Ceramics

Archaeological ceramics refers to products made primarily of clay and containing variable amounts of lithic and other materials as well. The term ceramic is derived from the Greek keramos, which has been translated as "earthenware" or "burned stuff." Ceramics include products that have been fired, primarily pottery but also brick, tile, glass, plaster, and cement as well. Since pottery is by far the most important archaeologically, and the methods of sampling and study are largely applicable to the others, this chapter is devoted primarily to pottery. Pottery then is the general term used here for artifacts made entirely or largely of clay and hardened by heat. Today, a distinction is sometimes made between pottery, applied to lower-quality ceramic wares, and the higher-grade product porcelain. No such distinction will be made here, so the term pottery alone will be used. Raw material that goes into the making of a pot includes primarily clay, but also varying amounts of temper, which is added to make the material more manageable and to help preserve the worked shape of the pot during firing. Of primary interest in ceramic studies are 1. the nature and the source of the raw materials—clays, temper, and slip (applied surface pigment)—and a reconstruction of the working methods of ancient potters; 2. the physical properties of the raw materials, from their preparation as a clay-temper body through their transformations during manufacture into a final ceramic product; 3. the nature of the chemical and mineral reactions that take place during firing as a clue to the technology available to the potter; and 4. the uses, provenance, and trade of the wares produced. Much of the information needed to answer these questions is available through standard geochemical and petrographic analysis of ceramic artifacts. Insight into the working methods of ancient potters also has been obtained through ethnographic studies of cultures where, because of isolation or conservative traditions or both, ancient methods have been preserved.

Archaeological Materials :Rocks and Minerals

This chapter is only a brief introduction to lithic archaeological materials. Archaeologists with but little knowledge of rocks and rock-forming minerals are urged to learn about them in greater detail than that presented here. Lithic resources are abundant in almost every archaeological site, and lithic artifacts are invariably the best preserved of any remains. Early societies learned how to exploit these resources, and the use and production of lithics go back to the earliest known sites, at least 1.5 million years. In fact, the earliest cultures are distinguished on the basis of their lithic industries and lithic artifacts. Horror stories in misidentification of lithics abound. Not only have misidentified artifacts proven embarrassing to the archaeologist, but also they have made it difficult to make meaningful comparisons of different societies using published descriptions. In addition, conservation strategies for historical monuments cannot be developed without an understanding of the nature of the material used in their construction. Some egregious examples of ignorance of the rocks and minerals from our personal experience include the following: 1. An archaeologist asked if a quartzite scraper was either flint or chert. When told that it was neither, he asked, "Well then, which is it more like?" (answer, still neither). 2. Egyptian basalt statues have been called limestone in publications (and several other rock types). 3. Sources for alabaster were searched to explain a trading link between a site and elsewhere when the geological map showed the site was adjacent to a mountain of gypsum, the mineral component of alabaster (the gypsum may have merely rolled down the hillside to the workshops, where it became the more salable alabaster). 4. Conservators searched for methods to preserve an allegedly granitic historic monument, or so it had been identified. Chemical analysis revealed only abundant Ca, Mg, and carbonate. Fossils were also abundant in the "granite," which dissolved easily in hydrochloric acid (the "granite" was clearly limestone). Petrology is the branch of geology that deals with the occurrence, origin, and history of rocks. Petrography is concerned with descriptions of rocks, their mineralogy, structures, and textures.

