How did I scorch a pizza in under 2 minutes?

[scott123]

Hmmmm... interesting.  Very interesting. I was expecting a small difference, but that's pretty dramatic.

Bake time? Oven temp? Stone material?

[Tampa]

Bake time ~5 minutes.  Oven Temp 675F.  Cordierite Stone.

Frankly, it's easy to replicate the results.  Just place foil on 1/2 of the peel before you lay the skin down.  Everything slides off easily into the oven.  (I know, there is a little thermal mass in the unheated foil, but that's minimal - and if I'm wrong about that too, I'd rather not know)

I still can't believe the browning difference between the cookie sheet and foil.  If you had seen how ductile that sheet was at 675F, perhaps you would have thought it was mostly in contact with the stone as I did - or at least it touched in some places - making the absence of ANY char on the sheet side of the pizza a likely predictor of the subsequent foil test.  Not so.  I knew about the thermal restance of a small air gap, but not such a tiny the gap.

[PizzaHog]

Yes, vewwy interwesting you wascally wabbit!
Some more numbers and SWAGs.

Porosity

Cordierite    0.15 / 15%
Soapstone   0.08 / 8%
Alum Foil    Unfound, but let's assume dang close to zero.

So perhaps porosity does play a part here, just not enough of one for Scott to note any differences between "stones", but enough for Tampa to note a diff when baking on foil.
Surface texture differences is also a thought, as well as the quasi contact adhesion between wet raw dough and smooth flexible foil.
One thing for sure - Don't bake your pizza on foil.
I love this stuff.

[November]

I couldn't resist commenting on this experiment, and provide the reason why the results were completely predictable.  As Scott pointed out, the experiment with the aluminum sheet was flawed in that it had an air gap between it and the stone.  The foil-based experiment was an improvement, but on the whole this line of experimentation is flawed as it does not cancel out all but the one variable being tested.  Aluminum has a very low emissivity coefficient (foil = 0.04) while cordierite is at least a magnitude higher.  This is why the pizza baked on the foil had more pockets of unchared crust on the bottom.  The crust will not brown nearly as rapidly where the dough does not come into contact with the aluminum as with cordierite or most stones.  Here it is not the porosity being tested as much as it is the emissivity of these materials.  To test porosity properly, you must eliminate as much of the material-dependent emissivity by selecting the same material, such as cordierite, and modify its porosity in a controlled and measurable manner.  Perhaps apply a high grit count sandpaper to a portion of a sacrificial stone.  This is the only way you are going to get conclusive results.

In short I can tell you porosity can make a difference.  However, not that it always will, and not always for the better under subjective qualifying conditions.  It is entirely material and temperature dependent.

- red.november

[Tampa]

Sorry, but I’m not buying your argument Mr. November.  (I know I’m likely to get schooled, but maybe this will be a good teaching moment for someone besides me.)

Emissivity is a measure of the percentage of heat radiated by a surface.  Aluminum foil with an emissivity value of 0.04 would reflect 96% of the radiant energy.  That’s nice, but the interface between the cooking surface and the pizza skin has little to do with radiation, but has everything to do with conduction.  As an example, I think I would scream almost as loud if I put my thumb on a burner covered with aluminum foil, as I would with the burner alone.  The foil conducts most of the heat but provides an excellent porosity barrier – which was the goal of the test. 

Sure we could argue about the insulation properties of a thin air gap between the stone and the foil, or between the foil and the pizza skin, but that misses the point.  I think the point is that the porosity barrier created by aluminum foil influenced the cooking of the bottom of the pie hence, porosity matters.

Maybe where there is confusion is that emissivity is also a measure of how much heat is absorbed in the material.  In that case, cordierite stores an order of magnitude more heat than aluminum foil.  Got that.  But if for sake of argument, we assume a small area of the stone holds 10 BTUs of heat, those 10 BTUs are available to cook the pie with, or without, the foil there as foil doesn’t store any heat but conducts nearby 10 BTUS very efficiently.

Dave

[November]

Tampa,

I don't understand why you are unaware radiation is a principle component of heat transfer in an oven.  Did you think the top of your pizza is cooking via conduction?!  Stick your hand in the center of a 550°F oven and tell me how long before you have to pull it back out.  Even more to the point, stick your hand about 1/16" above the surface of a preheated stone and see if that isn't too hot for you.

