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|Posted: Fri Dec 15, 2006 1:56 pm Post subject: (Chem) Spectroscopy: TR-XRF and Ancient Egyptian Colors
Total reflection x-ray fluorescence (TR-XRF) spectroscopy: Analyzing Ancient Egyptian Colors
By Sofi Bahgat, Georgetown University
15 December 2006
The accurate dating of artifacts is of paramount importance to understanding the history of ancient civilizations. Inaccurate dating of ancient objects can lead to serious historical misconceptions. For instance, the history of the ancient Egyptians – comprised of six main periods; the Predynastic Period (3150–2686 BC), the Old Kingdom (2686–2134 BC), the Middle Kingdom (2134–1782 BC), the second Intermediate Period (1782–1570 BC), the New Kingdom (1570–1080 BC), and the Late Period (525 – 380 BC) – can be seriously misinterpreted if the ancient Egyptian artifacts were assigned to wrong periods in history.
Their strong belief in the afterlife underlies the whole ancient Egyptians’ civilization. That is clearly reflected in the careful construction of their graves and the skillful crafting of highly durable objects they believed are necessary for their afterlife. For instance boat models were found in almost every grave: they believed the deceased must cross the Nile to meet the gods. Accordingly archeologists and historians are interested in dating these boat models in order to better understand and speculate about the underlying sequence of historical events.
Spectroscopic techniques proved very useful as non-destructive means of analyzing artifacts for their constituent materials, i.e. their chemical make up. The materials can then be traced to specific times in history. Spectroscopy is our means of “seeing” atoms and molecules that cannot be seen even by the most powerful microscope. To understand spectroscopy a little we may visualize it as “the science of jumps”. In electronic spectroscopy we probe electrons to make energy jumps while in nuclear spectroscopy we probe nuclei to make energy jumps. The magnitude of an energy jump depends on the chemical species present while the number of jumps depends on the amount of that species. Computers are needed both for collecting and analyzing spectroscopic data to provide useful chemical information.
A group of analytical chemists in Germany (Huhnerfub and Kurth of Hamburg university, and Bohlen of ISAS – Institute for analytical sciences) employed total reflection x-ray fluorescence (TR-XRF) spectroscopy to analyze the pigments and colors used in ancient Egyptian boat models in an attempt to date the pigments and therefore the boat models to their appropriate times. TR-XRF spectroscopy is a nondestructive atomic technique used for analyzing the surface of materials: a few micrograms of pigment gently scratched off the boat surface is sufficient for the analysis.
In this type of spectroscopy, monochromatic x-rays (x-rays of only one wavelength) are directed onto the sample at a special angle such that total external reflection occurs. The reflected x-rays constructively interfere with the incident x-rays producing a standing x-ray wave at the surface of the sample. The standing x-ray wave (x-ray photons) ionizes core electrons from the sample’s atoms. Now that the atoms are excited they need to release the extra energy they got from the x-ray photons; to do so a higher energy level electron jumps to fill the vacancy, emitting (fluorescing) an x-ray photon in the process. The following URL provides an animation of how x-ray fluorescence occurs: http://www-project.slac.stanfo.....theory.htm . The energies of the fluorescent x-rays reveal the identities of the elements present in the sample. The amount of element present is reflected in the intensity of the fluorescent x-rays. However TR-XRF is insensitive to lighter elements, it can only detect elements of atomic number 20 or higher. In the lighter atoms the x-ray photon is not emitted, instead it is used to knock off another electron and that process is known as Auger electron spectroscopy (AES).
TR-XRF analysis of the samples taken from the boat models revealed useful information about the elemental composition of the pigments, even when they had faded over the centuries. 45 samples, collected from boats stored in the Egyptology museum in Berlin and in the British museum in London, were analyzed.
The white colors used in ancient Egypt contained mainly one of two types of pigments, Calcite (calcium carbonate) and Gypsum (calcium sulphate dihydrate). The white pigment Huntite (CaCO3.3 MgCO3) was later used for ceramics in the second Intermediate Period (1782–1570 BC) and in the New Kingdom (1570–1080 BC). Unfortunately the huntite pigment could not be identified by TR-XRF since the technique in insensitive to Mg (atomic number 12). The white pigment obtained from the boat models was found to contain calcium, titanium and many other elements; this indicates that the boat models have been restored since the white titanium pigment was first produced in 1938.
The red colors used in ancient Egypt contained mainly the durable and brilliant red mineral Haematite (iron oxide). Realgar (arsenic sulphide) – a fading red pigment – was introduced during the New Kingdom period (1570–1080 BC). Red lead (Pb3O4) and Vermillon (HgS) have been brought to Egypt by the Romans. Boat models containing iron in their red pigment had been dated to earlier than the New Kingdom, while those containing arsenic in their red pigments were dated to the New Kingdom. In one instance a boat model had 71.3 (Pb3O4) which indicates that the model was made no earlier then one century B.C.
TR-XRF spectroscopy is an atomic spectroscopic technique used to analyze for elements of atomic number 20 or higher. It proved to be a very useful tool for archeologists and historians as it provides them with information that can be used to assign artifacts – such as the ancient Egyptian boat models – to their most likely historic periods.
Questions to further explore this topic:
What is light?
What are wavelength and frequency?
What is the energy associated with light?
What is Beer-Lambert Law?
How is light created?
How does light interact with matter?
How does light affect matter?
What is a spectrum?
What is spectroscopy?
What is TR-XRF spectroscopy?
What is Auger Electron spectroscopy (AES)?
What is the difference between XRF and AES?
How are xrays used in chemistry?
What is fluorescence?
What is phosphorescence?
What is chemiluminescence?
What is a laser?
Is spectroscopy used only in chemistry?
What is total external reflection?
What is a standing wave? (Interactive)
What is ionization energy?
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