Dr. Johnson C. Philip
In this second article we take a look at the more technical aspects of dating archeological finds.
Dating Techniques Used By Archeologists
Dozens of techniques are now applied, often more than one to the same object, to determine the dates as accurately as is possible. These techniques can be divided into two: relative dating methods and absolute dating methods. As is obvious from the name, the latter gives more accurate results compared to the former.
One should realize that not all methods can be used for dating all kinds of discoveries. Further, some of them, such as Cultural Affiliation Dating, can be used on a wide variety of artifacts whereas some such as Radio-Carbon Dating can be used only on things having a biological origin. But then this should not surprise any because tools of science have both their range as well as their boundaries.
The major Relative and Absolute methods of dating used in archeology are as follows:
Relative Dating Techniques
- Cultural Affiliation
- Cation Ratio
- Fluorine Dating
- Obsidian Hydration
- Patination
- Pollen Analysis
- Rate Of Accumulation
- Seriation
- Varve Analysis
Absolute Dating Techniques
- Archeomagnetism
- Astronomical Dating
- Dendrochronology
- Electron Spin Resonance
- Fission Track
- Opacity Stimulation Luminescence
- Oxidizable Carbon Ratio
- Racemization
- Thermoluminiscence
- Radio-Carbon Dating
Cultural Affiliation Dating: Each culture has its own discernible peculiarities, that change with time, so that an observer [who already knows about that culture] can see an object, hear about a practice, come across a word, and tell the approximate time-period to which it belongs. A good example is the English language.
The language of a drama, novel, or book written during the Elizabethan period will resemble the language in a remarkable way. Thus if one comes across an ancient manuscript that uses this type of a language, then one can safely assume that it is at least three to four hundred years. Of course, before coming to that conclusion once has to rule out many factors that can create apparent age. For example, one has to make sure that it is not a mock-writing done by a modern writer for some purpose such as a school assignment or a satire in older English but meant for consumption by the present readers.
Artifacts produced by various cultures, and similar tangible items, have come in so abundantly, and their styles and other discernible factors have been studied in detail that is so sufficient, that experts can often ascribe a tentative date within a day of the discovery. Surprisingly, this method of dating can be used on an extremely wide variety of artifacts, inscriptions, clothing, pictures, and even languages.
Cultural Affiliation Dating can give good results in the hands of a careful researcher, specially if he has plenty of experience with this kind of dating, and if he has a large number of artifacts [large sample, speaking technically] to conduct this study. Often CAD can be a good starting-point, and from there one can move on to more accurate and exacting techniques.
Cation Ratio: Rocks exposed to the elements of nature gradually become coated with a layer of special chemicals. This “rock varnish” is formed when calcium and potassium seep out of the rocks, which undergo predictable chemical changes, and quote the surface to a thickness that is somewhat proportional to the time for which the rock has been exposed to natural elements.
When someone carves a picture over such a rock, the “rock varnish” is removed from the area of rock that is removed through carving, whereas it keeps growing in areas untouched by the carving. Once the carved area is exposed to nature, a fresh coat of the so-called rock-varnish begins to form over the carved area. Thus if one is able to get to two surfaces, the one always exposed to elements and the one that has a younger coating after the carving, one can come to some conclusions about how long the carving has been there on the rock.
To do so, they scrape the varnish-like coating from the virgin rock surface and also from the carved surface, and obtain the ration between the positively charged ions [cations] in the samples. This ratio gives some idea of the age of the carving.
Cation Ratio dating is a relative method, and it has many uses, but it can never give absolute dating. Further, cation formation is affected by soil, moisture, and other chemical factors that affect it over the years. Thus tests are conducted on a large sample of rocks from a given geographical area [with some carvings, if possible, with a known date] and a calibration-chart is prepared. This chart then helps the investigators to come up with dates that are still relative, but which are more reliable.
