Stone Boiling - The History of the Ancient Cooking Method

How Do You Make Soup Hot Without a Stove Top?

Stones in a pile
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Stone boiling is an ancient cooking technique to heat food with directly exposing it to flame, reducing the likelihood of burning, and allowing the construction of stews and soups. The old story about Stone Soup, in which a glorious stew is created by placing stones in hot water and inviting guests to contribute vegetables and bones, may have its roots in ancient stone-boiling. 

How to Boil Stones

Stone boiling involves placing stones into or next to a hearth or other heat source until the stones are hot. Once they have achieved an optimal temperature, the stones are quickly placed into a ceramic pot, lined basket or other vessel holding water or liquid or semi-liquid food. The hot stones then transfer the heat to the food. To maintain a continued boiling or simmering temperature, the cook simply adds more, carefully timed, heated rocks.

Boiling stones typically range in size between large cobbles and small boulders, and should be of a type of stone that is resistant to flaking and splintering when heated. The technology involves a considerable amount of labor, including finding and carrying an adequate number of appropriately sized stones and building a large enough fire to transfer sufficient heat to the stones.

Invention

Direct evidence for using stones to heat liquid is a little hard to come by: hearths by definition generally have rocks in them (called generally fire-cracked rock), and identifying whether the stones have been used to heat liquid is difficult at best. The earliest evidence that scholars have suggested for the use of fire dates to ~790,000 years ago, and clear evidence for soup making is not present at such sites: it is possible, perhaps likely, that fire was first used to provide warmth and light, rather than cooking.

The first true, purpose-built hearths associated with cooked food date to the Middle Paleolithic (ca. 125,000 years ago). And the earliest example of hearths filled with heat-fractured round river cobbles come from the Upper Paleolithic site of Abri Pataud in the Dordogne valley of France, about 32,000 years ago. Whether those cobbles were used to cook with is probably speculation, but definitely a possibility.

According to a comparative ethnography study conducted by American anthropologist Kit Nelson, stone boiling is used most frequently by people who live ​in the temperate zones on earth, between 41 and 68 degrees latitude. All kinds of cooking methods are familiar to most people, but in general, tropical cultures more often use roasting or steaming; arctic cultures rely on direct-fire heating; and in the boreal mid-latitudes, stone boiling is most common.

Why Boil Stones?

American archaeologist Alston Thoms has argued that people use stone boiling when they don't have access to easily cooked foods, such as lean meat that can be direct-cooked over a flame. He indicates support for this argument by showing that the first North American hunter-gatherers didn't use stone boiling intensively until about 4,000 years ago when agriculture became a dominant subsistence strategy.

Stone boiling might be considered evidence of the invention of stews or soups. Pottery made that possible. Nelson points out that stone boiling requires a container and a stored liquid; stone boiling involves the process of heating liquids without the dangers of burning a basket or the contents of a bowl by direct exposure to fire. And, domestic grains such as maize in North America and millet elsewhere require more processing, in general, to be edible.

Any connection between boiling stones and the ancient story called "Stone Soup" is sheer speculation. The story involves a stranger coming to a village, building a hearth and placing a pot of water over it. She puts in stones and invites others to taste the stone soup. The stranger invites others to add an ingredient, and pretty soon, Stone Soup is a collaborative meal full of tasty things.

The Benefits of Limestone Cookery

A recent experimental study based on assumptions about American southwestern Basketmaker II (200–400 CE) stone boiling used local limestone rocks as heating elements in baskets to cook maize. Basketmaker societies did not have pottery containers until after the introduction of beans: but corn was an important part of the diet, and hot stone cookery is believed to have been the primary method of preparing maize.

U.S. archaeologist Emily Ellwood and colleagues adding heated limestone to water, raising the pH of ​the water to 11.4–11.6 at temperatures between 300–600 degrees centigrade, and higher yet over longer periods and at higher temperatures. When historical varieties of maize were cooked in the water, chemical lime leached from the stones broke down the corn and increased the availability of digestible proteins.

Identifying Stone Boiling Tools

Hearths at many prehistoric archaeological sites have a preponderance of fire-cracked rock, and establishing evidence that some were used in stone boiling has been tested by American archaeologist Fernanda Neubauer. Her experiments found that the most common fracture on stone boiled rocks are contraction-fractures, which exhibit irregular crenulated, wavy, or jagged cracks on the breakage faces and a rough and undulating interior surface. She also found that repeated heating and cooling eventually fractures the cobbles into pieces too small to use depending on the raw material and that the repetition also can cause fine crazing of the rock surfaces.

Evidence such as that described by Neubauer has been found in Spain and China by about 12,000–15,000 years ago, suggesting the technique was well known by the end of the last Ice Age.

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