. Heat Add oil. to thin it. let cool. The oil will soak in. As the pours open. When heated the oil will be forced out. You may still need add a light oil wipe. That in a black smith shop is the difference in iron & steel. The pours being hammered shut. to make steel but taking more heat to harden & much longer for the carbon to sink in. Today rolled steel is used. Stamped to shape, then sent to heat treat. Were it may be 3 days or more. Been 30 years last I was in a steel mill. May be more modern answers here. Grandfather was a blacksmith hammer & forge man. This could be what they are talking about. heat then oil so oil stays on top. Rather than some being forced out of cast iron as it heats.

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