The Ages of Stone and Clay

The High Priest of Kulaba formed some clay and wrote words on it as if on a tablet — In those days words written on clay tablets did not exist, But now, with the sun's rising, so it was! The High Priest of Kulaba wrote words as if on a tablet, and so it was!

— Enmerkar and the Lord of Arratá

Gazing across the stark, sunbaked land- and waterscape of Salmon Creek Reservoir, set in the sagebrush desert of southern Idaho, I was alert to any motion of the tip of my fishing pole, propped up by rocks. My family and I often visit my in-laws in Twin Falls, Idaho, and we always go fishing for rainbow trout. Erik, my younger son, back from exploring the barren cliffs, came running up to me, clutching a black stone different in appearance from the slabs of lava rock scattered along the shore about us.

"Dad, what's this?"

Turning the dull stone over in my hand, I told him it was obsidian. "It's glass — different from most rocks. More like a frozen liquid than a crystalline solid." When I started to explain that it had a different atomic structure than many other minerals, his gaze drifted away. I turned and threw the piece of obsidian against a nearby rock, shattering it into shiny, razor-edged chunks. "Native Americans around here and people in the ancient Near East used obsidian to make axes and arrowheads, because it splits into lots of sharp pieces." Glass is one of many materials that craftspeople used thousands of years ago that we still employ, albeit in very different ways. I told Erik that today phone companies were replacing copper wires with optical fibers made of very pure and ultra-clear glass.

"Well," Erik asked, perhaps less concerned with these facts than the rock he had found, "why is obsidian black?"

"Clear glass is made from silicon, oxygen, sodium, and calcium," I replied. "But obsidian contains dirt, small amounts of other atoms that make it black. The first people could make tools out of rocks like this, which is why we humans did so well. Glass was as high tech ten thousand years ago as it is today." Satisfied, Erik checked his rod and went off to look for other rocks.

A few days later, we were looking out across a large moraine at the spectacular vistas in Rocky Mountain National Park. Once the basin before us was clogged with glacial debris — large boulders and rocks — now hardly to be seen, though the U-shaped valley is the signature of a glacier melted long ago. The displays in the park exhibit at Moraine Basin remind visitors that rocks erode because slightly acidic water attacks the cement that holds minerals together. Most rocks do not have a uniform structure like obsidian, but are composites of several different constituents, similar to concrete, an artificial pourable stone, in which mortar bonds together sand and hard rocks. In nature, heavy loads are always supported by composite structures, not homogeneous materials. Mimicking nature, humans also use composite materials for their most advanced applications. We'll learn why later. First let's turn to the earliest tales of materials.

Early humans faced overwhelming obstacles to survival. They needed food for sustenance, weapons against predators — both animal and human — and shelter from an often brutal environment. In their desperate struggle, our ancestors came to realize that the gray-brown-black rocks they found scattered about them were useful for making weapons and tools; flint and obsidian were particularly desirable. Anthropologists have uncovered the earliest evidence of stone implements in the Rift Valley of East Africa. More than two million years ago, humans were first finding ways to master nature, and the earliest stone artifacts discovered in the Olduvai Gorge consisted of flakes, or thin chips, and the stones from which they were struck. No one yet knows what these stone implements were used for, although the fact they were frequently found near bone fragments suggests that our ancestors used them to butcher animals, ranging in size from elephants and hippopotami to rodents and tortoises. Sharp flakes could slice through tough hides, while stones broke open bones to get at the marrow. Since anthropologists believe the diet of these early folk was more than half vegetarian, they likely also used stone tools to dig up roots and tubers, and crack open hard-shelled nuts.

Perhaps not coincidentally, the separation of the genus Homo (man) from Australopithecus (southern ape) occurred at approximately the same time as the first appearance of simple stone tools. It is still unclear whether Homo fashioned rudimentary tools and Australopithecus did not; it is conceivable, however, that the ability to make tools gave Homo an advantage in the battle for survival over Australopithecus, which eventually became extinct. Nevertheless, Homo survived for other reasons, including the sharing of responsibility for gathering wild plants and hunting game. Cooperation within a hunter-gatherer group was the first step toward the specialization in crafts leading to innovations in both their world and, ultimately, ours.

Early development of simple collections of stone implements — or tool kits, as they are called — appears to have taken place entirely in Africa and progressed at a very slow pace, because food was readily available. Where there was enough to eat, there was no need to innovate. Pressure on the food supply by an increasing population forced these early folk to develop new and better ways to hunt and gather food. It was during the Lower Paleolithic period that early humans spread out of Africa to parts of Asia and Europe, perhaps 1.5 million years ago. At about this time, the ancestor of modern humans, the species Homo erectus (man who walks upright) emerged. Homo erectus, with a brain larger than such predecessors as Australopithecus, walked with a striding gait and differed from modern humans primarily through its larger jaws and teeth. With additional enlargement of its brain and changes in facial structure, a new species, Homo sapiens (wise man), appeared around 250,000 years ago. Further division into subspecies occurred, including the famed Homo sapiens neanderthalensis and culminating in Homo sapiens sapiens, modern humans who emerged 100,000 years ago, and who replaced all other human types on earth by approximately 30,000 years ago.