Fluorine Dating: Fluorine is an element found abundantly in groundwater in most places around the world. When men and animals are buried in soil rich with water content, the skeletal remains are inundated with a host of chemicals circulating through the ground water. Fluorine begins to accumulate in these bones through a complex chemical process. The accumulation of Fluorine is somewhat proportional to the time for which the skeleton remains buried underground.
This method for relative dating is now several decades old, and techniques are available for accurately assessing the amount of Fluorine that has accumulated in buried bones or implements made of bone [which are abundant in archeological remains]. Once a Fluorine profile of bones found in single location [or even an entire geographical area] is prepared, it can be used to place those skeletal remains in relation to each other in time. Thus though this method offers only relative dating, it offers a highly accurate method for establishing the general chronology of skeletal finds in relation to each other.
The famous Piltdown Hoax was exposed with the help of this method. Piltdown Man is the name given to the hypothetical “Missing Link” in human evolution discovered in Piltdown, England. More than 500 learned works appeared on this missing link and even a monument was erected in the place of its discovery to commemorate this find. However, once the discover of this “missing link” died, the bones [so far zealously hidden from the scientific community, who were given only plaster casts for their study] came into the possession of the scientific community for the first time. A series of Fluorine dating, and they found this “ape man” was a composite fabricated by mixing bones from various ages. Thus even relative dating have substantial scientific contributions to make.
Obsidian Hydration: This is a relatively new dating method, developed around 1960. Though the primary purpose of this technique is to find relative dates, at times it gives dates that are as good as absolute. It also happens to be more easy to conduct, and cost effective in most cases.
Obsidian is a natural glass, formed by volcanic activity. During earlier part of the human civilization, when hardened metal implements were not available, Obsidian was highly prized for its ability to cut and shape implements. It has been found in the mountains of Americas, Europe, Near East to India, North East Asia to Japan and even New Zeland. A large number of Obsidian artifacts have been discovered in all these places.
The Obsidian Dating is based upon a special characteristic of this material. As soon as there is a fracture in this material. that new surface begins to absorb water [hydration] at a level that is almost constant with time. The thickness of this hydration layer increases with time, and this time can be determined by cutting a thin piece of this material which is then examined under microscope.
The method yields only relative dates, but they are more reliable than dates given by many other methods. The cost is also less. But it needs to be used with care as the rate of hydration can be affected by many factors. Even in the laboratory the test is often repeated after one week on the same sample before the final result is declared.
Patination: Many ancient artifact develop a thin outer layer of chemicals called the patina. The patina is the outermost surface of the artifact. This layer differs in color, texture, luster or composition from the rest of the artifact. This layer is created as a result of chemical, physical or biological change in response to the surrounding soil and environmental condition. When a large number of artifacts with patina are found in the same location, this layer can be used to gauge their ages relative to each other.
A good example of patination would be a flint artifact buried in some type of soil or sediment that causes its surface to be chemically altered and become white, yellow or brown. Another example would be the green crust that forms on bronze. Similar examples are known to even those who do not work with archeological samples.
So far five types of patination have been recognized and investigation. The first is bleaching, which is caused by the leaching out silica and replacement of it with lime salts. The second is induration of exposed surfaces, which is caused by the leaching out of soluble silica and redeposition of it at the surface forming a substance called silcrete. The third is limonite-penetration and staining which involves limonite clays and salts in the soil being absorbed by stone artifacts. The fourth can be called “desert-varnish” which is a condition created by extreme dryness and intense solar radiation affecting artifacts. Fifth is the formation of a crust, caused by leaching out iron salts and redeposition at the surface.
With a site that has several layers of civilizations and a large series of objects from each layer, it is possible to observe obvious differences in the amounts of patina. With this knowledge the layer may be determined from the earliest to the latest civilization. However, work is going on to identify possible yearly-deposit-layers, so that more accurate dates might be obtained. However the work is still in the stage of growth.
