Do pan "pores" exist, what are they, and what are their effects?

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.

This is the microstructure of SAE 304, a steel type commonly used in pans:

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):

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

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.

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.

Mechanical engineer by trade here with a smattering of materials science background, I think I can weigh in a bit on this topic. There are a few questions that I think are getting conflated with this whole discussion.

First off, I'll open by saying that no, I have no reason to believe that carbon steel skillets or cast iron pans are "porous" in the same way that a sponge is, in the sense of having small voids in the material where liquids and such can seep clear through the material. The discussion about surface finish though has some merit to it - but it's very unlikely that you're having any effect on the microstructure (crystal alignment) or the macrostructure/shape of the crystals at cooking temperatures. Frankly, if your cast iron pan is at transition temperatures like that, you will see it in the formation of a black film growing on the outside of your pan as the carbon migrates out of the steel - and this would basically be soot, not long carbon polymers like your pan's seasoning.

The likeliest mechanism for improving the non-stick properties of a pan at temperatures actually has nothing really to do with the pan, but the oil. Commenter Wayne Short is correct that at the temperatures and sizes we're discussing, coefficient of thermal expansion does nothing. The oil however, will see a massive drop in viscosity. Think of your pan sauce's consistency at temperature versus after it's chilled on a plate. That marked decrease in viscosity will absolutely help with wetting the surface and filling in imperfections, and will make your cooking experience much better.

For those machinists who say that cast iron feels more porous, your instincts are correct, but the root cause is actually not pores. What you're feeling in machining/surfacing cast iron is the incredibly high carbon content that's dissolved into the pan - if memory serves right it usually exceeds 1.8% carbon by mass. As you work carbon steel on a mill or lathe, you temporarily generate enough heat and pressure at the tooltip that the carbon comes out of solution and basically applies graphite lube on your tooltip. This makes it cut very differently to normal steels. To add to matters, cast iron is extremely brittle compared to a more standard steel, so that even further changes the way it machines. Normal steels will form chips that tend to be longer and much more ductile for that reason - cast iron is basically shattering constantly, while steels act much more like you're peeling the surface...

Before someone says it: yes, there are absolutely steel/metal products which are porous. A previous comment mentions powder coating metal parts and seeing oil films baking out. This happens with metals, just not on your pans - it's a consequence of manufacturing technologies, not of any inherent porosity in steel. An extremely common manufacturing technique is called Powder Injection Molding (PIM), also known as Powder Metallurgy. The easiest way to describe this would be to think of building sand castles on a beach. On an industrial level, manufacturers take molds, fill them with grains of metal powder, and compress them into shape. These 'green' parts are highly porous and loosely held together (just like your sand castle), so parts are then sintered in a high temperature oven. The sintering process effectively welds each of those grains together - but because you never reach the melting point, the grains stay in the positions they were in when they were pressed in to shape. The consequence is a highly grainy structure - and this is porous enough that many designers take advantage of these pores by soaking oil into them. For moving parts such as bronze bushings, this results in a highly effective self-lubricating property. As the part wears slowly, oil is also released, which then self-lubricates. Off-the-shelf bronze bushings like this are often called oilite bearings, but the technique isn't only applied to bearings, it can be used on basically any PIM-produced part.

Edit: I thought of something else that may be much more of a factor than oil viscosity. Metal doesn't change much through that temperature range, and oil changes viscosity quite a bit - but gases and water change a LOT in density. It's possible that by pouring oil onto a cold pan, any gases and water trapped underneath form microscopic bubbles where the contact isn't perfect, and these bubbles of trapped gas expand as you heat up the pan. Water may drive off mostly, but trapped air won't do enough to form a bubble that boils out. That means the oil doesn't wet the surface perfectly.

Now contrast this with pouring oil onto a hot pan: the water is already driven off, and gases near the pan are at the lowest density they'll see through the cooking process. You pour cold oil onto them, and any gas that gets trapped under the oil will cool rapidly. The rapid drop in volume will suck the oil into the voids, significantly improving wetting of the surface. On a microscopic level, you'll get a whole lot better contact in the interface between your pan and the oil. This will likely improve not your non-stick properties, but also the strength of the seasoning layer you form.

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.

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".

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.

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.

Metals do NOT have "pores". Some answers are close, most are creative although wrong. Cast iron has rounded ( nodular and mealleable) or flakes of carbon/graphite . The carbon particles may be visible at low magnification and are black so may look like pores. Poor castings may have porosity voids but it would be exceptionally rare for any to be at a surface.

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%.

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.