Chemistry+in+Archaeology

Uses of Chemistry in Archaeology Archaeologists are using chemistry more and more in analyzing artifacts, remains, or obtaining their ages, especially as these techniques become more refined and accessible. Some of these chemical analyses and techniques will be introduced and explained here, and some examples of their applications will be provided. When an archaeologist approaches a site, or artifact within a site, they must first determine the //context// of the site or artifact - in other words, what its surroundings are, and what its use may have been. Some of the first questions that may follow are //how old is it//? or //what is it made of?// **Determining Age** The most common way to determine the age of an artifact is through context. What stratigraphic layer was it found in? Were there any objects (such as a coin) with known dates in that layer? Sometimes, these clues don't exist, or don't provide answers, and this is where chemistry comes in.

//Radiometric dating,// the use of radioactive isotopes in a sample to determine the sample's age, is one way of dating something. The most common radiometric dating technique is **radiocarbon** dating. This is one type of //cosmogenic radionuclide dating//, where detectable, radioactive isotopes are created in a material by cosmic radiation.

__Radiocarbon Dating__

In order to be able to use radiocarbon dating, there must be some organic material (wood, leaves, ashes, human or animal remains) present, within the sample or within its stratigraphic layer. It has, for example, been used to date the Dead Sea Scrolls, and to determine the age of the Ice Man, Otzi.



Here's how it works. Carbon-14 is produced in the upper atmosphere when neutrons (from cosmic sources) strike nitrogen-14 atoms. The nitrogen absorbs the neutron, then spits out a proton, turning into carbon-14. This carbon makes its way into the carbon cycle, and is used by living organisms such as plants and animals.



Carbon-14 is continuously being regenerated in the atmosphere, so you can think of all presently living things as having the most carbon-14 they'll ever have. When something dies, it stops getting carbon-14 from the carbon cycle.

Carbon-14 is an unstable atom, and will undergo radioactive decay (in the form of beta particle emission) to nitrogen-14 over time. The rate at which this happens is called its **//half-life//**, which for carbon-14 is 5,730 years. This means that 100 grams of carbon-14 today will decay, and in 5,730 years, there will only be 50 grams of carbon.



To determine the age of an object, the amount of carbon-14 must be determined. There are two ways of doing this: by beta counting, or by determining the ratio of 14 C to 12 C using accelerator mass spectrometry (AMS). Beta counting uses a radiation detector to measure the emission of beta radiation from decaying 14 C. AMS actually counts the number of 14 C atoms and 12 C atoms, providing a ratio. These techniques are then compared to calibration curves and the age of the sample is determined.

Some complexities with the radiocarbon dating method exist. For example, any items older than about 60,00 years will not have enough carbon-14 in them to provide a date. Second, the variation in Earth's magnetic field, and in the solar cycle, influence how much carbon-14 is produced. Variations in the carbon cycle, such as the rising amount of carbon dioxide in the atmosphere, play a role. Even the nuclear weapons testing in the late 20th century had an effect on the amount of carbon-14 in the atmosphere. All of these have to be calibrated for, and the data looks something like this: This video does a nice job summarizing carbon-14 dating. media type="custom" key="25599758" align="center"

One drawback to standard C14 dating is that it requires a small part of the artifact to be destroyed. Obviously, for small or valuable items, or in cases of cultural sensitivity, this is not always possible.

Fortunately, an alternative method is currently being developed at Texas A&M, where the sample is essentially unharmed during C14 dating. An extremely tiny 'layer' of organic material is stripped off of the object using a plasma, which is then analyzed to get an age. It is being used to date rock paintings, among other things. __Other Cosmogenic Radioisotope Dating Techniques__ As with carbon-14, cosmic rays can create other radionuclides. These can be formed in the atmosphere, or at the Earth's surface. They can be measured and used to determine the age of rocks. Here are some examples:

__Surface Exposure Dating: Beryllium-10__ When a cosmic ray strikes an atom in the atmosphere, it may produce a neutron (see the carbon-14 production diagram earlier). This neutron can then crash into the surface of the Earth. When it does, it can strike rocks or minerals that contain oxygen - quartz, for example. The neutron will **//spallate//** an oxygen atom (break it up), forming beryllium-10 and beryllium-7: Beryllium-10 has a half-life of 1.6 million years, so it sticks around for a while. On the other hand, beryllium-7 has a half-life of only 53.3 days, so it isn't used for long-term surface exposure dating. The amount of beryllium-10 in a rock sample can thus be measured (using accelerator mass spectrometry), providing an estimate of how long the rock has been exposed at the surface of the Earth. The following video provides a nice overview and use of beryllium-10 surface exposure dating. media type="custom" key="25599702" align="center"

Surface exposure dating has been used to date petroglyphs in Australia to approximately 40,000 years ago. Cosmogenic nuclide dating in Antarctic research **Thermoluminescence (TL) Dating** Thermoluminescence (TL) dating is mainly used to date pottery, but can also be applied to lithics. Almost any material that contains minerals, at any grade (fine or coarse), can be dated using TL, as long as the minerals have been heated to a certain temperature at some point of their use. Again, this is why TL is primarily used to date pottery, since it has to be fired to transform it from clay to porcelain. Minerals will absorb energy from sunlight or cosmic radiation over their lives. But once the mineral has been heated (usually over 500 C), all of this energy is released. This then 'resets the clock' on these minerals. By measuring the amount of energy a mineral sample has absorbed, a date can be obtained.