Pollen Analysis: As microscopic techniques became more refined, and as an increasing number of organic materials were microscopically analyzed, the researchers began identifying pollen grains embedded in these materials that might range from dried up varnishes up to woven clothes. Pollen analysis which led to relative dating was first done by the Swedish geologist Von Post around 1916.
Since that time many other researchers have analyzed samples of known and unknown origins and dates, and have made extensive charts about pollen distribution. Using such pollen diagrams, information can be obtained about vegetation, floristic and climatic changes, sea level variations, etc. which took place in the past. The difference in amount of pollen in 1cc can tell about the condition of the forest which produced it. If the amount is small, it might have come from tundra whereas larger amounts might indicate forests with vegetation which depends on pollen or spore reproduction.
Surprisingly, the tiny grains of pollen are very resistant to external factors, and survive almost unchanged for millennia, preserving an accurate record of history. Thus in addition to the relative dates, the pollen analysis, study of vegetation history using the microfossils (pollen grain and spores of size 15-50 um), can give us useful information about the target area’s condition in the present and past.
Thus once a pollen-containing sample is examined, and the types of pollen, along with their density is determined, it is checked against standard charts for that region to arrive at relative dates and other information. This is a highly useful method in spite of its relative nature.
Rate Of Accumulation: Archeological finds in most places are in the form of layers, with each layer representing one particular group/time of habitation. The topmost layer will be the youngest, and the successive layers below it will be older. The same will be the chronological pattern of the artifacts found, with the youngest one in the topmost layer and the oldest one in the layer at the bottom.
The depth of the debris/mud/other materials can give some idea of age, and on comparison the relative dates of the layers can be guessed to some degree. If there are mounds sufficiently close, and if some interdependence can be demonstrated, then the layers in one place can be compared to the other and more reliable relative dates can be fixed for both through cross-checking and comparison. If coins, or artifacts are discovered that can be dated more precisely, this relative dating can be made more accurate.
Rate of accumulation, though relative, has an important role to play in developing the history of succession in any given mound with multiple layers. It needs to be used with caution, because often only less than ten percent of a given large mound is excavated to expose layers. Second, intrusion of one layer into another is common where there has been digging by men or animals. The researchers needs to be careful to choose an area where this has not happened.
Seriation: This technique depends upon ceramics discovered from a given site. It has the advantage in that one does not need to know from which layer the ceramic came. For this kind of dating, the investigator takes all the available ceramics from a given place, and arranges them in a sequence that produces the most consistent pattering of that culture. This arrangement takes shape, technique, and usage-patterns of the ceramics to arrive at the relative chronology.
Ceramic artifacts all over the world have undergone change in shape, size, and manufacturing technology in a predictable pattern that has been understood relatively well due to the exceptionally large sample of ceramics discovered in situ for the last 150 years.
Among relative dating methods this has been a highly reliable method, used from the nineteenth century onwards, and has contributed much to the understanding of the relative dates of various cultures.
Varve Analysis: Sediments are deposited by the rain/water cycle in many places. A type of these deposits is known as “varve”. Counting the number of varves can give some idea of the relative age of a previously inhabited location and, by implication, of the culture. This techniques was developed by the Swedish scientist Baron de Geer in 1878, and has been refined further since then. Audio-visual equipment and computers are now used to make the counting accurate.
A varve is made up of two layers: a thick light coloured layer of slit and a fine sand layer. These layers are sufficiently distinct from each other to be counted. Once the varve profile of a place is prepared, it can be compared with profiles in nearby area for cross-matching, and comparing of chronologies.
Varve-formation is affected by a number of factors, and therefore it provided only relative ages, and a strict numerical accuracy is not possible.
Summary Of Relative Dating Techniques
One needs absolute dating techniques for developing a reliable and numerically accurate chronology in archeology, but often absolute dating techniques are more difficult and not available. Thus the importance of relative dating. In spite of this weakness, it is amazing the substantial increase in chronological understanding that has come with the help of relative dating techniques.
Methods Of Absolute Dating Will Be Given In The Next Article…..