Archaeogeophysical Exploration

Geophysical techniques are a commonplace tool in today's archaeology as a result of an extensive collaboration between scientists and archaeologists on both sides of the Atlantic. This "cross-fertilization" has produced growing subdisciplines, of which archaeological geophysics is one example. As may be recalled from our introductory chapter, K. Butzer defined geoarchaeology as archaeology done using a geological methodology. G. Rapp and J. A. Gifford describe archaeological geology as the use of geological techniques to solve archaeological problems. Fagan has called geoarchaeology a "far wider enterprise than geology," involving (1) geochemical and geophysical techniques to locate sites and features; (2) studies of site formation and spatial context; (3) geomorphology, palynology, paleobotany; (4) absolute and relative dating procedures; and (5) taphonomic studies. Archaeological geophysics is a major aspect of archaeological geology. The application of geophysical exploration techniques in archaeology is also known as archaeogeophysics. Geophysical methods of potential usefulness to archaeological geology fall within the following classes: 1. seismic: reflection/refraction 2. electrical & electromagnetic: resistivity and conductivity 3. magnetic 4. radar 5. microgravity 6. thermography All have been used on a variety of archaeological problems. The application of geophysical techniques has grown as (1) the access to the instruments and (2) the methodological understanding of the users have increased. Access to geophysical instrumentation has been made easier by the steady development in solid-state design and computerization, which has reduced size and costs as it has in almost every technical field. The beneficiaries are the geologists and archaeologists. The first to recognize the applicability of geophysical methods to archaeology were the geologists—more specifically, the geophysicists. Working in association with their archaeological colleagues, the earth scientists translated the objectives of the archaeologists into practice. Such cooperation was very productive but suffered from the same kinds of problems that dogged the early usage and acceptance of radiocarbon dating. The archaeologists' untutored enthusiasm, coupled with their lack of a true understanding of the physics and atmospheric chemistry inherent in that technique, led to a backlash of skepticism when dates reported by the first radiocarbon researchers were found to be in error.

Chemical Methods

Time is nature's way of keeping everything from happening at once" (anonymous). Time is a continuum—we sense this continuum as a succession of events. In archaeological matters it is one of the most salient attributes. To determine time accurately the archaeologist must rely on modern dating techniques. Age determination by chemical methods relies on the constancy or predictability of rates of chemical processes. For instance the oxidation of iron—rust—could be used for dating purposes if one could determine a chemical rate, in this case that of oxidation, that applied to more than the singular event. Unfortunately, the rate of the oxidation of iron is highly variable, being affected by temperature, available moisture, and the particular type of iron (mild, cast, stainless, etc.). Another common chemical change is the patination of certain types of glass. Yet here, too, the process is highly variable, making dating impractical. Still, there have been attempts to use patination and rock "varnish" for archaeological dating, as we shall see. In the main, chemical dating is used to determine relative ages since absolute ages require calibration for each sample and its find site using independent dating measures such as radiometric or dendrochronological techniques. We shall first discuss the relative techniques based on the uptake or decrease in fluorine, uranium, and nitrogen found in bone. This is most appropriate because these chemical techniques played a key role in unmasking one of the most famous frauds in the history of science: Piltdown Man. Next we shall examine the two most accepted chemical processes utilized in absolute age determination, which are based, respectively, on amino acid racemization and obsidian hydration. Finally, we shall examine a few techniques that show some promise for the dating of archaeological materials or deposits, such as those using patination ("varnish") and cation ratios. Our points of reference are those events we view as, in some sense, marking a change in the state of things. Stylistic or formal change in an archaeological facies can be a chronological landmark for the archaeologist and allows us to divide the continuum of time into discrete segments or phases.

Sediments and Soils

Sediments are not soils. Sediments are layered, unconsolidated materials of lithic and/or organic origin. Soils are mixtures of organic and lithic materials capable of supporting plant growth. Sediments that overlie bedrock and cannot support plant growth are termed regolithic. Soils begin as rocks. If they are eroded or altered by diagenetic processes, they can become paleosols. The study of sediments and soils has great importance for the archaeologist because these materials can tell us about the conditions that led to their formation, thus giving much information on paleoenvironments and climate. Not too long ago, the soils of Holocene archaeological sites and the paleosols of more ancient Pleistocene ones were no more than the annoying material concealing the objects of prime interest. More recently, sediments and soils achieved their own importance as the stuff of archaeological geology. In more narrow definitions of the field, they are generally the only subjects of study. The emphasis of archaeological geology is decidedly not the rock. In this text we define archaeological geology in its broadest context such that the linkages between the geological, sedimentological, and cultural (i.e., archaeological) can be explored in all their depth and texture. Ultimately, the linkage of these aspects of earth science and humanity can be categorized under the term ecology, as first coined by Ernst Haeckel in 1866. Here the interrelationships of all areas of the earth system are sought in order to conceptualize stability and change wherever they occur. Sediments and soils typically occur as strata, which are described by principles of superposition, horizontality, continuity, and succession: Superposition: In a series of layers and interfacial features as originally created, the upper units of stratification are younger and the lower are older, for each must have been deposited on, or created by the removal of, a preexisting layer. Original horizontality: Any layer deposited in an unconsolidated form will tend toward a horizontal position. Strata that are found with tilted surfaces were originally deposited horizontally or lie in conformity with the contours of a preexisting basin of deposition.