I also don't know why you have any kind of objection to the explanation.  It wasn't a dissent from your position on porosity.  As to all the extraneous information you gave about aluminum foil conduction, insulation properties of air, and radiation absorption: great.  It has nothing to do with what I talked about, but great.  I was just explaining the science behind why the aluminum foil baked crust was not as crispy as the stone baked crust, and why the experiment must change to make the results conclusive.

- red.november

[Tampa]

Sorry, if I was confusing and incomplete in my comments - I didn’t intent to offend anyone.  I remembered discussing heat transfer principles in an oven at length pertaining to pizza baking – but I forgot that this communication was in a private with TXCraig1.   The funny thing is that we, November and I, both used the example of “sticking our hand inside a hot oven” to describe various types of heat so we probably are not far off.  I just didn’t describe the test setup well.

I am aware of the radiation component.  For others interested, there is a pretty good discussion of convection, conduction, and radiation on Wikipedia at http://en.wikipedia.org/wiki/Heat_transfer.

I had a couple of options in the foil setup.  I could have used foil to cover ½ of the stone then fired up the oven and made the pizza – or placed the foil over ½ of the peel.  The difference is quite significant.  If I had covered a portion of the stone during warm up, the reflectivity of the foil would have directed heat away resulting in skewed results.  The test and I would be victims of emissivity and Mr. November was kind to correct me.

Instead, I placed the foil over half of the peel and slid the whole shebang in together.  It seemed like a good choice because I confirmed that the revolving stone was at a uniform temperature (refer to “rotisserie pizza grill” for setup), took advantage of the properties of aluminum foil – highly conductive, low thermal mass, and low porosity.  So given the setup, I think Mythbusters would assign “plausible” that porosity is a factor in cooking pizza.

Dave

[scott123]

Scotty wants big boom 

[November]

If I had covered a portion of the stone during warm up, the reflectivity of the foil would have directed heat away resulting in skewed results.  The test and I would be victims of emissivity and Mr. November was kind to correct me.

Again, this is not what I am talking about.  I don't know why you keep bringing up reflectivity.  Reflectivity has nothing to do with your experiment.  Low emissivity (the complete opposite of reflectivity) of the aluminum is why you had poor crust browning in the areas where the dough was not in contact with the foil.  Porosity would have nothing to do with the browning of any food when the food doesn't come into contact with the baking surface.  Browning only occurs where energy is being absorbed.  Porosity does not equal energy.  I can't brown something just because I put it on a porous surface.

I don't know how to make it any clearer.  The pockets of crust where the dough did not touch the stone browned because of ample radiant energy.  It wasn't because of conduction, invisible fairies, or porosity.  It takes actual heat to bake food.  On a good day the best porosity can do is dehydrate food.  Dehydrated food bakes faster but it doesn't magically bake itself.

- red.november

[November]

Attached to this post is a marked up image of the original image displaying the results of baking on a stone baking surface and an aluminum foil baking surface.  The ovals markup four example regions of the crust where the dough did not have direct contact with the baking surface.  The green ovals are where radiant heat was able to brown the crust.  The blue ovals are where radiant heat was present at a level ten times less than the stone, so the crust was not browned in those regions.

You can't with any validity call this difference on account of porosity.  How much porosity makes a difference can't be determined until you have the emissivity (and consequently radiant heat) equal on both sides.

[scott123]

Ah, I get now.  November, I have to admit that when you first mentioned emissivity, I was like 'what?'  but after reading your post a few times, it finally hit me.

That's fascinating.  I never really thought about radiative energy playing a role in the baking of the bottom areas of a crust that don't touch, but now that you bring it up, it makes perfect sense.  Where you have air, you have radiative impact.

Now how much radiative energy plays a role, that's the question.  We're talking about a fraction of an inch here. Radiative heat is a factor of distance. The closer the object, the less impact the poor emissivity of aluminum foil will have.

[November]

The closer the object, the less impact the poor emissivity of aluminum foil will have.

You have that backwards.  Radiation follows the inverse square law.  The further away the object, the less a difference there is by a factor of the distance squared.  So at very close distances, which is where these pockets are, the difference between aluminum and stone is huge.

[Tampa]

Thanks Mr. November, but enough with the condescending replies.

I’ve had the classes: physics, thermo, heat transfer and I’m good at burning pizzas.

I agree with you that heat energy causes browning.  In the non-contact areas, how do you know that radiation is the predominant form of heat transfer?  Did you run the calculations?  Did someone tell you that?  (Circle one)  If you ran the calculations, please share.

To me, we’re talking about a tiny gap and all three forms of heat transfer are involved to some degree.  I don’t believe radiation dominates.  But if it does, you are clearly right and I am clearly wrong.  I’d rather eat pizza, but I’ll eat crow.  Show me crow.