TL dating has been used to study the age of paintings in rock shelters, which is stirring debate over when humans arrived in America.



Here is a video explaining TL dating. media type="custom" key="25644752" align="center"

**Determining Composition** Another important question asked in archaeology is "what is it made of?" Knowing the composition of a stone (lithic) tool, piece of bronze, or pottery shard can prove invaluable at placing the object in its proper context in terms of age, origin, or craftsmanship. Sometimes, metals can be identified based on their corrosion properties. For example, artifacts containing iron will usually have a red-orange oxidation, and those containing copper (such as bronze) will have a green or blue oxidation. This can be difficult or unreliable if the matrix (surrounding materials) is complex and the chemistry is unique. Soils can have different acidities, or exposure to ocean water versus fresh water can alter the oxidation of metals. For lithics or pottery, identification becomes even more challenging. __Chemical Tests__ If a material is recoverable in its original state, there are certain chemical tests called //spot tests// that can help identify its composition. Certain materials react with specific chemicals to produce signature colors. For example, starches turn purple with iodine; iron turns orange-red with orthophenanthroline. Mystery Fiber is a good example of using spot tests, in this case on fibers found on an ancient Egyptian coffin. Another test that can be done on solid materials is a streak test, or //touchstone test//, where the color of the streak is used in identifying the material. This technique has limitations, especially in more complex situations.

__**Radiography**__ The use of X-rays to study archaeological materials has become fairly common. CT scans and X-rays of mummies, for example, have revealed much about their health, burial methods, clothing and jewelry, etc. without invasive techniques.

Radiography and CT from Cornell University's Project ArAGATS Ancient Skeleton Yields Earliest Complete Example of Human Cancer

__X-Ray Fluorescence__ One of the fastest ways to determine chemical composition in the field is with a portable X-ray fluorescence spectrometer. This device is capable of identifying and measuring the elemental composition of almost any material non-destructively. Here's how it works: an X-ray photon, being a high-energy photon, strikes an electron within an atom. This displaces the electron (kicks it out), and leaves a "hole" in the atom's electron structure. When this hole gets filled by another of the atom's electrons, it releases X-ray photons (see emission theory). These photons will have a specific wavelength and frequency, providing an identifiable "fingerprint" for that atom. Matching the fingerprints of these atoms allows for their identification.



Gadget IDs ‘fingerprints’ of ancient tools Neutron Activation Analysis (NAA) Neutron activation analysis is a very sensitive technique that can detect minute quantities of elements in a sample. It is used to study trace elements in pottery, or in metals or alloys, to help identify their production techniques and compositions. Small samples of the material are bombarded with neutrons in a nuclear reactor. This causes elements within the sample to become radioactive (similar to cosmogenic radioisotope production). The emission properties of the material are then analyzed and matched to a known database of emissions, allowing for identification of the elements in the sample.

[|Identification of the geologic origins of **archaeological** artifacts]
Chemical fingerprinting and source tracing of obsidian: the central Mediterranean trade in black gold. New archaeological ‘high definition’ sourcing sharpens understanding of the past


 * Strontium Isotope Analysis**

Strontium tracing is the use of the isotopic ratio of two strontium atoms, strontium-87 and strontium-86, in artifacts (especially pottery and bone) to determine their geologic origin.

Strontium is chemically similar to calcium, and is taken in to the bodies of humans or animals from foods, where it is then stored in teeth and bones. It is also part of the fabric (clays) of pottery, for example. If the strontium ratio of a certain artifact is known, it will match the 87 Sr/ 86 Sr ratio of the geology in which the plant, animal, or clay originated. When the 87 Sr/ 86 Sr ratio does not match, then questions arise about whether the object was moved from its original location, or whether the person whose bones contain the strontium migrated or moved from the original location. Geological and archaeological implications of strontium isotope analysis of exposed bedrock in the Chicxulub crater basin, northwestern Yucatán, Mexico Tracing Past human and animal migrations using strontium isotope analysis Written in Bone To determine the 87 Sr/ 86 Sr ratio in an artifact, a sample of the artifact is prepared and analyzed using mass spectrometry. media type="custom" key="25647484" align="center"

__Phosphate Analysis__ When humans occupy a site, they tend to deposit organic matter (in the form of animal or human waste, food waste, or otherwise) that contains phosphates. The levels of phosphate in the matrix (soils) tend to be appreciably higher in human-occupied areas, and detection of this elevated phosphate can be helpful in studying an archaeological site. After sample preparation, the phosphate ion is detected using molybdenum blue spectrophotometry.


 * Preservation**

__Cyclododecane__

Cyclododecane is an organic compound with really unique properties. It can be applied in liquid form to a surface, where it will solidify to form a protective barrier. Then, over some time, the cyclododecane barrier will sublimate, leaving the surface unchanged. It has been used to protect artifacts such as mummies, paintings, etc. during transport and study.



The Harvard Art Museum used cyclododecane to protect a fresco while moving it.