river rock for cooking project

December 2, 2007 7:35 PM

Where in nature can I find a dinner plate sized rounded flattish rock that can be heated to high temperatures without exploding?

Ideally one side would be flat, or best, concave. The other side doesn't matter so much as long as it is rounded. Which part of the river should I look in? What land features produce this type of rock? And how do I know it's safe for heating. This is for a cooking project and I live in Japan if that helps.

posted by Infernarl to science & nature (20 comments total)

What kind of rocks you will be able to find in rivers near you has to do with what the underlying land is made of -- not with what shape it has been carved into by water. Where are you in Japan?
posted by LobsterMitten at 7:38 PM on December 2, 2007

Here is a geological map of Japan. To see what kind of rock is at the surface in your area, click on the square that is closest to where you are (or where you would travel to, to find your rock). The legend is in the left column - click it and read what kind of rock each color on the map represents.
posted by LobsterMitten at 7:43 PM on December 2, 2007

Soapstone, if you can find it would be ideal.
posted by hortense at 7:55 PM on December 2, 2007

Generally you don't want to heat river rocks as they are more prone to exploding due to water expanding inside them. [/girlscout]
posted by fshgrl at 8:17 PM on December 2, 2007

I don't know about heating properties of rocks, but here's as much info as I can offer.

Searching online for "cooking rock", the ones I see are described as being either "lava rocks" or granite. (This site has some suggesting that you should not use granite from a river, because it may have water trapped in it that could explode when it's heated.)

On the legend of the geologic map, granite would be a "felsic intrusive" or "felsic plutonic volcanic" rock.
"Lava rocks" would be volcanic rocks that are not "pyroclastic" or "debris" or "intrusive". A drawback of granite is that it's really hard, so it will be shaped by a river much more slowly than other rock types. It also doesn't naturally become flat and dish-shaped with wear; it tends to wear in a more rounded sphere-like shape. Commercially cut granite would be a perfectly flat surface.

Rocks that naturally become flat and dish-shaped with wear include most sedimentary rocks (because they are made of material that was deposited in flat layers in the first place, so they naturally break down at the separations between these layers) and low-level metamorphic rocks (slate and schist -- these were existing rocks that were subjected to huge pressure, and their minerals re-formed with strong bonds in one plane and weaker bonds in another plane, so they break down naturally in flattish sheets). (High-level metamorphics like gneiss will not work, since they don't have the natural flat-sheet breakdown.)

Soapstone is a schist made mostly of talc, so it's very soft. To find soapstone, looking at that geologic map legend, you want "low-press type metamorphic".

Here are some quick rock identification photos.
posted by LobsterMitten at 8:22 PM on December 2, 2007

You might be able to reduce the risk of it exploding by baking it out slowly — cook it at 70°C for a while, then 80°, and so on, to drive off any moisture slowly enough that it doesn't crack the rock. I don't know how much time you'd need to spend doing that. If you can't find information on drying rocks, I'd start with 15 hours or so of gradual baking, and if the rock still bursts, try longer.
posted by hattifattener at 9:27 PM on December 2, 2007

I live in Osaka. On the geological map that is here.

Soapstone seems good except that it is so soft that I would be worried about trapped water. And I don't know if it's thermal properties are appropriate.

Does anyone know at which point in a river's run a dish shaped stone would be most likely to occur?
posted by Infernarl at 10:04 PM on December 2, 2007

soap stone is impervious,
posted by hortense at 10:29 PM on December 2, 2007

at which point in a river's run

There is not likely to be a simple answer to this question. Too much depends on the rocks the river runs over, how old the river is, how steep its descent is, etc. The closest to a general answer I can give (although, I am not a geologist myself) would be in the middle or toward the drainage of the river, not toward the source. You want a rock that has been in the river for a long time, and the ones near the source have not been in it as long.
posted by LobsterMitten at 10:52 PM on December 2, 2007

Also, once you find a rock, test-heat it outdoors in a place where it could explode safely.
posted by LobsterMitten at 10:54 PM on December 2, 2007

In Switzerland and Germany, it is somewhat popular to cook raclette (hot stone) style, using little grills with granite stone tops. The granite stones are first heated in electric ovens over a couple hours time, to very high temperature (500 to 600 ° F). Then, they are place in holders or on racks with Sterno can flame pots, to be brought safely to the table. Meat, cheese and sometimes vegetables like mushrooms are sliced or cubed, and grilled at the table, by each person, sizzling in little pats of butter. After grilling, each person pulls off his meat/cheese/vegetable chunks, and dips them in an assortment of sauces. Thus, most raclette "sets" also include a number of shallow bowls or trays for the requisite sauces.

Overall, it's a lot like fondue cooking, but instead of fondue's deep fat frying method using a central, shared pot of heated oil, raclette uses the grill-on-stone method, which adherents claim is "healthier."

Raclette stones are usually machined from natural granite stone blanks - they need to fit in the grills that safely hold them during use, and many have a shallow channel machined into them, around the perimeter, to contain excess grease that might cook out of meats, or be applied to cook vegetables. You wouldn't find modern raclette stones worn smooth in a river bed, simply because they'd never attain their useful, very flat shapes naturally, any more than you'd find granite counter tops, or tombstones (two other popular uses for this dense, hard, long wearing stone) laying around. It's common to machine granite for most human uses.