Metallic Minerals and Archaeological Geology

Economic geology had its inception in the ancient utilization of rocks and minerals. The first economic materials were nonmetallic and include flint, quartz, diabase, rhyolite, obsidian, jade, and other stones, which were sought for weapons, implements, adornment, and even art. Beginning with the Upper Paleolithic Aurignacian period, clay began to be widely used for simple figurines, then brick and finally pottery. S. H. Ball identifies 13 varieties of minerals—chalcedony, quartz, rock crystal, serpentine, obsidian, pyrite, jasper, steatite, amber, jadite, calcite, amethyst, and fluorspar—as economic within the Paleolithic. Add to this list the use of ochres and mineral paints together with nephrite, sillimanite, and turquoise. In the standard reference on the nonmetallic deposits, "Industrial Minerals and Rocks", 6th edition published in 1994, deposits are classified by use and the minerals and rocks described as commodities. The fourteen use groups include such items as abrasives, constructions materials, and gem materials; the 48 commodities include clay, diamonds, feldspar, etc. Metalliferous minerals as ore deposits are unevenly distributed throughout the world. The formation of a mineral deposit is an episode or series of episodes in the geological history of a region and reflects three broad categories: (1) igneous activity, (2) sedimentary processes, and (3) metamorphism. Table 12.1 summarizes general features of the three categories of mineral deposits. Admixtures of metals are by far the most common form of mineral deposits. Gold, silver, and copper occur either as native metals or admixed with other metals and compounds. Most ore deposits are actually mixtures of metals: silver commonly with lead, zinc with cadmium, iron with copper. Many metallic ore deposits are products of igneous activity. Conditions change in the magma chamber as the principal rock-forming minerals crystallize, temperature falls as the magma cools, pressure is lowered as the magma rises in the crust, and volatiles increase in the magma chamber.

Soil Phosphate in Archaeological Surveys

It has long been recognized that human activity chemically modifies the composition of the soil. This is especially true around ancient settlements that were occupied for relatively long periods of time. In areas that humans have inhabited, soil fertility is higher than in uninhabited areas because of an increase in plant nutrients derived from human and animal waste. Deep dark soils that contrast with neighboring lighter colored soils can define areas of intensive occupation with great precision. Phosphate (PO4-3), an important plant nutrient, is highly concentrated at ancient sites and makes for an increased soil fertility. Arab farmers in the Near East have been known to use soils excavated from archaeological sites to fertilize their agricultural land. The soil phosphate has been derived from animal and human excreta and bones and dead bodies. Phosphate will be especially concentrated where animals have been enclosed. Phosphate found in the soil can be bound chemically in a variety of ways. Since the soil is a dynamic system, its physical and chemical nature will constantly alter over time depending on local and temporal equilibria conditions. The first studies of soil phosphate were by agronomists as a tool for agriculture. The observation that human occupation increased the phosphate concentration was noted at least by 1911 in Egypt as a result of agronomic studies. O. Arrhenius, a Swedish agronomist, made the first attempt to apply phosphate studies to archaeology, in a series of papers beginning in 1929. He concluded that phosphate concentrations could be used to locate abandoned settlement sites, even where no visible evidence remained. Thus, the initial application of soil phosphate analysis to archaeology was as a geochemical exploration tool to locate ancient settlements. Human occupation should increase not only the phosphate found in the soil but also the nitrogen and carbon. These additions result from the decomposition of organic matter, principally human and animal remains and excreta. In desert or agricultural land, phosphorus in the soil ranges from 0.01% to 0.2% in the uppermost 10 cm and nitrogen ranges from 0.1% to 1%.

