Example: Historical Iron Slag from Minneapolis
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 Kent Bakken, involves the excavation of a historic site near downtown Minneapolis. For the last four years, the project has been a community event with archaeologists and local volunteers of all ages working together to learn about the past of this neighborhood. The excavations follow maps produced by the Sanborn Company, which made insurance maps for most of the country reaching back into the 19th century. The Sanborn maps are particularly useful because they show building-level detail and commonly show internal arrangements of buildings and industrial works as they relate to fire insurance.
The Sanborn maps indicate this area was a residential neighborhood in the mid-19th century. Consequently the excavators were surprised to find some slag, and they had no idea what type of slag it was. Eventually, they brought this chunk of slag to me and asked if it could investigate it using the microprobe.
I analyzed a sample in the microprobe, and it turned out to be a product of iron smelting. There were five major components to the slag: (1) silica grains; (2) small iron oxide particles; (3) globs of rhyolite melt with dendritic crystals; (4) a surrounding matrix of iron-rich melt; and (5) agglomerations of carbon material.
What does this suggest? My analyses indicated the slag from a puddling furnace. Puddling is a process used to make wrought iron from pig iron by removing excess carbon and silicon from the pig. The rhyolitic melt is basically the gangue from that process, and puddling causes significant loss of iron to the slag, so much of the glassy matrix is iron-rich. Why is the slag there? The Sanborn maps do not indicate ironworks anywhere near the site, and excavators felt that the slag distribution suggested a primary context, not secondary dumping. It remains a mystery...
Above: Another element map in which iron content is displayed in shades of green, carbon in blue, and silicon in red. The silica particles are bright red in this image -- silicon from the pig is removed during puddling and bound as an oxide. Other silicates are darker red. The green matrix is iron-rich glassy melt. The blue areas are carbon removed from the pig iron to produce wrought iron. The field of view is 1.5 x 2 millimeters.
Below: More element maps of iron, carbon, and silicon. The bright green iron-oxide spherule in the center of these images is about 200 microns in diameter. Carbon (blue) is concentrated in voids and cracks between the iron oxide crystals. The silica particles are bright red. Low-iron melt is the dark greenish-red matrix, and high-iron melt is dark green and also fills voids and cracks -- note a bubble of the left of the low-magnification image.
8/24/07
Added:
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
