The Fresh Loaf

News & Information for Amateur Bakers and Artisan Bread Enthusiasts

What is the rationale for "thermal mass"?

yankeedave's picture
yankeedave

What is the rationale for "thermal mass"?

Maybe this is an ignorant question. In fact, it is an ignorant question, since I don't know the answer. But I'm wondering. 


I've so often read that it helps to add thermal mass to your oven that I've never really questioned why. But now I wonder - what is the reason?


The most common rationale for adding thermal mass (usually by means of a pizza stone) seems to be that it keeps the temperature stable. In other words, when you open the door to remove or rotate the loaf or pizza, the oven temperature will not drop much.


If you are not opening the oven door much, though, or not baking many loaves or pizzas, how important is that?


Let's say I bake a pizza on a metal baking sheet. My oven heats pretty evenly, so I don't need to keep futzing with it during baking, so I just take it out after 8 minutes or so, quickly. If it takes the oven another minute to get back up to 550, before I put the next one in, what's the big deal? If you're going to be baking a lot of pizzas, maybe it matters, but with 1, 2 or 3, so what if you have to let it reheat for a minute?


And if keeping the oven temperature stable is the main issue, I can do that without a $50 pizza stone. I can just put some bricks on the other rack, right? More mass, and much cheaper.


Now maybe another rationale has to do with the stone itself. If you put a pizza onto a metal baking sheet, I imagine the sheet would cool down a bit more than would a stone, since it's thin and metal is a good conductor of heat. But if it's on the lower rack, near the heating element, and is good and flat, no concavities or convexities, and heats evenly, would it really make that much difference?

blaisepascal's picture
blaisepascal

It's not so much about what the oven does between pizzas, it's what the oven does when you open the door and dump all the hot air out.


If your oven is just thin sheet-metal walls, metal baking racks, and thin metal baking sheets, then most of the heat energy in the oven is actually going to be in the air, which leaves the oven when the door is opened.  The oven then has to dump a bunch of energy back into the new cooler air in order to bring things back up to temperature.


If your oven is full of bricks or a baking stone, it takes a lot of energy to bring that mass of stone up to temperature and it isn't lost when the air is dumped.  The hot bricks won't be cooled by the cool air and will reheat the air quickly.


Better still, when you put your cool, wet pizza dough on the metal sheet, the sheet cools down drastically and so the whole pizza has to be heated by the hot air and the oven directly.  When you put your cool, wet pizza dough on a heavy baking stone, the stone doesn't cool down as much and immediatly starts directly cooking your pizza.

yankeedave's picture
yankeedave

OK, that makes some sense. So having said all that, what if you laid regular bricks on the rack, and put a metal sheet on top of them? Plenty of thermal mass and no stone to worry about cracking from that cool, wet pizza dough?

cranbo's picture
cranbo

that sounds like it would work fine to me.


Be aware that if the sheet doesn't sit flat on the stone (as many baking sheets warp) you will have more temperature variations across the metal surface, because some parts will be in contact with the stone conducting heat, and some won't . But it might not make a big difference in the finished product. 

Chuck's picture
Chuck

...no stone to worry about cracking from that cool, wet pizza dough?...


With a good baking stone, cracking happens very seldom (0.5% in 10 years?). Don't worry about it (but don't let your mister drip cold water onto a hot stone, or drop barbells on it, either:-).

yankeedave's picture
yankeedave

But I've gone through 3 of them now. I heat the stone as the oven is preheating, I try not to get any water on them, but I've about had it with pizza stones. My current one, though cracked, is still usable, since the two pieces fit together well, but I'd like a cheaper alternative.

Chuck's picture
Chuck

Hmmm, that's atypical and weird.



  • What kind of stones are they (Fibrament, granite, tile, cordierite, etc.)?

  • When do they crack (during preheating, when you put any dough on, when you put pizza dough on, when you add pizza toppings, during cooling, etc.)?

  • What exactly do you mean when you say you "try not to get water on them"? Does it happen once in a while anyway?Are these "accidents" associated with cracking? Is the stone hot or cold when this happens? Where does the water come from? What (if anything) is the association between water and dough?

  • What oven temperature are you using (and have you "modified" your oven to reach that temperature)?

  • How close is the baking stone to the bottom element (bottom rack, five inches, etc.)?

