Ceramics at the Micron Scale - Investigating Chalcolithic Furnace Ceramics

Backscatter BSE image of a cross-section cut through a piece of chalcolithic age furnace ceramic. The left side of the image shows the original exterior of the ceramic vessel. The right side is the original interior, which would have held the ore during smelting. In the BSE image, bright areas are those with the highest concentration of dense elements (here copper). The red boxes show the locations of the compositional maps shown below. Samples provided courtesy of Prof. Aaron Shugar, Buffalo State College SUNY.

Ceramic objects count among some of the earliest evidence for manufacture in human history. With the oldest evidence for pottery dating back tens of thousands of years, with uses ranging from decorative items or figures of probable religious significance, to the more utilitarian items such as pots and other vessels used for storage or cooking. The raw materials used for ceramics vary widely and the technologies for production evolved through time, all of which are recorded in the resulting earthenware, stoneware or ceramic materials.

Understanding the types of raw materials, where they were sourced, and manufacturing practice used in ceramic production may be investigated in a number of ways – non-invasive techniques that allow characterization of the broad compositions (e.g., using handheld XRF), or compositions of individual components such as clay or temper (e.g., usinfg micro-XRF). However, to fully understand the levels of and changes to technologies used in ceramic production, microscopic investigations are commonly needed, using petrographic light microscopes or scanning electron microscopes. The latter allows imaging of details at the micrometer scale using backscatter or secondary electron images, and compositional analysis of even the finest components using energy dispersive or wavelength dispersive X-ray spectrometry (EDS or WDS, respectively).

Here we present examples from of a scanning electron microscopy investigation of Chalcolithic age furnace ceramics. In this period crucibles made of clay were used for smelting of raw copper ore prior to pouring into casts or other molds. The crucibles, which were commonly heated from above using blow pipes, kept the ore where heat was most concentrated and allow separation of impurities from the raw ore minerals. The high temperatures (>1000°C) experienced on the interior walls of the vessel lead to recrystallization of the clay-rich matrix and other components added to stabilize the pottery. Loss of volatiles (e.g., intergranular water and water bound up in the minerals themselves) may lead to development of bubbles as steam or CO2 was driven out. In addition, components of the molten ore may work its way into the ceramic wall, leaving a trace of the original purpose of the vessels.

Backscatter electron (BSE) imaging and compositional mapping was conducted using a small-footprint scanning electron microscope (Hitachi FlexSEM 1000) using Bruker's compact EDS detector, the QUANTAX Q80, with data collection using Bruker's ESPRIT software. The images show in great detail changes to the texture and mineralogy between the outer and inner walls of the vessel. These transitions allow an improved understanding of the technologies employed by early metallurgists of the Chalcolithic age.

EDS compositional maps overlain on BSE images showing the transition from the outer zone of the ceramic vessel through to an intermediate domain closer to the vessel interior. While the mineral types making up the temper (e.g., quartz, K-feldspar) do not change, the abundance of organic material (here mapped as C in red) rapidly decreases and the abundance of bubbles increases.
EDS compositional maps overlain on BSE images showing an area of the ceramic vessel close to the inner wall. Quartz and feldspar temper are preserved, along with minor residues of carbon. However, copper has been introduced to the matrix and is now preserved as grains of copper sulfide (pale yellow-blue in the map above). The copper sulfide has partially reacted to copper chloride (pink in the map).
An example of a furnace ceramic generated in an experiment to reproduce conditions used for iron smelting. The bright areas in the upper image correlate with high iron contents (red in the lower image), and show infiltration of molten metal into the vessel walls.

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