Electron Microprobe Analysis in Archaeology
Electron microprobe analysis (EMPA), also known as electron probe microanalysis (EPMA), is an analytical technique that combines scanning electron microscopy (SEM) and compositional analysis using x-ray spectrometry. The ability to determine structure and chemistry of samples makes EMPA very versatile. This is a dominant analytical technique in geology, but it is not as commonly used in archaeology despite similar materials in studied both fields. Here I will post about topics in EMPA, artifacts I have analyzed, archaeological studies that use EMPA, etc. If there is a topic you'd like to see posted here, please let me know.
Ellery Frahm
Doctoral Candidate, Archaeology
Senior Research Fellow, Geology & Geophysics
University of Minnesota - Twin Cities campus
Example: EMPA and Coronado's Musket Balls - Part 2
4/25/08
Above: My data illustrate that even musket balls recovered from a single Coronado battlefield have significant compositional variations -- each color represents an individual musket ball sample, each little cube represents an individual analysis point on the sample, and 20 points were analyzed on each musket ball sample.
I should've known that, if any post here could cause controversy, it would be Can EMPA Identify Coronado's Musket Balls?. Why is that? Like I mentioned in my earlier post,
Example: Quantitative Chemical Analysis of Ancient Glass
2/6/08
Above: Various glass artifacts from Egypt. Source: Malter Galleries.
I rarely post quantitative elemental data here because, well, a table of numbers is less interesting than pretty X-ray maps. I don't want people to forget, however, that, with a little work, an electron microprobe can acquire very good quantitative data. Our electron microprobe can determine the abundance of each element from beryllium (Z = 4) to uranium (Z = 92) using the wavelength-dispersive spectrometers (WDS). Using
Example: The Process of Analyzing Two (Modern) Coins
1/16/08
Above: A Viking silver penny circa 920... Or is it a fake?
Ancient coins were one of the first artifacts to be analyzed using instrumental techniques that arose in the 1950s and 1960s. One of these early researchers was C. M. Kraay, who used neutron activation analysis to measure silver and gold in Greek coins in the hope that differences would be detected between different mints. In 1958, his article in the journal Archaeometry describes his findings. For instance, the data showed the “
Example: Can EMPA Identify Coronado's Musket Balls?
12/6/07
Above: A painting by American artist Frederic Remington (1861-1909), best known for his paintings of the American West, titled "Coronado Sets Out to the North." Source: Texas Council for the Humanities.
Francisco Vázquez de Coronado had balls. Musket balls, that is. And like all 16th-century Spanish conquistadors, Coronado used his musket balls to "explore" Central and North America, leaving behind many of these lead bullets along the expedition's trail and for us to find in the
Example: Egyptian Scarab and Its Hieroglyph Pigments
12/5/07
Okay, okay... The Egyptian Museum in Cairo did not actually lend me a priceless gold scarab artifact. No, this is a reproduction loosely based on the few known gold scarabs. I suspect, though, that its look derives more from The Mummy movies than actual artifacts. Several years ago, underwater archaeologists excavating a late Bronze Age shipwreck found a real golden scarab with hieroglyphs indicating that it was owned at one time by Egyptian Queen Nefertiti. Below is a photo of this
Example: Investigation of Cast-Iron Technology
12/4/07
Above: Micrograph of Ming Dynasty cast iron statue. Credit: Lehigh University.
Cast iron dates back to the first millennium BCE -- its first use is attributed to China in about 550 BCE. It was poured into moulds in order to manufacture weapons, figures, and other objects. There are controversial claims that ancient Greeks also had cast iron, but there is no evidence of cast iron in Europe until medieval times. Two sites in Sweden, dating between 1150 and 1350 BSE, are among the first in
Example: Historical Red Bricks from Two Manufacturers
12/3/07
Above: A collection of old red bricks. Credit: Salvo Architectural Salvage.
Four or five years ago, as I discuss here, I examined brick samples from the wreck of a famous Civil War blockade-runner, Denbigh. In 1865, while trying to reach the Confederate port at Galveston, the ship ran aground on a sandy shoal and was sunk by the fleet. Bricks were among the supplies carried by Denbigh, and a researcher brought me brick samples, still soaking in water, from the shipwreck. He wanted to
Example: Examining the Microstructure of Flint and Chert
12/2/07
Above: Chert, flint -- it's just all microcrystalline silica. Credit: John Vigay.