Among our ancestors' tools, hand axes — large cutting implements with two planar faces meeting at a shallow angle — were the primary product of stoneworking, though early peoples also fashioned scrapers for cleaning animal hides, as well as knives and toothed implements. With time, artisans developed sophisticated flaking techniques, which they used masterfully to craft flint tools. Instead of laboriously chipping individual blades, they first carefully prepared a plump cigar-shaped flint core, from which they then rapidly struck several blades — an early version of mass production. As the final step in this "Levallois technique," as it is called, stoneworkers retouched the edges of the flint blades with well-aimed blows.

Cementing hand axes into wooden handles with tree resin or bitumen (allowing ingenious early artisans to take advantage of the lever principle and increase the velocity of the axblow), a crucial innovation. They fashioned bone, ivory, and antler into new tools. One such class of new implement was the straightener for wooden shafts. These straighteners enabled the invention of the arrow, which revolutionized both hunting and fighting by allowing killing at a distance. By this time, high-quality stones were much in demand and were frequently traded over long distances. Flint, for example, was shipped eighty miles to sites in the former Soviet Union.

To survive the harsh winters of middle and northern Europe, Homo sapiens stitched together animal skins using bone needles. Finds from north of Moscow, dating from 22,000 years ago, include leather caps, shirts, jackets, trousers, and moccasins. Fox and wolf furs provided additional insulation. These people also built substantial shelters, fashioning the walls of their huts from large numbers of mammoth bones. They made stone floors, lamps holding animal fat to light the long winter nights, and fireplaces for heat and cooking. Excavations show they understood the role of draft in making hotter fires, for their hearths were frequently complex structures, with corrugated floors to increase the flow of air to the wood fuel. Many thousands of years later, this simple idea would lead to furnaces hot enough first to fire clay pieces, then to extract copper and iron, and finally, to melt and blow glass.

The stone tools in use just before the start of the Holocene, or Modern Era, usually dated ten thousand years ago, indicate that humans were still hunter-gatherers. Archaeologists have unearthed spear and arrow tips fabricated from both stone and bone, with long grooves to make wounded animals bleed more heavily, thereby speeding their death. Together with wooden shafts found in northern Germany, they demonstrate that hunters had added bows and arrows to their arsenal. Fishhooks made of bone originate from the same period, as do weapons such as the harpoon and the spear thrower, a leather-thonged sling, allowing hunters to increase the range and velocity, and therefore the impact of their spears, encouraging them to stalk larger and more dangerous game. Mammals that had survived earlier ice ages, such as the woolly mammoth and great deer in Europe, and the giant buffalo and giant Cape horse in Africa, became extinct toward the end of the last Ice Age, possibly because of the increased sophistication of the hunters and their weapons.

An extraordinary discovery occurred roughly 26,000 years ago, when artisans at a site in what is now the Czech Republic mixed clay with loess, rich soil left behind by retreating glaciers, and then fired it in ovens (the word ceramic comes from the Greek keramos, meaning "burnt stuff"). Clay was the first substance that humans totally transformed by heating. Many modern materials undergo similar metamorphoses when heated. Soft doughy clay is remarkable because in a kiln or oven it becomes a hard, heat-resistant ceramic that can hold liquids. Along with the discovery that grains could be cultivated, the advent of ceramics led to what some have called the Neolithic ("new stone") Revolution, laying the agricultural basis for the first cities of the world that emerged in Mesopotamia, between the Euphrates and Tigris Rivers in what is now Iraq.

Artisans at the Czech site fashioned Venus-like figurines from clay. Curiously, most of these were found shattered, and scientists speculate that they were fired so as to break purposely, perhaps as part of a religious ritual whose meanings are lost to us. Ceramic pots were first fired in Japan twelve thousand years ago, but only much later, during the seventh millennium B.C.E., were potters able to make them in sufficient quantities to have any significant impact on the economy of early villages.

Discovering how to produce a ceramic by firing clay was a milestone in our quest to master nature, and its importance can't be overstated. Up to that time craftspeople had transformed stone, bone, and wood into tools and weapons by altering their shape, not their intrinsic properties. With ceramics from clay, humans learned that the physical properties of materials could be dramatically improved.

In antiquity, most advances involving materials were based in some manner on this simple concept. (They frequently are today as well.) Early humans had discovered that heating flint and chert allowed them to be more easily cleaved, so, strictly speaking, clay was not the first material humans modified. Clay is, however, the earliest example of a complete change in the property of a material that involved a major technological advance; it was, in this sense, the first truly man-made material.

