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This was something I noticed right away in our new kitchen, I love that it's so easy to defrost something on granite!

Perhaps it is the radon in the granite?? (sorry, I couldn't resist...)

Another way to do it (if you don't have granite) is to put your frozen item in a cast-iron pan which, oddly enough, does conduct heat well.

Ok, let me take a stab at the science... granite is going to be slow to change temperature even when heat is applied or (in this case) taken away. That temperature is higher than the freezing point of water; ergo, the ice melts. If my logic is faulty (or if someone can explain it better) please jump in.

Granite doesn't conduct heat?

I always thought that the opposite was true, and that this was the reason why granite feels cold to the touch, because it transports heat away from you more quickly than for instance wood.

Aluminium, which conducts heat very well, is also great for defrosting.

Are there any scientists or physics teachers here who can give us an answer?

It would seem that Granite is actually a reasonably good heat conductor, although much less so than the metals listed.

http://www.learn.londonmet.ac.uk/packages/clear/thermal/buildings/building_fabric/properties/conductivity.html

So granite has a low *specific heat,* which is the amount of temperature it can absorb or needs to have taken away before it actually changes temperature, meaning you can put something really cold or hot on it and it still won't change temperature as much. Or, it absorbs all the coldness from your frozen chicken without getting colder, so it can continue to warm the chicken up. It's a bad energy conductor, but it's a good energy absorber.

If you want to get technical, and I remember this from freshman year Physical World well enough, the bonds between the granite molecules are very strong so each molecule is very stable. You have to put a LOT of heat in, or cold, to make the molecules shake faster (warm up) or slow down (get cold). http://en.wikipedia.org/wiki/Specific_heat_capacity

Physicist here ... although I've never given granite a thought before...
Katti is absolutely right, if something feels cold to the touch (at room temperature of course) then it conducts heat rather well. And if it quickly transfers heat away from your hand, then it can also quickly transfer heat to the frozen food. This is why metal feels cool etc. The heat conductivity depends on the molecular structure so each material has its own, and although granite's is lower than metals' it's still higher than many other materials'.
In this case the smooth surface also helps: notice how coarser materials feel warmer to the touch (eg. cotton vs. silk). This is due to their molecules being a bit farther part, so their individual motion cannot be as easily transfered from one to the next. And molecule motion = heat.

IT works well for a couple reasons, some of which are explained above. All things in nature tend towards an equilibrium. In the case of the granite and the frozen item, there is a temperature imbalance. Nature wants to equalize this imbalance, and in short dissipates the cold of the frozen item throughout the large mass of the countertop. The countertop is so large that the surrounding areas can then interact with the warm air dissipating its cold (technically absorbing the energy of the warmer air, but same basic idea), thus allowing the countertop to absorb even more cold (again actually transmitting its "warm energy" to the frozen item, but...). In other words, the countertop acts like a giant heatsink.

I hate my granite countertops, but the lemonade is a) defrosting and b) rolling out crusts. Fab for both. If only it wasn't hard enough to shatter acrylic, ugly, and a fussy material to care for.

You can't pull cold or absorb cold - cold is just the absence of heat. The energy from heat transfers from warm things into cold things, not the other way around.

I'm guessing that granite is a reasonably good conductor of heat. It's also probably affected by the fact that you generally have one big heavy contiguous slab of granite. As the heat moves from the granite countertop beneath the frozen food, the heat from nearby parts of granite keep moving into the cold spot, and therefore into the frozen food.

So to sum up, granite is not as good of a conductor of heat as metal, but because it has so much more mass, the net effect is that it is a better surface to defrost on. (i.e., a larger metal surface would be more effective than a smaller surface of the same metal because it has more mass). I usually defrost items on my stovetop because I have formica counters which do not absorb heat/cold very well at all, but I do need to move the item being defrosted to a less cold area of the stove so it can draw give off more heat than the area the frozen stuff cooled down (stainless is also not a great conductor by itself).

There are three ways to heat something (ie thaw something frozen): conduction, radiation, and convection. If you were to suspend the bag of stock from a string, only radiation and convection do anything. But when you put it on top of something else, you now have conduction, which is the most efficent. How efficient it is, depends on how good a conductor you have, and how much heat there is to conduct into the thing you're thawing.

Granite actually has a High specific heat. Specific heat is how much energy it takes to raise some mass (it's not per volume, but per mass) of the material some temperature. (typically measured in joules per gram per degree celcius or kelvin).

This has nothing -directly- to do with ability to conduct heat, or how heavy or dense it is, but these things commonly go together: gold is heavy, conducts well, and has a high specific heat. Aluminum is light, conducts well, and has a medium to low specific heat.

Wood has a low specific heat, and low conductance, and also low density. It's not a conductor, but more of an insulator (hence wooden handles on pots). This means that when you put the chicken stock on a wooden cutting board or counter, the only heat entering the stock is from the air.

The sink has a medium to high specific heat, mediumish conductance, but is fairly small, so when you but the stock in the sink, the sink quickly gives much of its heat to the stock, but there just isn't enough of it to make a much of a difference. And since stainless is a pretty bad conductor (hence aluminum cores in high-end cookware) most of the sink isn't helping; the heat to defrost the stock is coming from only a small part of the sink.

Now, granite has a high specific heat, a really high density, and lowish conductance. So a 3" thick bit of granite holds a lot of heat. When you put the stock on the counter, that heat will be sufficient to defrost alot of it. Even tho the granite doesn't conduct very well, the granite is so massive that it has enough stored heat to thaw a pretty big amount of stock before become too cold to do much good.

If you had a solid 3" steel countertop, that would defrost stock even faster than the granite, as it would have a huge amount of stored heat, and because it's able to conduct it better than granite, more of the steel's heat would be available for thawing.

But the best way to thaw something is to seal it in a waterproof bag, and dump it in a sink full of water. The hotter the water, the faster it thaws. Change or heat the water for faster thawing. Just keep in mind that for big things, you need to conduct heat into the center of what you're thawing, and there's no hurrying that. All you can do is work on the outside.