When we unloaded our last reduction-cooled train kiln wood firing at Cobb Mountain Art & Ecology Project, we found several pieces at the top of one stack covered in silvery, plasticky-metallic flakes as seen in the short video clip above. We were of course curious to know what these were! Carbon from excessive reduction? Iron from the grate bars? My best guess is that when we closed up the kiln with a full firebox, large amounts of carbon started depositing on some pots whose surfaces were still a bit molten from the hot finish of the firing, and these flakes that we’ve seen are what formed when that carbon-silicon-aluminum-sodium-potassium melt cooled. Here are some studio photos of the same piece from the above video, progressively more close-up (24mm limit of my zoom lens, 70mm limit of my zoom lens, 85mm macro lens):
If you click on the embedded images here (anywhere in this post) you should be able to view a full-size 40 megapixel image, but please be patient as it will take a few minutes to download. Just by color you could imagine this being something like graphite, or possibly a metal but iron would presumably have oxidized at the end of the reduction down-fire… Below is an optical microscope image of a small flake of this stuff, taken at 200x:
Of course we can go to much higher resolution using a Scanning Electron Microscope. The following gallery shows a set of representative images of what this stuff looks like at 2500x, 5000x, and 20000x:
The stuff is full of holes! If you look closely you can also spot some little spherical beads here-and-there. Here are two more images, at 50000x and 120000x, zooming in on the sphere near the center of the middle image from the above set:
Note the scale bars on these micrographs — the sphere is only 2 or 3 microns in diameter! Using a tool called an Energy Dispersive Spectrometer (EDS) incorporated within the electron microscope we can estimate the elemental composition of the holey matrix and these scattered little spheres. This first screenshot from the EDS control software shows that measurements taken at a point in the holey substrate yield readings of mostly carbon, plus a bit of oxygen:
These readings are taken at the point marked with the crosshair and “4” in the image in the upper-left window; the estimated elemental composition is shown in the table in the lower-right window: something like 99% carbon. The next screenshot shows readings taken at the crosshair marked “1” right on one of the little spheroidal beads:
Here we have silicon, aluminum, potassium, sodium, calcium and magnesium — very clay-like — in addition to oxygen and a bit of carbon. So the little spheres are more like a ceramic material, possibly a zeolite (just guessing from past studies of different samples)?
The next screenshot shows some data taken with a Raman microscope:
This isn’t the greatest data and it’s hard to explain Raman spectroscopy in simple terms, but the pair of prominent peaks near the middle of the spectrum are consistent with what you’d expect if you were looking at a sample of carbon soot or pyrolytic carbon.
One last fun bit of speculation regards how the spheroidal beads might be related to all those holes in the carbon. I happened upon the following little micro-scene while exploring my flake sample with the scanning electron microscope (12000x view on the left, 65000x view on the right):
I would speculate that what we’re seeing here is a sphere that started forming but didn’t finish, and it is curiously sitting right in the middle of a hollow whose dimensions are very similar to those of the ubiquitous holes…
Part of this work was performed at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under award ECCS-2026822.