Clay had the additional advantage of being malleable, making it easier to use than stone in the fashioning of pots and sickles, both rather complex objects. Clay can be easily shaped because its layered atomic structure leads to the formation of thin plates called lamellae, each weakly bonded to its neighbor. Individual layers are built up by periodically repeating an octahedral, or eight-sided, arrangement of oxygen atoms enclosing an aluminum atom (imagine an eight-sided cage of oxygen atoms, with a triangular array of three oxygen atoms making up each side, and with an aluminum atom at the center of the cage), and a tetrahedral, or four-sided, arrangement of oxygen atoms with a silicon atom at its center (imagine a four-sided cage of oxygen atoms, with a triangular array of three oxygen atoms making up each side, with a silicon atom at the center of the cage). Both the octahedra and tetrahedra are tilted over on their triangular sides. Larger platelike clay crystals are built up by interweaving layers of octahedra with layers of tetrahedra and water molecules. These thin crystals move easily past one another, like playing cards in a deck, allowing clay to be shaped. Clay's remarkable transformation to a ceramic occurs when water and hydroxyl molecules (made up of a hydrogen and an oxygen atom) are driven off upon heating. As the water evaporates, clay shrinks significantly, and new atomic structures are created.

When the atomic structure of clay changes, so do its properties. The element carbon demonstrates this change even more dramatically. As graphite, which has a layered structure similar to clay, carbon is slippery, soft, and black, hence its use as a dry lubricant and as the lead in pencils. Under extremely high pressures and temperatures — found in nature only deep within the earth — graphite transforms into diamond, which has quite a different atomic arrangement: a three-dimensional network instead of a two-dimensional, layered structure. This network makes diamond transparent and extremely hard; in fact, it is the hardest substance known on earth.

How was the firing of clay first discovered? Perhaps it was by a mason observing the effect of the sun on his bricks. Or by a woman idly shaping clay during cooking, dropping her handiwork into the campfire and then finding a rock-hard solid among the ashes. From such mundane events, lost to recorded history, are born revolutions in technology.

Once our ancestors had learned how to transform clay into ceramic, they applied this valuable lesson over and over again, developing new ways to manipulate materials' properties to their benefit by using both heat and mechanical energy (in other words, hammering). Sometimes the structure would be changed on the atomic level, as when clay is fired to a ceramic, and sometimes on a larger scale, as when metals are forged into new shapes. While the process of firing clay to obtain ceramics is commonplace (schoolchildren do it every day), its discovery made possible innumerable innovations in materials.

Most historians consider the end of the last glacial period, about ten thousand years ago, as the start of the Modern Era. As the average temperature on earth increased, ice fields receded and oceans rose from their lowest point of 400 feet below today's levels, engulfing vast stretches of coastline and severing the land bridge between Asia and North America. Loess deposited by retreating glaciers became fertile ground for meadows and pioneering trees such as birches, creating verdant pastures for animals and a rich bounty for hunters. Humans still made tools and weapons from stone, and so the beginnings of the Modern Era overlap with the end of the Stone Age (defined as the Neolithic and Chalcolithic — "copper stone" — Periods). Of course, different civilizations begin using particular materials at different times; indeed, some native tribes in the Amazon still use stone implements. For my purposes, I will define the start of an age as the moment when a material first began playing a significant role in the life of a society.

The transition from a nomadic hunter-gatherer lifestyle to a sedentary existence was crucial and first occurred, so far as we know, in the Near East. Evidence for this gradual shift is found in the Natufian culture (10,300 to 8500 B.C.E), which derives its name from Wadi en Natuf, wadi being Arabic for "streambed," where traces of its existence were first uncovered in the environs of present-day Israel. The Natufians established permanent settlements, sometimes exceeding one hundred inhabitants. These settlements typically overlooked marshy areas, which attracted game in search of water and provided ample opportunities for hunting, as well as for fishing and gathering vegetables. Their front yard was their larder. Caves overlooking the sea at Kebara at Mount Carmel and an open-air site facing the Jordan River at Eynan in the Hula Valley were all hospitable locations.

Archaeologists investigated three levels of villages at Eynan. Each consists of fifty round houses built of stone, with floors below ground level, walls three feet high, and roofs of conical or hemispherical shape. Since there was no evidence for either the domestication of animals other than dogs or the cultivation of grain, their inhabitants must have still lived by hunting and gathering. Enormous quantities of gazelle bones were found in caves at Mount Carmel, indicating that gazelles were the primary source of meat. The discovery of large numbers of flint sickle blades and grinding utensils, including large stone mortars, provides strong evidence that Natufians gathered wild grains. Sickle blades with edges that show considerable wear are occasionally found mounted in beautifully carved bone handles. Relatively easy access to food gave the Natufians spare time to lavish rich artistry on their implements. They also developed new tools for hunting and fishing, including harpoons, hooks, and net sinkers. Highly prized obsidian was not present in Palestine and Syria, so artisans fashioned their stone tools and weapons out of local materials.