Let's take a journey through time. Envision the entire timeline of human history. Now zoom in to the first point in time during which humans began to develop an understanding of materials. What time period is this? The Stone Age.
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In modern times, reference to the Stone Age evokes the image of people with limited intellectual capacity. However, our Stone Age counterparts knew more than perhaps we credit. As noted by the name of the era, the Stone Age was a period during which humans relied on various types of stone as a fundamental support to provide for our most basic needs—food and shelter. Let us take a moment to consider stone from a materials science perspective by understanding some key material properties and characteristics.
Material Property:
Stress & Fracture
During the Stone Age, humans who had access to stones with good fracture qualities had a technological advantage—they were able to fabricate the most sophisticated tools. In order to understand why, we look to basic materials fracture science.
Frost cracking
A unique type of fracture that occurs with no physical contact in materials like concrete.
Occurs when water freezes (and thus expands) inside concrete and a pressure higher than the fracture threshold causes cracks to form in the concrete.
It turns out that the crack causes stress amplification in its vicinity. That does not mean that we are somehow striking it with greater force if we happen to be near a crack; but, rather, the fracture stress threshold changes. Let us examine two identical specimen of a material that have only one distinction: one of them has a crack. When struck with exactly the same force, the force “felt” by the cracked material will be higher. Translation: the presence of the crack lowers the fracture stress threshold.
Our Stone Age counterparts could do much more than just break stone into pieces. Some societies, in fact, developed fine tools that contained intricate features with techniques such as lithic reduction, a technique in which natural stone is chipped away to form a desired tool or weapon.
A crucial material characteristic relevant in the Stone Age is the fracture property. In materials science, fracture is defined as the separation of a material into two or more pieces. In material designs today, we consider fracture to be an event of material failure, when a structural component can no longer perform its desired function. For example, cracks in the wall of a building are signs of fracture in the building’s foundational materials, and a crack in the windshield of a car is fracture in the glass that impairs its ability to provide protection.
Hand axe
a sharp piece of stone adhered to a wooden handle using a rope
Most fracture comes from stress applied by some sort of physical impact, such as hitting or striking. Stress is defined as the force applied to a given area. The stress at which a particular material breaks is, unsurprisingly, called fracture stress. That means that we can strike the material all we want, but if the force is not higher than the fracture stress threshold of a given material, it will not break. Only once we reach a force great enough to pass the material’s fracture stress threshold will we be able to break the material. But there’s one key exception.
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We know from experience that if something already has a crack in it, it will be easier to break. And by “easier” we mean: less force is required. Why is that?
Lithic reduction
used to make intricate tools
Material Property:
Grain Size
depiction of grain boundary
Ok, let’s back up a little. To get a better picture, we need to understand what rocks are and how they are formed. Before rocks become “rocks” (i.e., hard, solid substances), they start out in the molten state (i.e., melted, liquid). Molten rock within the earth’s surface is called magma, and it is molten because it is very hot down there—between 1,300 °F and 2,400 °F! The cooling of magma into rock happens very slowly. This slow cooling process provides time for large crystals to form, resulting in a coarse grain size within the solid rock.
diagram of magma etc.
But why does grain size matter? The finer the grain size in the raw stone, the easier it is to create tools with fine characteristics because the stone will break more easily at the grain boundary, which is the interface between two grains. That means people who lived in regions on earth that were limited in fine-grained stone were also limited in their ability to fabricate fine tools. When our Stone Age counterparts tried to work stone into fine tools, such as a hand axe, they undoubtedly would have figured out that some stones, such as obsidian, flint, or jasper (fine-grained stone), would prove more suitable than coarse-grained quartz.
Once they put in time and energy to acquire and understand valuable stone, and then apply skill in manufacturing a specific tool, they most likely were going to hold onto anything deemed useful, both raw stone and fabricated tools. In those days, our materialistic culture came from a place of need and resource allocation rather than a place of superficial material attachment. Cultures that evolved in regions that contained copious amounts of stone with fine grain size were more likely to fabricate fine tools with specified functions, accelerating the rate of societal development.
If our Stone Age counterparts tried to make tools just using knowledge of the effects of cracks on fracture stress, they would be unable to make fine tools. They would have had to rely on a much more prominent characteristic to do fine stone work: grain size.
Grain size refers to the individual particle sizes present in a larger chunk of stone or mineral. Stone can have either coarse grain size or fine grain size. The type of grain size present depends significantly on the local geology of the region and whether the rock formed inside or outside the earth’s crust.
Hottest Place on Earth:
Furnace Creek, California
According to the World Meteorological Organization, the highest temperature ever recorded on earth was 134 °F in the aptly named Furnace Creek, California. That is no match for the temperature of molten rock: 1,300-2,400 °F!