  • Are you using any steam, and if so how do you generate it and for how long?

yankeedave's picture
yankeedave

Frankly I don't know what they've been made of. They were all ordinary commercially sold pizza stones for home ovens. The last one cracked while I was baking pizza. I don't recall if there was a pizza on it at the time, as I made four pizzas that day. I heard a loud noise and when I looked in the oven it had cracked. At that time I was not using any steam at all. It's conceivable that some sauce dripped onto the stone but that would be it. It was maybe 4 inches from the electric heating element, and the oven was at 550. I had previously used the stone, without incident, to bake bread, using a preheated pan underneath for steam, but I was careful when pouring in the hot water not to get it on the stone. Never washed it.


It's been some time since I had the other two stones, so I don't remember the precise circumstances. The first one did last me for a couple of years or more and in fact had been accidentally dropped by a family member, and one corner chipped off, so maybe there was already a hairline crack in it when it cracked in two in the oven. The second one didn't last long, and broke under similar circumstances, I think, to this latest one, for no apparent reason. Had the first stone not been damaged, maybe I'd still be using it today, but I'd still like to find a cheaper alternative without sacrificing the quality of my breads and pizzas.

mrfrost's picture
mrfrost

After reading several of your posts regarding your cracked stones, I noticed you never mentioned the stones' sources, or materials, or thicknesses. Your latest post seems to acknowdledge those were probably not considerations.


I'm sure many of us probably started out using the cheap(no matter the actual cost), generic, 3/8" or less "ceramic"  "pizza stones". You can(or could) pick them up at Walmart for about $10. Some seem to have pretty good luck with these. Mine lasted about four years with occasional use for pizzas. But as soon as I got deeper into more frequent, serious baking,  it gradually, but quickly became a jigsaw puzzle in just a few more months.


After doing some research, I settled on a more serious baking stone made of 5/8" thick cordierite. The thickness, and thermal properties seem to be turning out to be ideal for my needs(though approaching only 18 months use, so far). Depending on your needs, they can get even a lot more serious and expensive, but a thicker stone comprised of  known materials, with good thermal properties, seems to make the most sense after one's first failed "pizza stone".


Also, if you do end up getting another stone, make sure to read the documentation and/or research the proper use and care of the stone; including the initial break in(tempering).


Although there are perfectly good breads made and sold without being baked on stones, I personally don't think there is some "hidden secret" way of replacing the results of a good baking stone in a home oven. If there is, I will certainly be on the lookout for it if my present one ever fails.

Chuck's picture
Chuck

Cracking the next time a stone is used after it's been damaged (rather than right away:-) is quite quite common, which often makes it relatively difficult to figure out what went wrong. 550F is fairly hot for a home oven, and it sounds like the baking stone was quite near the element. Can you move the rack with the baking stone up one notch?


Also, I suspect the "steam" when baking bread may have gotten water into the stone, which then caused it to crack the next time. It seems to work better if you can somehow get the steam source beside the baking stone (or failing that as much distance away as the heating element) rather than right under the stone.


Many commercial pizza stones are rather thin (3/8 inch?); something a little thicker (more than 1/2 inch?) would be less prone to cracking (and contrary to expectations possibly less expensive too, it just depends on what's available in your area). A material called "cordierite" works very very well (it's what's used in those commercial "brick ovens";-). It's probably not easily available to the general public in small enough pieces  ...except in disguise. Find a pottery supply store and get a "kiln shelf" (be sure there's some space [an inch or more?] on all four sides of the stone in your oven). Another possibility is the sink cutout from a granite countertop  ...except you have to be careful to only get UNsealed pieces (ground smooth is okay, maybe even a good thing). (And be sure the supplier doesn't try to be "helpful" by gluing on little non-skid or padded feet, as they won't stand up in the heat.)


(One caveat though: not all bricks are the same, and some will "explode" the next time they get hot after they've gotten wet. Firebrick or very old brick is better; be a little careful of newfangled bricks. Put a drop of water on the brick and wait five minutes; if the water has entirely soaked in, take more care with that brick. It may be safer to completely wrap the bricks in layers of [waterproof] tinfoil.)


Also be aware that in most home ovens the "ready" beeper that tells you when pre-heating is done does not work right with extra thermal mass. Typically, however long the oven says it takes to preheat, double that time before you put anything in the oven.


