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When cooking outdoors, stones are often used around an open fire. These help contain the coals and provide surfaces for balancing food. They can also get quite hot.

Are there any rules of thumb to follow when selecting such stones to avoid them splitting at high heat?

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  • This doesn't sound like an especially useful question. First, if the answer is on the lines of "syenite is better than granodiorite", would you be able to ID them in the wild? And second, even if you can recognize different types of stones, you usually get the same type of stones in any given area.
    – rumtscho
    Aug 12, 2023 at 9:12

2 Answers 2

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If you are cooking outdoor like in a camping set up, you can't really afford to go looking around for good refractory stones. You use what you have at hand.

If you are worried about the stones splitting, you have to do the opposite of what our ancestor used to do to split stones, that is be gentler with the heating and the cooling, which are the moments when thermal stress is higher: start the fire slow, and don't quench it with water immediately after it was burning fiercely.

Of course if you are in a situation where the fire must be put off quickly, splitting the stones around it is a lesser concern.

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By way of background, my mother is an archaeologist. Starting in around 2000, she started working in the Great Basin, where evidence of past human habitation tends to be pretty sparse, and not associated to permanent settlements. The most common kinds of artifacts found in that region are stone tools (ceramics weren't introduced until relatively recently, and other kinds of artifacts don't generally preserve all that well). Because it was a thing which was possible to study, she started getting really interested in fire cracked rock. Her interest was in studying how rocks break from thermal stress as they are used in hearths, in boiling baskets, and in other cooking applications. Over the last 20 years or so, she has build hundreds of fires and worked with many kinds of stone and cooking methods in order to determine how the rocks break down (we've had a few Thanksgiving turkeys cooked using these techniques, for example).

A few observations from her work:

  1. Thermal stress will break down your rocks over time. However, we've never experienced stones "exploding". They crack, and they split, but it not extraordinarily violent. They just kind of fall apart.

  2. This is probably obvious, but the more rapidly the temperature of the rock is changed, the more quickly it will break down. If you line a hearth with rocks, then build a fire on top of that, the rocks will generally heat fairly gently. If you dig out the hearth while the rocks are hot, put in food to cook, then bury the whole thing and let it sit for a couple of hours, the food will cook slowly, and the rocks will experience little thermal shock. On the other hand, if you drop the rocks into boiling water (was you would want to do when making soup, for example), they experience a great deal of thermal shock, and break apart pretty quickly.

  3. Rocks with a finer grained structure last longer. Rocks like basalt, rhyolite, and "chert" tend to last quite a long time (they can go through 10+ heating/quenching cycles and remain pretty much intact). On the other hand, rocks with a larger grain tend to break down much more quickly. Granite and sandstone fall apart rapidly, often the first time that they are used.

The take home here, I think, is that your rocks will eventually split as they go through thermal cycles, but rocks with a finer structure are likely to last longer. Also, if you can reduce the thermal stress the rocks experience, they will last longer (so, for example, don't quench your fire with water, but bury it with dirt when you want to put it out; try to avoid putting the rocks directly into the fire; etc).

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