As our civilization grows, there is also a growing passion for knowledge of the past. Archaeology is fast becoming one of the popular sciences. Most archeological finds are composed of shreds of pottery or ceramics. As archeology and modern science progress many dating techniques are being developed to be used on these fragile artefacts in order to give us a brief glimpse of our past. Many techniques are now available which allow the detailed physical and chemical characterization of ceramic artefacts.
Given a suitable archaeological sample it is now possible to determine many aspects of technology, provenance, and, in some cases, its use. Several dating techniques used in archeology are superposition, artifacts of known age, stratigraphy, dendrochronology and thermoluminescence. This paper shall focus on Ceramic Analysis using three of these techniques: Dendrochronology, Radiocarbon dating and Thermoluminescence. Carbon 14 Radiocarbon dating or Carbon-14 dating is the determination of the approximate age of an ancient object, such as an archaeological specimen, by measuring the amount of carbon 14 it contains.
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Radiocarbon dating was discovered by a team of scientists led by Prof. Willard Libby of the University of Chicago after World War II. Since plants and animals take up carbon-14 during their lifetime. The amount of carbon-14 in them exists in equilibrium with the C14 concentration of the atmosphere and the ratio of C-14 to C-12 remains about the same as the ratio in the atmosphere. As soon as a plant or animal dies, they cease the metabolic function of carbon uptake; there is no replenishment of radioactive carbon, only decay.
This rate of decay was calculated by Libby’s team to be 5568 years per half-life, this means that half the C14 in the original sample will have decayed after 5568 years and after another 5568 years, half of that remaining material will have decayed, and so on. This rate of decay is what is now know as Libby’s half-life which he measured at 5568±30 years. The true advantage of the radiocarbon method is its capability to be uniformly applied throughout the world , this is probably the primary reason why it is one of the most widely used and best-known absolute dating methods.
However, it is not without its flaws, radiocarbon dating has the following limitations: 1. that after 10 half-lives there is a very small amount of radioactive carbon present in a sample, at about 50,000-60,000 years radiocarbon dating becomes inaccurate. 2. the ratio of C-14 to C-12 in the atmosphere is not constant. This variation is due to changes in the intensity of the cosmic radiation bombardment of the Earth, the 1950’s nuclear testing and the depletion of the Ozone layer.
3. in rare cases, a "reservoir effect" will give C-14 dates that are much older than the true age of the sample because “some plants and animals live in very unusual environments whose C-14 content is much lower than normal. ” 4. Contamination of the sample also limits the accuracy of Radiocarbon dating as when porous samples contain recently living material with a full "charge" of C-14. Finally, Radiocarbon dating can only be used on living organisms. Dendrochronology
Dendrochronology is the science that uses tree rings dated to their exact year of formation to analyze temporal and spatial patterns of processes in the physical and cultural sciences. Its main function in archaeology is to use tree rings to date when timber was felled, transported, processed, or used for construction or wooden artefacts such as a beam or pole of an ancient structure. The principle behind using tree-rings is that “tree-ring sequences from trees that grow in a seasonal climate, i. e.
, with one growth increment per year, with the size of that growth dependent upon some climatic stimulus such as cold in the Polar regions, drought in the Aegean, and various combinations of the two stimuli in regions in between, can be compared so that these increments, more popularly known as "rings," can be dated to the calendar year in which they were formed. ” Aside from the principle of using tree-rings, the use of Dendrochronology is governed by several other principles, these set of scientific rules must be adhered to in order for the result to be flawless.
There is the Uniformitarian Principle which states that “physical and biological processes that link current environmental processes with current patterns of tree growth must have been in operation in the past,” the Principle of Limiting Factors which states that “rates of plant processes are constrained by the primary environmental variable that is most limiting,” The Principle of Aggregate Tree Growth which states that “any individual tree-growth series can be "decomposed" into an aggregate of environmental factors, both human and natural, that affected the patterns of tree growth over time,” The Principle of Ecological Amplitude stating that a specie of trees “may grow and reproduce over a certain range of habitats, referred to as its ecological amplitude,” the Principle of Sites Selection which states that “hat sites useful to dendrochronology can be identified and selected based on criteria that will produce tree-ring series sensitive to the environmental variable being examined,” the Principle of Cross-dating which states that “matching patterns in ring widths or other ring characteristics (such as ring density patterns) among several tree-ring series allow the identification of the exact year in which each tree ring was formed. ” and the Principle of Replication which states “that the environmental signal being investigated can be maximized, and the amount of "noise" minimized, by sampling more than one stem radius per tree, and more than one tree per site,” the application of which is not limited to Dendrochronology only.
The methodology used in Dendrochronology is cross-dating, or matching patterns of ring-growth from one tree to another and assigning rings to specific years, however, this possible only among trees growing in the same general climatic region. The good thing about dendrochronology on the other hand, is that cross-dating can sometimes be achieved in spite of human interference to ring-growth such as thinning of stands, resin-gathering, fire damage, and other traumas such as severe weather effects, pollution or lightning damage, not to mention shaping of the wood at the time of construction and decay afterward. Furthermore, visual and statistical techniques are employed to guarantee the accuracy of the matches and in addition to simple ring-width analysis, X-ray densitometric methods are used to reconstruct past environmental conditions.
