Example: Experiment to Distinguish Different Slag Types
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-products created by purifying ore into metal. Metal oxides comprise a large portion of slags, although sulfides and silicates are common also. Different ores and smelting processes produce different slags. Different smelting processes can be utilized to remove iron, copper, lead, and other metals from oxides and silicates, leaving behind waste.
Below are backscattered (BSE) images and elements maps for the four slag samples. You'll see how the slags are different in structure and composition, and it is clear which two came from the same location.
Slag #1
I seem to have misplaced my notes for Slag #1, and we've upgraded the microprobe workstation since I did these analyses, so I cannot look up the data I collected -- I will have to rely on my memory for this specimen. The slag is glassy and brittle, as evidenced by cracks that do not follow any cleavage planes. This vitreous slag has chemical zoning -- there are regions high in silicon and aluminum (dark areas in the grayscale backscattered-electron image below, and there are regions high in iron and other heavier metals (brighter areas in the image).
Below: Backscattered-electron image in grayscale and false color; the field of view is 670 x 670 microns.
Slag #2
Slag #2 had two major compositional zones, both of which appear in Area #1 below. One zone, shown in Area #2, occurred as roughly spherical blobs within a matrix, shown in Area #3, that had a different composition. The "blob" zones were high in copper, lead, and sulfur. The surrounding matrix was high in iron and arsenic. Other elements included nickel, silicon, molybdenum, and antimony. Numerous dendrites, crystal phases, and melts with different compositions are present, and describing all the complex structures would require a lot of text here. Based on the composition of the blobs, it seems this might be a product of copper or lead smelting.
Below: Area #1 - Field of View: 2 x 2 mm
Below: Area #2 - Field of View: 1 x 1 mm
Below: Area #3 - Field of View: 1 x 1 mm
Slag #3
Slag #3 is comprised of two main phases. Both of them are silicon- and iron-based, and the phases are metals, not oxides. The brighter phase is ferisilicite (FeSi). Its composition is iron (66.3%), silicon (32.1%), manganese (0.8%), nickel (0.4%), chromium (0.3%), and aluminum (0.2%). The darker phase is ferdisilicite (FeSi2). The composition is silicon (53.3%), iron (45.9%), manganese (0.4%), and aluminum (0.3%). This darker phase also had bands, high in aluminum, calcium, chromium, manganese, and nickel. The bright spots in the backscattered-electron (BSE) image correspond to molybdenum inclusions, and titanium inclusions were also present.
Below: BSE image and X-ray element maps for Slag #3; the field of view is 2 x 2 mm.
Slag #4
This slag had the same features as Slag #3 -- Slags #3 and #4 are the two samples from the same location.
Below: BSE image and X-ray element maps for Slag #4; the field of view is 2 x 2 mm.
11/24/07
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