When molten rock reaches the earth’s surface, we call it lava. As we know, temperature on the earth’s surface can vary significantly depending on the geographic region, but even the hottest recorded temperatures at the earth’s surface are much cooler than the molten rock that spews out as lava.** That means the cooling of lava into rock will happen much faster, leaving nominal time for crystal growth. Hence, small grains make up the rock formation (i.e., fine grain size).
Obsidian
Flint
Jasper
Quartz
Machu Picchu: Relic of the Great Incan Empire
As a case study, we will consider the great Inca feat of engineering: Machu Picchu. The presence of such a colossal fortress within a Bronze-Age empire is incredible. There are many crucial engineering concepts that were applied together (e.g., heavy materials transport, architectural design, etc.) to result in such a vast structure that still stands strong, centuries after it was abandoned. Considering the property of stone grain size, the choice of building materials used is key. The stone blocks used in building the structures in Machu Picchu were ones of granite, which is mostly composed of quartz, a coarse-grained material. Geological factors in the region made granite available in large sections, broken from enormous pieces of igneous rock (called plutons) due to high-pressure tectonic activity in the vicinity of precursor joints, which are natural fractures in a body of rock. In other words, enormous grain-size granite was broken into finer grain-size granite by means of natural geological activity. Breaking the granite into yet smaller chunks was possible with the presence of existing cracks, combined with the skilled use of other specialized tools. The surface of granite blocks could be smoothened using a direct percussion technique with hammerstones, which were specialized tools often made from fine-grained obsidian. The smooth granite blocks were fitted into the adjacent stones via drystone masonry, eliminating the need for the use of a binding material (e.g., mortar). The secure fitting, in combination with the energy required to initiate cracks into coarse-grain granite, is the reason that Machu Picchu still stands today.
Beyond Stone:
Bone, Wood, Bark
While stone was the primary material used in the Stone Age (hence the name), there were other materials that proved useful, such as bones, wood, and bark. If these materials were equally important, why is there limited evidence of these materials in use? Because they are biodegradable and are long gone, transformed into other materials like fossils or soil. Nonetheless, our Stone Age counterparts were utilitarian and creative in using anything they could find.
Origins of the Materialistic Culture
The locations of “good” tool stone often defined prime Stone Age-era real estate. That is, until people learned how to transport stone.
This knowledge of stone-transport capability provided two things: (1) more freedom in where to settle, and (2) opportunity for trade of material goods.
As trade became increasingly important in human societies, all material things held the potential for utility to someone. It made sense to cling to anything of potential value. And by accumulating material things, humans established one of the first forms of financial security. Possession of quality material objects increased one’s chance of survival. Thus, the material culture was established.
Imagine a Stone Age woman whose errand for the day was to collect large pieces of stone from a nearby quarry. She would be wiser to use her limited strength and energy to apply force near existing cracks, rather than at regions of continuous material. It would require less work on her part to break off chunks of stone when she strikes near a crack. Considering that physical human energy was one of the core sources of power in the Stone Age, there’s a good chance that Brenda and her Stone Age colleagues figured out the concept of stress amplification and its effect on fracture stress, probably all without knowing how to put words to the phenomenon.
Vocabulary
BIODEGRADABLE
Material capable of being decomposed by bacteria or other living organisms.
DRYSTONE MASONRY
A building method in which structures are constructed from stones without any mortar to bind them together.
FAILURE
Separation into two or more pieces of a specimen or component that is loaded beyond its capacity.
FRACTURE
The physical separation of a part of component. In many instances it is synonymous with failure.
FRACTURE STRESS
External force per unit area applied that results in materials fracture.
GRAIN BOUNDARY
Boundary that occurs when two crystals of arbitrary orientation to each other are joined along an arbitrary surface.
GRAIN SIZE
Dimensions of grains, or crystals, in a polycrystalline material that result from grains nucleating at different sites
HAMMERSTONES
A hard, smooth stone (approximately 2-10 inches in size) that is used to strike off lithic flakes from a lump of tool stone during the process of lithic reduction.
IGNEOUS ROCK
The type of rock formed from the cooling of magma.
LAVA
Molten rock that has been expelled from the interior of the planet onto the surface through a volcano or a fracture in the crust.
LITHIC REDUCTION
Process of manufacturing tools by removing parts of a stone object.
MAGMA
Molten state of the natural material from which igneous rocks are formed. It is found in subterranean layers of the Earth’s surface.
PLUTONS
Molten state of the natural material from which igneous rocks are formed. It is found in subterranean layers of the Earth’s surface.
PRECURSOR JOINT
A fracture of natural origin in a layer or body of rock.
STRESS
Force per unit area.
STRESS AMPLIFICATION
Reduction in driving force required for crack propagation.
THRESHOLD
A magnitude that must be exceeded for a given event to occur. For example, fracture stress threshold is the amount of force that must be applied to induce material fracture.