Microcrystalline silica -- chert, flint, jasper, novaculite, chalcedony -- was a desirable material for making stone tools. These rocks are all principally composed of quartz or its polymorphs (minerals with the same chemistry but different crystalline structure). They essentially exist on a continuum, so it is difficult to delineate geologically what is a "flint" versus what is a "jasper" unless one resorts
Example: Examining Antique Porcelain for Lead Glaze
12/1/07
Above: Ming period (1368-1644 CE) porcelain bowl. Credit: AskAsia.org.
My friend Donald loves old things, which is possibly why he studies rocks. He'd be thrilled if nothing in his apartment was less than one-hundred-years old (except his television and DVD player for watching old films). He sews his own clothes using patterns from a century ago and all natural fabrics and materials -- we actually analyzed a button in the electron microprobe because he suspected that it was actually
Example: Does the Tool Match the Identified Source?
11/29/07
Above: A variety of lithic materials. Credit: University of Hawaii.
Last year I was brought a stone tool from a Paleolithic archaeological site as well as a sample of what geologists had identified as the source of the raw material. Details, names, etc. will be left out to avoid embarrassment. Suffice it to state that the archaeologist who brought me these materials doubted the source identification -- the two rocks looked similar macroscopically, but he thought that the suspected raw
Example: Red Stone Tool -- Is It Pipestone or Shale?
11/26/07
Above: George Catlin's 1836 painting of pipestone quarries. Credit: Smithsonian.
In 2003, I was brought a thin section of a stone tool unearthed in Wisconsin. The stone flake was red in color, and a geologist hadn't been able to conclusively identify the material. This blade had been cut, and the thin slice had been mounted and polished. Examination with optical petrography was able to rule out a few materials (such as quartzite, jasper, and siltstone) but still left two fine-grained
Example: Experiment to Distinguish Different Slag Types
11/24/07
Above: A common archaeological material: slag. Credit: University of Calgary.
Several years ago, as a test, I was given four slag samples from three different locations. The question was: would these samples be different enough that I could identify which two came from the same location? Archaeometallurgy hadn't been a focus of mine, and this was my first chance to examine slag microscopically.
Slags are waste materials produced during smelting. The term is applied to a wide range of by-
Example: Distinguishing Novaculite and Metaquartzite
11/23/07
Above: Various colors and textures of novaculite. Credit: USDA Forest Service.
Metaquartzite and novaculite are very fine-grained and were used for lithic tools, and both are gray, tan, brown, and pink, making them sometimes hard to identify. Because both rocks have a very fine texture, differentiating between them macroscopically, or even with a hand lens, is not always possible. I used the electron microprobe to show the mineralogical differences in novaculite and metaquartzite with
Example: Sourcing Japanese Sanukite Stone Tools
11/22/07
Above: A backscattered-electron (BSE) image of Japanese sanukite. The brighter areas correspond to minerals with a higher average atomic number than the darker ones. The field of view is about 1.0 x 0.8 millimeters.
EMPA is a dominant technique in geology because it permits the analysis of individual minerals in situ. Rocks are mixtures of minerals, so a bulk technique can provide no information about the distribution of elements among the minerals. One possible approach to sourcing lithic
Update: New Research on Sourcing Nephrite Artifacts
11/21/07
Above: Examples of nephrite earrings sourced in a new study. Credit: PNAS.
In July, I discussed on this page some tests that I conducted on sourcing New Zealand nephrite -- you can read that discussion here. I explained the "jade" artifacts in New Zealand are nephrite, not jadeite. Nephrite is a calcium- and magnesium-rich mineral in the amphibole group, a variety of actinolite specifically. Nephrite commonly occurs in low-grade, regionally metamorphosed rock, and it is generally
Imaging Example #2: Neolithic/Predynastic Egyptian Point
10/17/07
Above: Image of a hollow-base Egyptian point; field of view is 50 x 35 mm.
This is a secondary entry about the topographic imaging capabilities of electron microprobe. Scanning electron microscopes (SEMs) and electron microprobes are both capable of secondary-electron imaging, which can generate high-magnification images of specimen surfaces. As shown above, the electron images also have greater depths of field than visible-light microscope images, meaning that more of the specimen
Imaging Example #1: Neolithic Point from Saudi Arabia
10/15/07
Above: Image of a Neolithic point from Saudi Arabia; field of view is 24 x 18 mm.
