DATING AN ANCIENT CHINESE URN by Joan K. Yanni

DATING AN ANCIENT CHINESE URN

by Joan K. Yanni

The Chinese funerary urn (99.56) in the New Acquisitions Gallery is made of earthenware with painted designs. Its broad shoulders, small loop handles, and narrow flat bottom is typical of the Banshan type of ceramic ware encountered in the western central part of Gansu province.  It dates from 2500-1700 BC.

According to the label, Neolithic Yang-shao ceramic ware was usually made by molding built-up coils of earthenware and finishing the work on a hand-turned wheel. Pieces such as this jar were fired at a temperature between 760 and 1020 degrees centigrade. The banded and crosshatched decoration was made by pressing a variety of materials, such as cord or textile, into the clay and then brushing on red and black pigments.  Many urns such as this were discovered in burial sites during the early 1900s and were probably used for the storage of grain or cereals.

Before its acquisition by the Gallery, this funerary urn underwent thermo luminescent testing, a process that determines the authenticity of the object by providing an accurate range of dates for the ceramic’s last firing.

Arthur Tweet, husband of docent Thea Tweet, has graciously agreed to describe the process of thermo-luminescent dating. Dr. Tweet is a Xerox retiree who received his PhD in physics from the University of Wisconsin. His explanation is as follows:

Thermo luminescence dating is a process for determining the age of an object by measuring the amount of light given off by a small sample of the object when it is heated under carefully controlled conditions in the laboratory.  The process is used in archaeology for dating artifacts made of ceramic or glass.

The process is based on the idea that when radiation from radioactive elements or from cosmic rays interact with any object, they transfer some of their energy to the material the object is made of.  If the material is ceramic, some of the transferred energy will be stored in it.  The longer the object is exposed to the radiation, the  energy will be stored—the ceramic becomes a sort of energy bucket that fills up at a certain rate.   time spent catching the radiation means  stored energy.

If a sample of material is taken from the object and heated in a chamber, some of that stored energy will be released from the sample in the form of light whose very feeble glow can be detected by sensitive instruments.  The amount of light emitted by the sample is proportional to the  amount of  energy stored, and therefore, to the length of time the object—a ceramic pot for example—was exposed to radiation after it was made. However, the radiation can come from several sources:

1. Radioactive elements of various kinds in the pot itself,
2. Radioactivity in the soil or other environment where the pot has lain, and
3. Cosmic rays, whose effect on the pot is dependent on how much earth has covered it and for how long.

So in order to find out when the pot was made, we must take into consideration what the pot is made of, whether soil and other objects in its environment are also sources of radiation, and whether it has been underground in a tomb or lying on the surface exposed to cosmic radiation.

Because all these factors determine how much light is given off by the sample when it is heated in the lab, it is necessary to calibrate the measurement.  An example may make this point clear.

A pot is found in a Middle Minoan tomb on Crete, and it is clear from other evidence that it was made within a few years of when the tomb (whose date is known from other evidence) was established.  The thermo luminescence of this pot is then measured and recorded.  From this measurement we know that a pot of the same date as our pot, made in the same way, of the same material, and kept in similar surroundings for the same length of time, will emit the same amount of light when heated in the same way in the laboratory.  Hence, if we ever find another pot just like our calibration sample, we can be confident that when it is heated, it will give off the same amount of light if it is the same age.

Further, if it gives off twice as much light, we will be tempted to believe that the newly-discovered pot is twice as old.  However, the skeptic will say, “Not so fast! You can say that your new find is very old only if it satisfies all the conditions (is made of the same material, was stored in the same environment, etc.) as your calibration sample.”

These conditions are usually hard to know for sure, and so there is considerable uncertainty about most thermo -luminescence dating.  For this reason conservative users of the technique will usually give a date within a range of +/- 100 – 300 years for archaeological finds.