Dave

[scott123]

You have that backwards.  Radiation follows the inverse square law.  The further away the object, the less a difference there is by a factor of the distance squared.  So at very close distances, which is where these pockets are, the difference between aluminum and stone is huge.

Wait a sec. Radiative impact is decreased as you move further away.  Shiny materials create a barrier to emission. As you move closer, that barrier should become less significant because of the increased radiative impact.  Am I missing something? Are you saying that if I have a shiny radiator and a black radiator, if I hold my hand far enough away, the heat will feel comparable, but if I move my hand closer, the poor emissivity will kick in and cause my hand to feel proportionately cooler? The closer my hands gets to the poor emitter, the more proportionately cooler it will feel?

While we're on the topic of radiation...

I'm curious, are better conductors more powerful/faster emitters? In, say, a typical 550 degree environment, would a pre-heated slab of steel brown a pizza from above any better than a firebrick?

My gut feeling is that steel's superior conductivity will cause it to lose energy faster to the air around it. It will glow (in this instance non visible IR) more intensely for a shorter amount of time than the firebrick.  Assuming that that 'shorter amount of time' is the time it takes to bake a pizza, shouldn't the pizza brown more under the steel than under the brick?

For the sake of argument, let's assume the brick and the steel are the same color.

[November]

In the non-contact areas, how do you know that radiation is the predominant form of heat transfer?

You can't be serious.  You've turned from saying the extra browning occurred because of porosity to asking how I know radiation is the predominant form of heat transfer in an oven where conduction couldn't possibly take place?!  I've never in my life seen a larger straw man argument.  Porosity is not being tested here.  That's the bottom line.  I'm sorry you can't accept that.

On top of that, I already gave you the numbers: 10x difference in emissivity; 0x difference in convection (same air, vapor, and volume); 0x difference in conduction.  As a side-note on the convection difference, if your theory of porosity actually had any validity, the difference would be negative, as there would be less water vapor to transfer energy to the crust, and not to mention aluminum would transfer heat to the air and water molecules faster than stone.

I'm sorry, but I don't have time to help you with this anymore.

[November]

Wait a sec. Radiative impact is decreased as you move further away.

http://en.wikipedia.org/wiki/Inverse-square_law

If the radiation energy emitting from the aluminum foil is 100W and the radiation energy emitting from the stone is 1000W, both at a distance of 1 unit length, doubling the distance would put the two respective energy levels at 25W and 250W.  The difference at 1 unit length is 900W whereas the difference at 2 unit lengths is 225W.  Because the activation energy for chemical bonding is a constant, this change in levels becomes less significant the greater the distance.

I'm curious, are better conductors more powerful/faster emitters? In, say, a typical 550 degree environment, would a pre-heated slab of steel brown a pizza from above any better than a firebrick?

Thermal conductivity has little to do with emissivity.  Water for example has a thermal conductivity of only 0.6 W/m K.  Water's emissivity however is in the high 0.9s depending on wavelength.

[Tampa]

What, no numbers and no crow?  I’m sorry too.  I guess we’re just steaming up the forum.

I’m sure we are all good guys at the end of the day – just got off to a bad start. 

I’m hoping to have a phone conference with an engineering professor that consults in oven design in the coming weeks.  If the call happens, and if I’m wrong about radiation/emissivity being a dominant factor, I’ll post up and come clean.

Dave

[sear]

damn, i just got my popcorn and everything .. 

[Tampa]

I’m sure your popcorn is stale by now.

I did have a phone conference with the good professor.  For what it is worth, he is an Ivy-league PHD engineering professor with twenty years of experience, including oven design.  In short, he didn’t know the answer to the question of which mode of heat transfer dominates between the bottom crust and stone – radiation or conduction.

Although a disappointment, this shouldn’t be a surprise to most of us.  Professors tend to be highly specialized in their research and knowing what happens in the boundary layer between dough and stone isn’t widely known, even by some oven designers.

For now, I’m back to eating pizza.  But the other guy could be right, or not, I don’t know.

[Tscarborough]

Steam from the dough looks to be the difference in the pic above.  On the left the stone absorbs or at least diffuses the steam, on the right, the foil does not.  Don't know about the emis... whatever, or conductivity, but steam generation is an important part of the cooking of any dough.

I generally start on a pan for 30-40 seconds, then remove it at the first turn of the pie, and get no charring at 650-900 degrees hearth temp on a 2 minute to 4 minute bake.