A good way to minimize damage to a baking stone is leave it in the oven all the time. There are very very few things it will interfere with; almost everything more or less ignores it. The extra thermal mass is nice for a lot more than just pizzas and bread. And the oven itself is a good storage space.


...


For pizzas, at least some kind of special surface and also some "thermal mass" seems to be quite important. A pan with a whole bunch of small holes in it (to let the heat into the crust and the moisture out of the crust) seems to work well as the special surface. Thermal mass is hard though. Maybe open and close your oven as little as possible and very fast, or maybe your idea with bricks will work (there are even pictures here of bricks wrapped in tinfoil).


For bread on the other hand, it may work okay to bake on something like an inverted metal sheet pan (or two) without any baking stone at all. If you look persistently here on TFL, you can find reports and even pictures of baking bread without any baking stone. (It's "heresy" and contradicts longstanding conventional wistom  ...but some reports suggest it works much better than you'd expect. Maybe thermal mass matters less at the somewhat lower temperatures used for bread loaves.)

yankeedave's picture
yankeedave

thanks Chuch, MrFrost, et al. for the thoughts and ideas. I am thinking that my last two stones may simply not have been very good stones to begin with. And I did put them right above the water pan so they were getting hit with steam when I would bake bread.


I've also seen the kiln shelf idea suggested although after some very cursory online research it seemed as if finding a useable size might be a little difficult. Many of them are too big or too small for what I want - I've seen 20x20, 9" diameter, etc.


For now I am having an unglazed quarry tile and a travertine tile tested for lead and if they come back clean, I'm going to try using them, along with some foil-wrapped clay bricks in the oven for mass. But if they prove unsatisfactory the kiln shelf idea would be next, followed by a good thick pizza stone.

Chuck's picture
Chuck

I am having an unglazed quarry tile and a travertine tile tested for lead and if they come back clean...


I'm confident they will come back clean. Lead is sometimes a problem in ceramic glazes (especially brightly colored ones), but not in the ceramic itself.


 


I expect (but please post your results) the unglazed quarry tile will work better than the travertine tile for a baking stone. (Although unglazed quarry tile can be an inexpensive and excellent material for baking stones, it's become so hard to obtain in some countries [one has to avoid "sealed" and "stain resistant"] that it doesn't suit the purpose any more.)

suave's picture
suave

Heat capacity of air is ridiculuosly small, far less then 1% of equal volume of a typical solid or liquid.  Say you replace the entire volume of a 5 cft oven heated to 450 F with room temperature air.  Typical 3 kW heating element can generate energy due to this convective loss in slightly less than 15 seconds.  Much more significant heat losses occur due to radiation.

leucadian's picture
leucadian

A convection oven is an important part of the equation, because the circulating hot air (only when the door is closed) means that the oven will be less affected by opening the door, especially when there is added thermal mass. The convection fan willl transfer the heat of the bricks and the heating element quickly to the air.


Another factor is the burst of heat on a conventional oven after the air has been cooled by opeing the door. The oven measures the air temperature in the oven, so when you open the door the heating elements come on until the air is back up to temperature. You put your perfectly proofed bread in the oven and the first thing it sees is a red-hot element inches away, trying to heat the air (convective), but doing a better job of heating the stone and your bread (radiant). I think this is why we are advised to lower the temperature after loading the oven: it's just to keep the heating element from coming back on too quickly. In a WFO, the temperature is not reduced, but there is no instantaneous heating element to burn things either.

Nickisafoodie's picture
Nickisafoodie

If you prefer to stay with metal pan and bricks in bottom rather than a stone - then find a perforated pizza pan.  Usually holes about 1/4" in size, every inch or so.  This allows the hot air to get into the bottom of the crust for a browner crisper bottom.  Place on your normal rack a few inches from the floor (or above the brick if you add for mass) so air can get under.  This is as close to a stone as you can get without using one...

yankeedave's picture
yankeedave

Do you preheat the pan and slide the pizza onto it? I've had pizza from pizzeriaas baked on those kinds of pans, but they usually make the pizza on the pan and stick the whole thing in the oven. I've never found those crusts to be too good since the pan has to heat up at the same time as the crust. Plus it seems to me that holes would mean you're using convective or radiant heat, which doesn't seem like the best way to bake a pizza.

ehanner's picture
ehanner

I've never cracked a stone but I have broken the glass in the oven door from dripping water on the glass. I'm wondering what kind of stone you have cracked 3 of? Also, what is happening to the stone between uses? Are you by chance exposing the stone to water by cleaning it? That would do it.  If you want a stone, I suggest buying a good one from a reliable source and not a retail store or discount operation. I bought my stone from King Arthur about 5 years ago and have spritzed it, poured water on it while at 500F, placed 500 degree cast iron pots on it when it was cold. All of those things would not be suggested usage but still, it looks as good as the day I bought it. And I just have a cheap GE oven, nothing special, a coil in the bottom and top.


