The C has a half-life of about years, i. In the disintegration process the Carbon returns to nitrogen emitting a beta particle in the process. The quantity of the C remaining is measured by counting the beta radiation emitted per minute per gram of material. Modern C emits about 15 counts per minute per gram, whereas Carbon which is years old, emits about 7. Specimens for C Dating: Specimens of organic material which can yield good amount of carbon can be collected for C dating.
Quantity of samples sent for radicarbon dating should be sufficient enough to give proper results. As quite a bit of sample is lost in the pre-pigmentation process one should try to collect as big sample as possible. Bones are generally affected by ground water carbonates and are therefore least reliable for dating. Charred bones are better preserved and are therefore relatively more reliable. Charcoal is best material specially if derived from short live plants. How to collect samples: While collecting samples for radio carbon dating we should take utmost care, and should observe the following principles and methods.
Sample should be collected from and undisturbed layer. Deposits bearing, pit activities and overlap of layers are not good for sampling. The excavator himself should collect the sample from an undisturbed area of the site which has a fair soil cover and is free of lay water associated structures like ring wells and soakage pits. Samples which are in contact or near the roots of any plants or trees should not be collected because these roots may implant fresh carbon into the specimens.
Handling with bare hands may add oil, grease, etc to the sample. Therefore, it is better to collect samples with clean and dry stainless steel sclapels or squeezers. It may also be collected with the help of glass. Stainless steel, glass, polythene and aluminium are free from carbonatious organic material. Therefore sampling should be done with such material only. Samples should be sundried before pacing in aluminium thin foils and placed in a glass jar or secured safely in thick polythene covers. Before pacing the soil should be removed while it is wet at the site.
Method of Sample Recording: Before removing the sample from the site we should note down the data or the environment of the sample. We have to fill the data sheets, which should be done at the time of sampling and should be submitted along with the sample to the dating laboratory. These sheets require data on environment and stratigraphy of the sample, and archaeological estimates of its dating. This data help in obtaining and objective interpretation of dates. Limitation and Errors of C Dating: There are a number of technical difficulties inherent in this method of dating.
The first difficulty is that the quantity required for a single determination is comparatively large.
Dating Techniques In Archaeology
It will be difficult to obtain sufficient quantities of samples, especially in the case of valuable museum specimens. The second difficulty is that the radio active decay does not take place at a uniform rate but is a random process, and is therefore, governed by the laws of statistical probability. Another difficulty that has to be taken into serious consideration is the possibility of uneven distribution of radio carbon in organic matter. If the specimen is analyzed after having been exposed to contamination by carbon compounds of an age younger than its own, radio carbon age is liable to be reduced.
The best results can be obtained from specimens, which were preserved under very dry conditions, or even enclosed in rock tombs of the like. Very dangerous contamination is done, very often, by the growth of fungus and bacteria on the surface of the specimen which even when removed from the specimen may falsify its actual age. Though there are some drawbacks and technical difficulties, the radiocarbon method is a reliable, efficient and most useful method of dating the archaeological specimens.
We are helpless in the case of contamination done by the natural agencies in the past, but we can overcome most of the difficulties by paying sufficient care and attention while collecting the samples. It is the duty of an archaeologist to study with care the condition of preservation of specimens submitted for analysis and, in fact, to submit only specimens that can be regarded as fool-proof as is possible in the circumstances.
Dendrochronology is a method that uses tree-ring analysis to establish chronology. A major application of dendrochronology in archaeology, as a tool for establishing dates from the samples of wood and articles made out of wood is not only in working out primary chronologies but also in cross checking the already known dates by other methods.
Often, the tree-ring analysis from a site can give strong clues about the length of occupation, certain periods of building or repair activities at the site. Another application of tree-ring analysis is the inference of past environmental conditions, which is extremely useful to the archaeologists. The modern science of dendrochronology was pioneered by A.
Tree ring analysis is based on the phenomenon of formation of annual growth rings in many trees, such as conifers. These rings are shown by the trees growing in regions with regular seasonal changes of climate. As a rule trees produce one ring every year. When growing season rainy season begins, sets of large, thinly-walled cells are added to the wood. This process repeats in the following years also.
The formation of rings is affected by drought and prosperous seasons. In the years with unfavourable weather the growth rings will be unusually narrow. On the other hand, during years with exceptionally large amounts of rain the tree will form much wider growth rings. Most of the trees in a give area show the same variability in the width of the growth rings because of the conditions they all endured.
Thus there is co-relation between the rings of one tree to that of another. Further, one can correlate with one another growth rings of different trees of same region, and by counting backwards co-relating the inner rings of younger trees with the outer rings of older trees we can reconstruct a sequence of dates.
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By comparing a sample with these calendars or charts we can estimate the age of that sample. Thus it is possible to know the age of the wood used for making furniture or in the construction work. The main disadvantage with the system is that, we require a sample showing at least 20 growth rings to make an objective estimation of its age.
Hence smaller samples cannot be dated. This method can date the sample upto the time of cutting the tree, but not the date when it was actually brought into use. This method is based on the fact that the magnetic field of the earth is changing constantly in direction and proporationate intensity, and that these changes lead to measurable records. The magnetism present in the clay is nullified once the pottery, bricks or klins are heated above degree centigrade. This implanted magnetism can be measured and the date of its firing estimated.
The dating of ancient pottery by Thermoluminiscence measurements was suggested by Farrington Daniels of the University of Wisconsin in America Thermoluminescence is the release in the form of light of stored energy from a substance when it is heated. All ceramic material contain certain amounts of radioactive impurities uranium, thorium, potassium. When the ceramic is heated the radioactive energy present in the clay till then is lost, and fresh energy acquired gradually depending on the time of its existence. The thermoluminescence observed is a measure of the total dose of radiation to which the ceramic has been exposed since the last previous heating, i.
The glow emitted is directly proportional to the radiation it received multiplied by the years. It is present in nearly every mineral. During rock formation, especially lava, tuffs, pumice, etc. Virtually all argon that had accumulated in the parent material will escape. The process of radio-active decay of potassium continues and the argon accumulated again which when measured will give a clue as to the age of the rock.
The application of this method to archaeology depends on locating the widespread distribution of localities that have recently in the last half-million years experienced volcanic activity forming layers over the culture-bearing deposits. The city of Pompeii in Italy is a good example of the destruction caused by volcanic activity. This method is more useful in dating the prehistoric sites.
The starting phase of the Palaeolithic period in India is pushed back by atleast one million years from the earlier dating of about 5 lakh years B. This unique example comes from a sit known as Bori in Maharashtra, where it was found that a layer yielding flake tools is overlain by a layer of volcanic ash. When this ash was subjected to Potassium-Argon dating it yielded a date of 1.
Initially this method was developed to date the meteorites and other extra-terristrial objects, but it is now being applied to archaeological purposes as well. It is known that may minerals and natural glasses obsidian, tektites contain very small quantities of uranium. Through time , the uranium undergoes a slow spontaneous process of decay.
This method of dating depends upon the measurements of detectable damage called tracks in the structure of glasses caused by the fission. Unless tied to historical records, dating by archaeological methods can only be relative -- such as stratigraphy , typology , cross-dating , and sequence dating.
Absolute dating , with some reservation, is provided by dendrochronology , varve dating , thermoluminescence , potassium-argon dating , and, most important presently, radiocarbon dating. Some relative dating can be calibrated by these or by historical methods to give a close approximation to absolute dates -- archaeomagnetism, obsidian hydration dating , and pollen analysis. Still others remain strictly relative -- collagen content , fluorine and nitrogen test, and radiometric assay. The methods have varying applications, accuracy , range , and cost.