It is also possible that wood or charcoal samples taken from standing buildings or excavated from archaeological sites be crossdated with each other and with wood from living trees to extend the tree-ring chronology beyond the date of the oldest ring of the oldest living tree in the region. Scientists believe that the best advantage of dendrochronology is that it is the only archaeometric technique where determination of absolute dates accurate to the year is either theoretically or practically possible, but, just like any other method of dating artefacts, it suffers from several limitations. The following limits the use or accuracy of Dendrochronology: 1. in some areas of the world, particularly in the tropics, the species available do not have sufficiently distinct seasonal patterns that can be used 2.
in cases where the right species are available, the wood must be well enough preserved that the rings are readable to the point that there must be at least 30 intact rings on any one sample. 3. in order to produce an accurate result, it is necessary to have samples of timber that retained their bark, so that it is clear which ring was the outermost when it was felled. 4. it can only be used if there is an existing master strip for that area and species; if the only master chronology available for the region is oak, cross-matching with timbers of these other species cannot be relied on. 5. use of this technique is also limited on how far back in the past things can be dated with tree rings although bristle cone pine trees can live to 9,000 years, this is a very rare phenomenon. 6.
“sapwood is highly susceptible to decay particularly by beetle larvae and as a “result all sapwood may have been removed from the accessible surfaces of timbers during building repairs and conservation work, making it impossible to determine when the timber was felled. ” 7. it may give inaccurate results on the actual date of the structure if it so happens that the date the timber was felled is not necessarily the date that the building was constructed or that the timber used was imported from another area. 8. in some places, prehistoric people may have built their structures using timber however, in most of the world that did not begin to happen until about 4,000 to 5,000 years ago.
Thermoluminiscence Thermoluminescence dating is the determination by means of measuring the accumulated radiation dose of the time elapsed since the material containing crystalline minerals was either heated or exposed to sunlight. The application ranges from “Lower Paleolithic to Neolithic archaeological sites, with a major focus on the Middle Paleolithic, which is often beyond the range of the radiocarbon method. ” The principle behind this unique dating technique is based on the “storage of information about the absorbed radiation in inorganic crystals. ” It is based on structural damage and faults to the crystal lattice of minerals by ionizing radiation.
The sources for this omnipresent radiation are radioactive nuclides from the surrounding sediment and from the sample itself, as well as secondary cosmic rays. Thus a radiation dose accumulates in the crystal in the form of electrons in excited states, of which some are metastable and thus resident over periods of time long enough to allow a dating application. During the first heating of the artefact, if the temperature is high enough (400° C), the drainage is sufficient to relax all electrons relevant to the luminescence method used; that is, the clock is set to zero and through the years it starts to accumulate natural radiation, upon its second heating it releases all this stored radiation in terms of thermoluminescent light.
The released light is then correlated to the absorbed radiation which is then correlated to the archeological age. In practice, knowledge of the composition of the artefact is of importance since the nature or chemical composition of it, as explained above, determines the amount of natural radioisotopes present in it. These natural radioisotopes are responsible for the greater part of the radiation that is absorbed. If the composition of the artefact is known, then the archeological age of the artefact can be computed by using the “total amount of absorbed radiation divided by the absorbed dose through internal and external radiation per year. ”
The greatest advantage of Thermoluminescence dating over other methods is not only the direct association of the event with past human activity on a linear time scale, but also its smaller vulnerability to unknown variation of certain parameters. However, Thermoluminescence dating is still prone to errors and inaccuracies. The evaluation and publishing of results must be done with due care and must meet with certain standards which includes the presentation of glow curves, heating- and DE-plateaus, growth curve(s) and the determination of the alpha sensitivity of each sample. Also, equal care has to be taken in the evaluation of parameters prone to variation with time. Conclusion
Each of the three dating techniques discussed has its own peculiar way of finding the archaeological era that a sample was created or may have died: Radiocarbon depends on decay, Dendrochronology depends upon growth and thermoluminescence depends upon absorption. The three techniques however different they may be are still connected by a linear timeline, when the accuracy of one technique ends, another begins. Dendrochronology can only be used as long as there are master strips which may reach only up to a maximum of 9,000 years old, results obtained through Radiocarbon, on the other hand, can only be accurate up to 50,000-60,000 years old while Thermoluminescence dating ranges from 10,000 to 230,000 years old. The three dating techniques discussed have their own advantages and disadvantages.
In the end, the scientist is the one who weighs all the pro’s and the con’s of any technique and he is given the discretion on what technique to use which he thinks is more appropriate and shall give out more accurate findings. BIBLIOGRAPHY Berger, Thomas. Thermoluminescence Dating: A Brief Overview. http://www. ati. ac. at/~vanaweb/papers/archview. pdf Fagan, Brian M. and George H. Michaels. Dating Techniques in Archaeology. http://www. mc. maricopa. edu/dept/d10/asb/anthro2003/archy/dating/datingtech. html#Dendrochronology Freestone, Ian. Ceramic Analysis. http://ads. ahds. ac. uk/catalogue/adsdata/cbaoccpap/pdf/117/11710001. pdf Grissino-Mayer, Henri D. Principles of Dendrochronology. http://web. utk. edu/~grissino/principles. htm Higham, Thomas. The Method. http://www. c14dating. com/int. html Hirst, Kris K.
Radiocarbon Dating Method. http://archaeology. about. com/od/rterms/g/radiocarbon. htm Kuniholm, Peter Ian. Dendrochronology. http://www. arts. cornell. edu/dendro/ajatext. html Richter, Daniel. Advantages and Limitations of Thermoluminescence of Heated Flint from Paleolithic Sites. http://www. eva. mpg. de/evolution/staff/richter/pdf/07-RichterGeoarchaeology. pdf Robinson, B. A. How does Carbon-14 Dating (C-14) Work? Is it Accurate and Reliable?. http://www. religioustolerance. org/c14dats. htm Taylor, Jonathan. Dendrochronology in Dating Timber Framed Building and Structures. http://www. buildingconservation. com/articles/dendrochron/dendro. htm
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