Example: Investigation of Cast-Iron Technology
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 Europe with cast iron. A favorite use was making cannon balls until the 1700s, when it started being used for pots, steam engines, and structures.
An iron smelter capable of 1130º C was the critical advancement which allowed metalworkers of the ancient Eastern Han Dynasty to manufacture cast iron. At such temperatures, iron melts when combine with about 4% carbon. This means that iron objects, like spearheads or statues, could be made by pouring molten iron into molds -- the Chinese metalworkers no longer had to forge each piece individually, saving time and effort.
Most people are familiar with grey cast iron, from which our cast iron cookware is made. Silicon is critical for making grey cast iron, which is roughly 2% silicon. Silicon is less important for making white cast iron, like the Ming Dynasty cast iron above and the sample analyzed below. White cast iron still contains sufficient silicon, though less than one percent, that it should be considered a Fe-C-Si alloy. Cast iron solidifies as a eutectic, which means the constituents crystallize simultaneously and form a mixture with phases of different composition.
In white cast iron, one of the phases is cementite, Fe3C, which can be seen in the carbon element map below as the bright green areas. In this particular sample, chromium is also concentrated in this phase. The other major phase is austenite, that is, a solid solution of iron and some other alloying element, usually silicon -- this phase is the greenish areas in the silicon map. In this sample, minor amounts of copper and nickel are also concentrated in this phase. A third phase, manganese sulfide (MnS), occurs as small inclusions. These three phases can be individually analyzed using the wavelength-dispersive spectrometers to establish their exact compositions.
The structure and compositions of the constituent phases depend on the ingredients, temperatures involved, and so forth. Therefore, backscattered-electron imaging and X-ray analysis can reveal the techniques people used to make cast-iron objects: Ming Dynasty cast iron is different than medieval European cast iron.
Below: BSE images and element maps of white cast iron; the field of view is 500 x 500 microns.
12/4/07
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
Research Fellow, Geology & Geophysics
University of Minnesota - Twin Cities
