Have you ever wondered what causes “flashing” colors on bare clay in atmospheric (e.g. wood, salt or soda) firings? Research by a team of Japanese materials scientists, focused specifically on hidasuki color markings on traditional Bizen stoneware, reveals that orange/red/brown flashing can be caused by nano-scale crystals of hematite (Fe2O3, also known as “red” iron oxide) that grow along the edges of microscopic crystals of corundum (Al2O3, also known as alumina) while pieces cool at the end of a firing. If you have access to professional scientific journals through a university library, you may be able to download a review paper about their research from this link [Y. Kusano et al., “Science in the Art of the Master Bizen Potter,” Accounts of Chemical Research 43, 906 (2010)].
The featured image of this Micro Monday blog post, which was captured using a high-power optical microscope, shows a jumble of hexagonal corundum platelets, many of which have distinctive orange-red edges. The individual corundum platelets are approximately ten microns across, corresponding to something like one-tenth the width of a human hair. Following the conclusions of the paper by Kusano and co-workers it seems reasonable to surmise that the orange-red edges seen in this image are caused by the nucleation of tiny hematite crystals, which are too small to see individually (even with a high-power optical microscope) but present in sufficient number to give rise to strong orange-red pigmentation. It is possible to demonstrate more conclusively that the hexagonal platelets in this type of “micro-landscape” are corundum and that their edges are rimmed by hematite, using electron microscopy techniques–more on this in a future Micro Monday NCECA blog post…
K-12 STEAM Connections: Ceramic surfaces undergo rather extreme heating and cooling during the firing process, and this creates opportunities for chemical constituents of the clay, glaze (if present) and kiln atmosphere to melt, fuse and re-solidify into crystalline (atomically ordered) or glassy (disordered) substances. You may be aware of the crystals that form in so-called macrocrystalline and microcrystalline glazes, which are large enough to be seen by the unaided eye. Microscope images such as the one featured in this blog post illustrate that crystals too small to be resolved by eye can play a key role in the development of color on bare clay surfaces in atmospheric firings. Here we suggest some related questions about this image that you might discuss with your students; a high-resolution version of it is included at the end of this blog post: How can we identify the hexagonal platelets in this image? How large are the corundum and hematite crystals in this image compared to ants, bacteria, or viruses? Why do crystals grow in geometric shapes such as hexagons? Are many types of crystals hexagonal? Do we encounter corundum as a material in any day-to-day contexts? Hematite is commonly used as a pigment in ceramics and other industries but can vary significantly in color—why? How can crystals create color? What is the smallest feature you can see using optical microscopy? What is focus-stacking in photography and microscopy?
Technical Details: In order to obtain the featured image of this blog post, a small piece was cut from the wall of an unglazed stoneware vessel fired in a kazegama-style gas reduction kiln with blown wood ash (as developed by Steve Davis) and briefly etched using hydrofluoric acid (warning: hydrofluoric acid is extremely super HAZARDOUS–exposure to even a tiny amount can be fatal–so do not try this yourself unless you are experienced and have appropriate facilities!). Etching removes glassy material that encapsulates the crystals, making them directly accessible for study by electron microscopy techniques (more on this in future posts). A small area within a “flashy” region of the sample was imaged using a Nikon Eclipse optical microscope with 100x objective. Focus stacking was used to compensate for the surface profile.
Acknowledgments: Part of this work was performed at the Stanford Nano Shared Facilities (SNSF) of Stanford University. Thanks to Josh Green and Cindy Bracker for helping to launch the Micro Monday series! Josh and Eric Rempe provided valuable feedback on earlier drafts of this post.