The magnetic fingerprint

Back in 1963, Frederick Vine and Drummond Matthews came up with a groundbreaking idea (pun very much intended); that the Earth's crust is divided into a bunch of chunks called plates that are all moving relative to each other, and that this is what causes virtually all earthquakes and volcanoes.

The main evidence for this dramatic paradigm shift in our understanding of how geology works came from the discovery on the ocean floor of regions of hardened lava that have opposite magnetic signatures.  When molten rock freezes, tiny magnetic particles that were free to move when they were in a liquid become locked into place, acting like billions of little compass needles recording the direction of the Earth's magnetic field at the time.  As you undoubtedly know, the positions of the magnetic poles flip, on average every three hundred thousand years (although the actual intervals vary greatly, for reasons that are still unknown).  So the rocks Vine and Matthews studied, on either side of the Mid-Atlantic Ridge, which showed symmetrically-arranged parallel stripes of magnetic signatures, showed that new oceanic crust was being formed all the time at the ridge, driving the plates apart and gradually widening the Atlantic Ocean.

Well, it turns out that lava isn't the only thing that can record what the magnetic field is doing.  According to a study last week in Proceedings of the National Academy of Sciences, so can pottery.

When clay is fired, its chemical structure changes, fusing into ceramic.  Different clays fire to different temperatures; in our kiln we fire our work to 1220 C (2232 F), which works for the clays classified as stonewares and mid-fire porcelains.  If we were to fire a high-fire porcelain to that temperature, it would still be brittle and not water-tight; fire an earthenware clay to that temperature, and it (literally) would melt.  (The difference is in the formulation of the clay, which is a complex subject about which I am still learning.)

But when you fire any clay to the correct temperature for that type, it effectively turns to stone.  The particles fuse together, giving it strength and resistance to breaking.  And this has the effect of locking into place any magnetic particles the clay may contain -- same as with Vine and Matthews's solidified lava on the ocean floor.

White stoneware vase with a cobalt splatter glaze

The reason this topic comes up is the discovery by a research team out of University College London of the fact that some earthenware bricks dating to the reign of Nebuchadnezzar II of Babylon (605-562 B.C.E.) show a magnetic particle pattern indicating a strange and sudden surge in the strength of the magnetic field -- something that has been nicknamed the Levantine Iron Age Geomagnetic Anomaly.

"It is really exciting that ancient artifacts from Mesopotamia help to explain and record key events in Earth history such as fluctuations in the magnetic field," said study co-author Mark Altaweel.  "It shows why preserving Mesopotamia’s ancient heritage is important for science and humanity more broadly."

Noting this odd magnetic fingerprint -- the cause of which is as yet unexplained -- has another added benefit; once they've identified it in items of known age (as with the bricks, that had an identifying stamp), it can be used to date ceramic items that have no such marks.

It makes me wonder what kind of record I'm creating in my own pottery.  When we have pieces with too many flaws to be worth keeping, we shatter them against the cement wall along the back of our house (there's now a pile of pottery shards at the base of the wall).  We think of it as our ongoing effort to confuse future archaeologists.  But supposing they do piece together some of our failed attempts at bowls and mugs and various sculptures, maybe they'll find out something more than our dubious skill at making pottery -- but what the Earth itself was doing in 2023.

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