This entry focuses on (Ha, ha, pun intended!) the topographic imaging capabilities of electron microprobe. The scanning electron microscope (SEM) is a close sibling to the electron microprobe, and both instruments can produce highly magnified images of specimen surfaces. Such images commonly have greater depths of field than visible-light microscope images, meaning that more of the specimen is in-focus
Example: East Java Mosaic Glass Beads
8/24/07
Above: A backscattered-electron (BSE) image of the surface of a polychromatic East Java mosaic bead. The bright areas corresponds to a glass that contains lead as an opacifier. The field of view is 3 x 4 millimeters.
East Java mosaic beads are not only found in Java but also Malaysia, the Philippines, Sumatra, and Kilimantan. These beads are among the heirloom items on the island of Palau and were used as a form of currency. They were principally made in eastern and central Java,
Example: Historical Iron Slag from Minneapolis
8/24/07
Above: A color-combination element map of historical iron slag. This map was produced by collecting the characteristic X-rays emitted from three elements, assigning each element its own color, and overlaying the three element maps into one that shows the distributions and concentrations of three elements in one image. In this image, silicon is red, iron is green, and carbon is blue. The field of view is 400 x 500 microns.
The Elliot Park Neighborhood Archaeology Project, headed by
Example: Identifying an Inclusion within Dacite
7/25/07
Some time ago I was knapping a bifacial point out of dacite. During this particular attempt, a strike removed a flake in an unexpected way. One can see the ripples in the photo below where the flake removal suddenly changed direction. When I looked closely at the ripples, I noticed an inclusion there, about a millimeter in each dimension. It turns out that this inclusion altered the crack propagation within the flake and sent the flake awry. I placed the entire flake within the
Thoughts on Sourcing New Zealand Greenstone
7/25/07
The "jade" artifacts found in New Zealand are nephrite, not jadeite. Nephrite is a calcium- and magnesium-rich mineral in the amphibole group. Specifically, it is a variety of actinolite. The other mineral commonly called "jade" is jadeite, which is a variety of pyroxene. Nephrite has been used as an ornamental stone by cultures around the world: China, the Pacific and Atlantic Coasts of North America, Europe, southeast Asia, and New Zealand. Nephrite is more common than jadeite
Select Bibliography of EMPA in Archaeology
6/7/07
Below are archaeological articles, books, and talks involving electron microprobe analysis. I am always collecting new references, so future posts will include publications that either I find or visitors submit to me.
Abbott, D.R. and D.M. Schaller. 1985. Electron Microprobe and Petrographic Analyses of Prehistoric Hohokam Pottery to Determine Ceramic Exchange within the Salt River Valley, Arizona. In Materials Issues in Art and Archaeology, edited by P.B. Vandiver, J. Druzik and G.S.
Example: Buttons from Fur-Trade-Era Minnesota
6/4/07
These buttons from the fur-trade period in Minnesota were analyzed for an interesting project that unfortunately was cancelled due to disagreement among the collaborators. Historical archaeologists know local historical societies and clubs can sometimes be excellent resources and offer a wealth of useful information. Other times, however, they can be sources of inaccurate information and unfounded lore. Local "experts" can be individuals with experience and insight, people with
Example: A Lost (and Found) Brass Spoon
6/4/07
This spoon was brought to me by a pleasant woman whose husband graduated from the University of Minnesota about 40 years earlier. She found it on the shore of Lake Michigan, protruding from the sand, while out for a stroll and recognized that it was old. The handle was broken, probably near its mid-point, and the bowl was warped. There was some black corrosion across some of the surface. She wondered about the composition of the spoon and was curious to learn more about it, but
Example: Brick from the Denbigh Shipwreck
6/4/07
Some time ago I was brought samples of brick from the wreck of a famous blockade-runner of the American Civil War, Denbigh. The swift vessel first made the run from Havana to Mobile, Alabama and later to Galveston, Texas. In 1865, while trying to reach the Confederate port at Galveston, Denbigh ran aground on a sandy shoal. When seen by the Federal flagship Fort Jackson, the gunboats Cornubia and Princess Royal started firing, and sailors from Seminole and Kennebec were sent to
Example: Sherds from Mari, Syria and Lassy, France
5/30/07
