Example: Historical Red Bricks from Two Manufacturers
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 determine whether the bricks were British or French and needed to know their ingredients, which we could establish with the electron microprobe. He planned to take that information to the Brick and Tile Museum in England and do research there.
Examination of the brick samples revealed that they were more than a little worse for the wear. The clay matrix had eroded away in some areas from around the temper, and cracks had developed due to the initial firing, saturation in ocean water for a century, and subsequent drying. The clay matrix was so eroded and altered by sea water that the measured composition was not likely representative of its initial composition. The temper was mostly quartz, usually several hundred microns in diameter, with a few other silicates, indicating that it was probably sand. Backscattered-electron images additionally revealed magnetite inclusions, likely naturally occurring within the clay. The researcher took this information -- the chemical composition of the clay (albeit altered by sea water), minerals in the temper and their sizes, and details about the magnetite inclusions -- and left for the Brick and Tile Museum in England to search their records for the compositions of nineteenth-century French and British bricks.
I was curious if I could more easily distinguish historic-period bricks from two different manufacturers if they were in better shape. I tracked down red-colored bricks from two different manufacturers, and I looked at samples from two bricks from each manufacturer. Backscattered electron (BSE) images, like those below, and EDS analyses showed these sets of bricks had similar ingredients but their sizes and abundances differed.
Below: False-color BSE images of two bricks from Company A -- the field of view is 1 x 1 mm.
Below: False-color BSE images of two bricks from Company B -- the field of view is 1 x 1 mm.
The temper in bricks are almost exclusively quartz. In the two bricks from Company A, the quartz grains were a few dozen microns in diameter and fall within a narrow size range. The bricks from Company B, on the other hand, have grains with a bimodal size distribution. There are abundant grains a few microns in diameter, and there are also rare grains hundreds of microns in diameter. The quartz is purple in the images above.
The red color of these bricks comes from iron oxides in the form of hematite (Fe2O3) -- these are bright yellow in the BSE images above. Like the quartz, the hematite grains are generally smaller and more abundant in the bricks from Company A, and they are larger and less abundant in the bricks from Company B.
The clay matrix is fuschia-colored in the images above. Clay minerals are hard to identify precisely with the electron microprobe. Most clay minerals have similar compositional ranges and differ just in crystal structure, requiring X-ray diffraction or a similar analytical technique to identify exactly. Such minerals are hydrous sheet aluminosilicates and usually have some amount of iron, magnesium, alkali metals like sodium and potassium, alkaline earths like calcium, and small amounts of other cation elements. When brick is fired, these minerals alter in the high temperatures, often producing the mullite (3Al2O3 • 2SiO2) and a few other metamorphic by-products. Lime, that is, crushed limestone or chalk composed mainly of calcium carbonate, is also sometimes added to aid the chemical reactions. Analyses with energy-dispersive spectrometry (EDS) and wavelength-dispersive spectrometry (WDS) revealed that the brick matrix compositions were consistent with sheet aluminosilicates and lime added as a flux.
It seems the bricks from different manufacturers can, at least in this case, be distinguished with electron microprobe analysis. These bricks had similar ingredients, but the sizes and distributions of these ingredients differed. Like the other ceramic materials discussed here, the microprobe can analyze each ingredient individually. It also seems that, much like other ceramics, bulk chemical analyses of brick would have limited usefulness.
12/3/07
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
