SEM
I was recently offered the incredible opportunity to prepare my own samples for analysis in a Scanning Electron Microscope.. Fellow nerds and geeks out there perhaps wont need me to explain just how significant this is, or how excited I was to learn that the SEM is also equipped with EDX analysis, (Energy Dispersive X-ray spectroscopy) which provides data on the proportions of atomic elements present in a given specimen.
It seemed obvious to me that I would attempt to analyse some of the ‘wild’ clays which I have gathered from Norfolk and the South East of England over recent years. The chance to know anything additional, especially anything empirical, about these clays is unlikely in the extreme! When I say “additional” I mean beyond what can be seen and felt from processing the clays in different ways, firing them to different temperatures, and exploring their working (mechanical/physical) properties.
I chose four foraged clays and a ‘control’ a clay, from France, called St Amand described as follows:
“A natural stoneware from Saint Amand en Puisaye, a famous potters clay, from the Burgundy region. A fine grained, plastic, naturally occurring clay with excellent throwing and modelling characteristics. A low iron, white/buff firing stoneware in oxidation and slightly toasted in reduction firing 1260°C.”
It was particularly important to me that the control clay be a “naturally occurring” clay rather than one blended from heavily processed ingredients imported from across the globe. Additionally the St Amand clay is “low” in iron.
The plan was to test the chemical make-up and the Particle Packing of these five clays at raw, bisque (1000°C) and stoneware (1250°C) temperatures.
In order to study the Particle Packing of a clay I have assumed that it might be wise to cut and grind a cross-section through the fired clay. Using a series of ever finer diamond pads I hope to grind clean through any and all particles in the fired ‘matrix’ of the clay regardless of their size, thus revealing the ‘terrazzo’ effect or mosaic appearance. I am expecting to see voids at raw and low temperature, and the glassy matrix beginning to ‘glue’ the particles together in closer proximity at stoneware. It may be possible to see the results of off-gassing in the form of small bubbles in the glass matrix too at stoneware.
In the case of the Norfolk cliff clay, which melts entirely at 1250°C, this glassy matrix will appear extreme, no doubt.
The Energy Dispersive X-Ray analyser (EDX) will hopefully show the high proportion of Calcium (Carbonate) in the Norfolk ‘Ostend’ Cliff clay, as this would help support my hypothesis that this is causing the heavy fluxing of the clay at around 1150°C. With luck, the proportions of Iron, Sodium, Potassium, and Magnesium with also cast some light on how the matrix fluxes at stoneware, and produces a more or less close-knit structure.
An old sandwich box fitted with foam inserts, meant my samples could be stored and protected from contamination. Each of the four columns shows a different clay. (top row = raw clay, middle fired to 1000c, bottom to 1250c.)
Each ‘stub’ has one fragment whose surface is simply rolled, another shows a cross-section through the clay diamond ground to 200grit and showing the positions and sizes of the grains or particles. And a third fragment showing a snapped cross-section (with particles intact). - It is tiny, fiddly work!
Having not allowed myself to hope for much, it was hugely exciting to take these first tentative images (initially of the raw clays) and see the tell-tale hexagonal platelet shapes of individual CLAY particles (!!!)
By using the BSE (back-scattered electron) sensor, especially on the cut and ground samples, I was able to explore the size and distribution of particles within the clay matrix. The three images below show that, by using different electron beam intensities, different sensors, and samples prepared in different ways, even the same clay fired to the same temperature can reveal quite different qualities.
These five images show how varied the clay structure can be when scanning the fired, broken surface. Bear in mind that these are all scanned at the same magnification, and detector settings, and that each clay is a secondary, transported, sedimentary clay. The grain size and structures are quite individual even at 1000c.
Much of my lifetime I have marvelled at what science has revealed to us about the underlying patterns, mathematics, and forces which seem to almost universally govern the structures and systems of the known universe. It feels incredible to have had this grainy glimpse into the microworld of the clays I use daily. So, whilst the EDX data may prove the most ‘useful’ part of this fascinating opportunity, the SEM images will surely stay with me, activating my imagination and curiosity, for years to come…