Above: Ceramic sherds from Tell Haririr (Mari) in Syria (left) and a medieval site in Lassy, France (right)
I was brought the two sherds pictured above awhile ago. The sherd on the left is circa 1850-1650 BCE and came from Tell Hariri, the ancient city of Mari, on the banks of the Euphrates River in Syria, approximately 120 km southeast of Deir ez-Zor. Mari was at its height between about 2900 BCE and 1760 BCE, when the Babylonian king Hammurabi sacked the city, so the sherd came
Example: North American Copper Fragments
5/30/07
Two copper fragments from the American Midwest, circa 1650 CE, were sent to me for analyses. Because these fragments were from a post-contact archaeological site, they could have been (1) native copper or (2) European trade metal, either smelted copper or one of its alloys. Before European contact, Native Americans did not use metallurgical casting or smelting techniques. Instead, they used copper in its native metallic form. Such copper found as a metal in nature is known as
Example: Unknown "Ball" from a Viking Site
5/30/07
Archaeologists recovered this unknown sphere in association with a Viking site. It looked metallic and felt heavy, though not as heavy as iron or lead. Nothing quite like this "ball" had been reported at Viking sites before. It seemed plausible that this object was the product of Viking metallurgy, but there were actually three possibilities:
(1) The object is artificial in origin and a product, either deliberate or not, of Viking metallurgy.
(2) The object is
Analysis in Archaeology, Part 2: Provenance Studies
5/29/07
For artifact interpretation, it is an enormous advantage to know from where an artifact or its raw material has come. Information on the source of artifact material can be utilized to model trade networks, and questions about the procurement and use of the material by prehistoric societies can also be addressed. One hope is to answer, if instance, whether a specific material was traded locally or over large distances. The exchange mode by which the raw material or finished artifacts
Analysis in Archaeology, Part 1: Technology Studies
5/29/07
Above: Backscattered-electron images of waste slag produced by two different smelting techniques.
Herz and Garrison (1998) state that most archaeological materials either “occur naturally in nature and are used in the form in which they are found, with no or minimal processing needed, or... require manufacture before their final use” (194). The first category was discussed in the previous section on provenance studies, whereas the second involves issues of ancient technologies. It is, of
EMPA in Archaeology: Examples from the 1950s to 1990s
5/25/07
Smith (1958) states that his article “Technological Research on Ancient Glass” could have instead been called “Ancient Glass Enters the Atomic Age” because “our increased knowledge of atomic structure and behavior is about to make ancient glass more meaningful” (111). He expects that, with advancements in chemical analysis, glass will be easier to date. One of the techniques that he discusses is EMPA. Smith contends that the electron microprobe is a “useful new refinement of X-ray
History and Development of Electron Microprobe Analysis
5/25/07
German physicist Wilhelm Röntgen first observed X-rays in 1895, two years before the discovery of the electron. Röntgen found that cathode rays (later identified as a stream of electrons) produced a new form of radiation when they impacted a target. He temporarily called his discovery X-rays (“X” for unknown). The term caught on and is still widely used, though X-rays are known as röntgenstrahlen in Germany. He found that X-rays move in straight lines and cannot be focused with a
Principles and Physics of Electron Microprobe Analysis
5/25/07
Electron microprobe analysis (EMPA), or electron probe microanalysis, is an analytical technique that is used to establish the composition of small areas on specimens. EMPA is one of several particle-beam techniques. Particular, although not unique, to EMPA is bombardment of the specimen with a beam of accelerated electrons. The electron beam is focused on the surface of a specimen using a series of electromagnetic lenses, and these energetic electrons produce characteristic X-rays
Recommended Books on Electron Microprobe Analysis
5/23/07
Below are my recommendations for books about electron microprobe analysis:
Title: Electron Microprobe Analysis and Scanning Electron Microscopy in Geology
Author: S.J.B. Reed
Publisher: Cambridge University Press; 2nd edition, 2006
Description from the Publisher: Now fully updated to cover recent developments, this book covers the closely related techniques of electron microprobe analysis (EMPA) and scanning electron microscopy (SEM) specifically from a geological viewpoint. Topics discussed
