The Science of Archaeomagnetism
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Today's photo tour offers a lesson in Archaeomagnetism - the science of dating archaeological sites by the use of material that has been fired and cooled - such as a tabun (bread oven). 

As a common method of dating, archaeologists typically  use the pottery found on location to affix a date to an excavation site (by looking at such specifics as coloring, markings, and materials used). 

The Specifics of Archaeomagnetism:
Within material that has been exposed to very high temperatures (e.g. sand and ashes in a cooking fire), the magnetic fields of the ferromagnetic particles such as hematite and magnetite align themselves like little compass needles according to the lines of flux present in the magnetic field of the earth. 

When the surrounding material cools, the magnetic fields of the ferromagnetic particles remain fixed in orientation to the magnetic field lines of the earth.  The magnetic field lines of the earth change over time, and can be documented by the discipline of archaeometrics.  By recording the orientations of the "frozen" magnetic fields of the particles in old fire pits which have been accurately dated by other means (such as pottery dating), a dating scale of magnetic orientations can be developed.

Egon H.E. Lass, our stratigraphic analyst, is one of the few people sampling tabuns and other fixed features of the Middle East today. He has gathered samples from as many as 90 tabuns within Israel. His goal is to generate enough data points to create a trajectory  that will allow future archaeologists to use archaeomagnetism as a precise method of dating excavation sites.

First, a tabun is discovered in one of the squares.  A tabun is a bread oven. The rim of the oven appears like a piece of pottery - almost ceramic from the amount of use and repeated high temperatures.

All pottery sherds are removed and used by the archaeologists to estimate the date of the site.

Pictured here is 
Egon H.E. Lass.

In the rim of the tabun, small pillars are cut. These pillars will become the samples.

Small rings of clay are placed around each pillar.

In this tabun, ten samples will be taken.

 

Into the rings of clay, aluminum molds are pressed and precisely leveled.  Aluminum molds are used because they contain no magnetic particles which  would interfere with the azimuth readings.

Next, plaster is mixed that will be used to encase each sample.

The plaster is poured into each mold, over and around each sample.

A drawing is made of the pillar location within the tabun. This drawing will be used to record the azimuth (magnetic bearing) readings that are taken with the compass.

The compass reading shows how the cube is oriented, and a precise magnetic bearing (azimuth) is taken.

This photo shows the record of the area drawing and the compass recordings. This record will be sent to the lab with the samples.

Once the plaster has had some time to harden, each mold is carefully removed with its encased sample.

The outer edges of the mold are scraped clean

More cleaning of the sample.

A small portion of the sample is carved away so that more plaster can be poured over it - completely encasing the sample in plaster.

Each final sample is marked with the date, sample number, site name, and orientation (so that the lab knows how to process each sample).

The mold casing is removed, leaving just the sample encased in plaster.
The samples are processed by a lab in Maryland, USA using a very sensitive and precise magnetometer.  The magnetometer measures the orientation of the "frozen" magnetic fields of the ferromagnetic particles in each sample, compared to the magnetic azimuth recorded for the sample (on-location as shown above). 

As Egon continues his work, he is building a database of knowledge that will someday allow archaeologists to date excavation sites through the process of archaeomagnetism.
 

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