But there are many other ways of using stone as a heat absorber/re-radiator tool, where a river rounded shape would be no great impediment. However, as noted by Lobstermitten, granite is among the hardest and densest form of igneous rock. It weathers from mountains very slowly, and because of it's weight and density, doesn't easily fracture into small stones. Here's a link to rock identification guidelines for granite, to aid you in your search.

It would take a long, long time for granite in a river to be worn smooth, and you would best look for your rocks in very fast flowing river channels, where the current is strong enough to jumble rocks one upon another regularly, or at least to carry and churn large quantities of abrasive sediments such as sand. Mountain streams with seasonal whitewater chutes are your best bet for finding such rocks, both because they are probably headwater streams nearer the source of granite fracture outcrops, and because their seasonal flow energy can produce the rock-against-rock tumbling action needed to smooth granite effectively. If you do find a suitably shaped and sized natural granite stone, I don't think you need be too worried about it exploding due to trapped water - granite is very dense and hard, and simply won't hold much water. Just heat it slowly the first time, over a couple of hours, and you should be fine.
posted by paulsc at 11:21 PM on December 2, 2007

Hangi stones are an important cultural item here in New Zealand.

The best ones are igneous. They should make a ringing noise when you hit them when a hammer.

River stones can be ok, but sometimes they have invisible fractures with water inside, which can lead to very nasty surprises when they are heated.
posted by i_am_joe's_spleen at 11:47 PM on December 2, 2007

Not only is soapstone relatively nonporous, it's very soft. You should be able to grind quite a nice hollow into one side of a flattish piece with a disc sander.
posted by flabdablet at 12:00 AM on December 3, 2007

Soapstone seems good except that it is so soft that I would be worried about trapped water. And I don't know if it's thermal properties are appropriate.

You can buy soapstone bakeware and pizza stones, so it probably would work fine thermally. They might be pre-seasoned in some way, though.
posted by advil at 12:16 AM on December 3, 2007

My mom has a soapstone pot. Similar to this one. Never cooked in it but it is supposed to be excellent on a wooden stove.

We also have flat pieces of soapstone that we use as cooking surfaces when we barbecue.
posted by uandt at 1:55 AM on December 3, 2007

The Chumash tribe in my area used soapstone and serpentine bowls and pots in olden times so you might check archaeological resources for your area to see if archaic people did the same.
This might give you info on sources.
posted by Iron Rat at 7:19 AM on December 3, 2007

Soapstone seems indestructible. We have it as countertops and routinely place pots on it straight from the range and oven. My mom used to put a slab about 8x6x1 inch thick in the oven as a bedwarmer for us as kids.

Since it's not clear from your post [apologies if it's inside here somewhere] how badly this has to be a "found" item, I would consider going to a tile/stone/granite dealer and asking them. Alternately, lots of people carve soapstone, so an art studio might guide you to a source.
posted by docpops at 8:08 AM on December 3, 2007

These are excellent answers; soapstone is used as a cooking surface traditionally all around the world, and I can hardly contain my envy of soapstone countertops. Whatever stone you choose, I want to re-emphasize i_am_joe's_spleen's point about tapping it to make it ring (I would use a wooden spoon) every time before you use it. Ringing with a clear tone tells you it's all in one piece with no major cracks, and so less likely to explode under heat.

I'd guess you are very unlikely to find this, but I think a big piece of amorphous quartz would also be ideal. Quartz, according to my old Merck Index, has the lowest known coefficient of thermal expansion for a solid material.

Granite can contain up to 3-4% water.
posted by jamjam at 8:56 AM on December 3, 2007

In Catalonia (south of France and northeast Spain) slate has been used for centuries as a cooking surface over an open fire.

This writer claims that it imparts a unique flavor.

I have enjoyed meat cooked on slate many times in the Pyrenees mountains.
posted by Geo at 5:48 PM on December 3, 2007

Just a quick note re: finding this stone in a river. Probably not going to happen. There are two opposing effects here. Something that is well rounded (worn smooth) will typically be found fairly far from the sediment source, as it has been in transport and worn down for quite some time. Opposing this is the fact that the further you move from the sediment source, the weaker the transporting medium (in this case a river) becomes. Thus when you are far from the source, you are unlikely to find large cobbles or boulders in the river. You may be able to find something large in the upper reaches of a stream that has been in the stream bed a while, and then worn down. But again, you run the risk of getting a 'wet' stone.

You might try a beach. I have been to some beaches in Japan where there were fairly lagre, rounded basalt cobbles and boulders lying about. They are near their sediment source, but are exposed to very high energy surf, and so they are often quite round. They are not often concave, however. If memory serves, the beach I was at was on the Izu Peninsula. Even if you don't find what you're looking for, it's a beautiful place to visit. But again the rock will very possibly, be 'wet'.

I am a grad student in geology, for what it's worth.
posted by HighTechUnderpants at 11:11 AM on December 10, 2007