Other Chronological Methods

Dendrochronology relies on the seasonal changes in the wood growth of trees that result in the annual production of rings; each ring starts with large cell elements associated with spring and ends with small cell elements associated with summer and autumn growth. The age of the tree is known by counting these rings. The sequence of rings produced over the years is distinctive and shared by trees of the same species over a broad region. In the western and southwestern United States, the bristlecone pine from the White Mountains of California and the eastern Great Basin has allowed the establishment of a tree-ring chronology of 10,000 years. The California bristlecone pines are found west of the Sierra escarpment's White Mountains, on the Trans-Sierra Valley slopes. The oldest groves of the trees are at an altitude of 13,000 ft (3936 m), with a few hundred trees. The oldest living tree is "Methuselah," at 4,700 years, while some of the dead trees have ages of 8,000 years. Shaped by the wind, their silvery trunks have tightly packed ring sequences. The growth of trees, which occurs from spring to autumn, is marked each year by the formation of a new ring of wood cells. The thickness of the rings is a function of the temperature and precipitation at the time of their formation. The trees of a region experience the same variations in climate and, therefore, present the same series of growth rings for the same data (period) sequence. In 1911, an astronomer, A. E. Douglass, was studying tree rings to correlate them with s spots and climatic changes. He succeeded in establishing one of the most precise dating t hniques used in archaeology. In order for the technique to be used, the tree rings must contain an arrangement of both narrow and wide rings that vary considerably in width. Each of the rings found within the cross section is called an annual ring. A wide annual ring signifies plentiful moisture in the soil, whereas a narrow ring signifies insufficient moisture in the soil for robust growth.

Radiation-Damage, Cosmogenic, and Atom-Counting Methods

Fission-track dating, one of the more recent techniques involving the use of radioactivity, has developed one of the widest ranges of applications. Dates of objects have been obtained ranging from 6 months to 109 years BP. Volcanic tephra, obsidian, man-made and basaltic glass, meteorites, and mica have been dated. A more apt term is nuclear-track dating because fissionable elements do not have to be present in the material. Fission, which produces one form of nuclear track, is a rare mode of radioactive decay. A more common decay is alpha decay, which produces a different type of track. Uranium 238 fissions spontaneously and has a well-defined half-life. It also fissions in the presence of neutrons such as are produced by reactors, accelerators, or neutron "howitzers." About 99.27% of all uranium is uranium 238. Robert L. Fleischer, Paul B. Price, and Robert M. Walker, who have done most of the original work in this field, have determined that most minerals contain this isotope in amounts from a few parts per billion (ppb) to many parts per million (ppm). These researchers devised a chart which characterizes the ease of use of this technique as a function of the uranium concentration. A high uranium concentration allows an "easily measured" age where the observer spends an hour at the microscope counting chemically etched fission tracks. For "considerable labor," 40 hours of such work is assumed. Ancient synthetic glass typically contains 1-2 ppm of uranium, so most glasses older than 8,000 years are datable. Most pottery clay contains about 5 ppm of uranium in either the clay itself or other minerals that occur as inclusions. It is very probable that some pottery clays or the mineral inclusions, such as zircon, might contain higher concentrations than this, which would make the age measurement lie between "easily" and "with considerable labor." It is important to point out that mineral inclusions such as zircons or micas act as solid-state detectors in that they register fissions as a track on the surface in contact with the pottery clay. Both fission and alpha events can do this.

Radioactive Methods

Radioactive decay can best be explained by accepted models of atomic or microscopic structure. By the mid 20th century most people understood that the atom was the smallest particle to which a homogeneous macroscopic sample could be subdivided and retain the physical characteristics of the original sample. The atom was called a microscopic particle because its dimensions were on the order of 10-8 cm. It is now known as mesoscopic, and the microscopic world is the subnuclear, or less than 10-13 cm, the distance across most nuclei. Physicists develop their intuitive feel for nature using such characteristic distances as 10-8 cm, called an angstrom (abbreviated Å). If you lived at the mesoscopic or microscopic level, you would choose this distance unit because it would be convenient. Today these levels are discussed as the nanoworld or nanostructure level.1 We shall refer to the microscopic/nanoscopic under the general term microworld. The energy required to separate two atoms coupled together is of the order of 0.1 electron volts. This is an energy unit characteristic of atoms, abbreviated as eV. In the macroscopic world, whose dimensions are most familiar to us, characteristic distances are of the order of 1 cm, which is 100 million times that of the microworld. The macroworld energy unit we are most familiar with is the food calorie, which is 1,000 heat calories, which is, in turn, about 4,180 joules. In the microworld, the electron volt is 1.6 X 10-19 joules. In the microworld, atoms consist of nuclei, which are about 10-13 cm across and which contain almost all of the mass of the atom. The nuclei, in turn, consist of protons and neutrons. These are two of the four elementary particles with which we will be concerned. Nuclei can be thought of as built up of nucleons or baryons, members of the larger class of elementary particles, hadrons. Baryons are composed of even smaller particles known as quarks. Particles like electrons, muons, and neutrinos are known as leptons.