On the subject of thermal mass and the need for a stone to get good results. A couple years back I bought a IR laser thermometer to measure the temperature on my outdoor pizza blast oven. This is a converted Weber with a gas burner in place of the charcoal bottom. I cook at 700F on a stone and I wanted a way to measure the heat and even heating of the stone. Since then, I have found many uses for the laser temp sensor and I use it many times every day. I would say that considering how important it is to know and manage your dough temperature, it is the most important tool I have in the kitchen. They don't cost much and once you have one, you will understand how valuable it is from measuring the deep fryer temp to the water for the levain. One click from a distance and you have an accurate temp.


One of the first things I learned with the IR temp gauge is how long it takes to get the stone up to full oven temperature. In my electric oven it takes a full 45 minutes to raise the temperature to 450F if the oven is set at 500F. Count on an hour at least to reach 500F. Then when you load bread or pizza, it drops to 300F roughly and stays there. Baking Pizza the stone temp recovers slightly better because it is so thin but if you're baking multiple pizza's, the second one will be baked on a cooler stone, even if you wait a minute or two.


The point to all this is to try to understand the need for thermal mass or extra mass in the oven in the form of bricks/stones/tiles/steam pans and so forth. It is possible to bake breads and pizza with out any extra thermal mass. You might need to consider which shelf level you use and adjust the time somewhat, but this can be easily done. Personally I bake breads on sheet pans all the time, especially if it will be easier to load a large loaf. I use the stone for hearth style breads and pizza and pita. Hope this helps.


Eric

yankeedave's picture
yankeedave

I have also wondered - if you are baking several pizzas in quick succession, wouldn't a metal sheet reheat faster than a stone, in between pizzas? Given its heat conductivity I suppose a sheet's temperature would also drop more when you put a room-temp pizza on it, but if it's at 550 or more to begin with, the pizza is still going to be hitting a very hot surface when you put it in. An experiment would be best but ideally it would utilize a stone side-by-side with a sheet, baking pizzas at the same time, and I don't have room for that.

cranbo's picture
cranbo

Yankeedave, someone has done a similar test.


See this thread on the steel sheet vs baking stone


Yes, steel sheet heats significantly more quickly than baking stone. 

cranbo's picture
cranbo

Double post sorry

polo's picture
polo

Take a look at the Lodge website. They have a multitude of choices in cast iron cookware. I use the pro grid/iron griddle for bread baking in my oven. I can bake two Boules side by side on this griddle. A decent sized Baguette will also fit.


https://secure.lodgemfg.com/storefront/product1_new.asp?menu=logic&idProduct=3943


They also sell a 14" cast iron pizza pan (if that is large enough for your needs)


https://secure.lodgemfg.com/storefront/product1_new.asp?menu=prologic&idProduct=3984


The thing I find nice about cast iron is that I can quickly preheat on the gas range top, before placing it in the oven. Also shouldn't have to worry about cracking. They are more expensive than a baking stone, but also multi functional.


By the way, I am not affiliated with the company. Just a fan of the equipment.

highmtnpam's picture
highmtnpam

See www.bakingstone.com/  -for a  fibrament stone-  $10.00 charge for custom size. Lots of information about baking stones.  Love mine


Pam

Pioneer Foodie's picture
Pioneer Foodie

I broke a pizza stone by carrying it hot on the manufacturer-provided wire carrying rack, and then setting that wire rack on a cold tile counter top.


After that I bought a remnant of a granite counter top, 2cm thick, and had it cut to size.  $20.  Cheaper than a pizza stone and much much bigger and better.  The pizza stone was smaller than the oven and I had lots of wasted space.  The granite fills the whole rack and I can bake four free-formed loaves (1.5 lbs. ea) instead of two.  Just get the plainest looking piece they have as any beautiful streaks running through the stone are signs of weaknesses in the stone.