A different type of clay memory
How fired clay retains information about the Earth's magnetic field
A few days ago, I found myself explaining the concept of clay memory to my students. The concept of clay memory that I was talking about has to do with clay particles aligning in a specific way after their direction was altered. If you are making a flat plate and lift the rim, even after you flatten it back, there is a good chance that as it dries, the rim will attempt to go back to that original position.
When I talk about clay memory to my students, I talk about the need to be mindful about how much we touch the clay, the need to consider our gestures, because they may leave an unintentional mark. When I talk about clay memory to my students, I am talking about the mark our hands today will leave on the fired clay in the future. I talk about intention. The clay may remember something we did not intend to immortalize.
Clay also carries the memories of the past, in a literal sense. Clay has a different type of memory, and it’s a planetary one. It’s more than simple bends and pleats. It is something embedded and immortalized in its depths. It turns out, clays particles can tell us about our planet’s history.
In 1863, Silvestro Gherardi, an Italian scientist who was studying electromagnetism, wrote a letter to Giuseppe Fiorelli, a friend who also happened to be the director of the excavation in Pompeii:
I am certain that this material has always kept alive its own magnetism, this ethereal movement, or this eminently subtle, imponderable, inconceivable substance [. . . ] of which we can well say (without heresy!) that it forms, that it is, its soul, its spirit.1
Gherardi had been studying ancient ceramic roof tiles from Pompeii, after spending time measuring the magnetic properties of the bricks that made up the building of the Department of Physics of the University of Turin. What Gherardi was studying, following in the footsteps of his predecessor Macedonio Melloni, was the chemical transformation that is left in baked clay (due to volcanic activity or heat in the kiln), in the form of magnetization.2
The Earth’s magnetic field
Our planet’s magnetic field originates in the outer core, the third layer from the surface. The molten iron and nickel that compose the outer core swirl and churn, generating electrical currents that flow through the magnetic north and south poles, following invisible lines around our planet. The Earth’s magnetic field shields us from solar winds and cosmic radiation and has been used for centuries for navigational purposes.
The intensity and direction of the magnetic field are not static over time. This means, among other things, that the magnetic poles are moving, with the north magnetic pole moving northwest towards Siberia by about 50km per year, and the south magnetic pole moving northwest by around 5km a year.3
The movement of the magnetic poles periodically results in the reversal of the Earth’s magnetic field. This reversal has happened 183 times in the last 83 million years, with the last one taking place around 780,000 years ago.4
Evidence for this reversal - and in general for understanding the intensity and direction of the magnetic field - come via the study of archaeomagnetism, a discipline pioneered by Italian scientist Giuseppe Folgheraiter, who was building upon the knowledge and experiments carried out by Gherardi.
Archaeomagnetism
Archaeomagnetism is an interdisciplinary science that studies the intensity and direction of the Earth’s magnetic field as recorded in heated ferromagnetic materials. It is a science with deep ties to archeology, as artifacts with pre-existing archeological dating can be used to pinpoint the status of the magnetic field at that particular time. Conversely, if the magnetic age of the object is known, the information can be used to date the artifact.
When Gherardi wrote “I am certain that this material has always kept alive its own magnetism”, he was referring to the ways in which heated clay can immortalize information about the Earth’s magnetic field. Unlike the clay memory we learn about in pottery class, this type of memory is not dependent on our intention. It follows the laws of physics.
Most clays contain a certain amount of ferromagnetic materials (magnetite or haematite - two types of iron ores), which are capable of being magnetized by the Earth’s magnetic field, essentially aligning to it. The magnetization occurs upon cooling: when the materials are heated above 500-600C (the so- called Curie temperature), the magnetic information is lost. As the pieces cool down, the magnetic materials align with the planet’s magnetic field and this information is “stored” unless a new process of heating and cooling occurs.5
Two types of measurements can be taken from fired artifacts: the intensity of the geomagnetic field and the direction of the field, measured in terms of inclination and declination. It was Folgheraiter who first understood that the information stored in various pottery artifacts could be used to provide with anchor points for the creation of an archaeomagnetic curve: a way to track the development of the intensity and direction of the Earth’s magnetic field over time.6 The current version of this curve can be found in the GEOMAGIA50 database, an international collaboration to provide access to published archeomagnetic studies and data for the past 50,000 years7.
The archaeomagnetic study of pottery artifacts presents some challenges when it comes to the measurement of the direction of the magnetic field. In order for the direction to be correctly measured, the pieces need to be resting in the exact same position they were in when they were cooling, which is a rare occurrence. This is why most of the archaeomagnetic studies of ancient pottery sherds are focused on only measuring the intensity of the magnetic field. In 2021, a group of scientists published an article in which they reported on the testing and analysis of numerous pre-pottery and pottery artifacts from the Neolithic period (7752 to 5069 BC) in Jordan.8 They concluded that around the 7600 BC, the magnetic field in that area was at its near weakest, gained strength until 7000 and weakening again until 5200 BC.9
The correct dating of the artifacts is an important element in archaeomagnetic studies. If pieces can be precisely dated, the magnetic information is invaluable. That’s why a lot of research has been conducted on bricks. Fired bricks, especially when they feature inscriptions that date them back to a specific time and place, can be even more useful in the reconstruction of the intensity of the Earth’s magnetic field. A 2023 study focused on ancient Mesopotamian bricks from the 3rd to 1st millennia, revealed that the magnetic field was very strong around this time in Mesopotamia, in line with previous studies conducted in the Levant and Syria10.
When it comes to the measurement of the direction of the Earth’s magnetic field over time, static structures are more useful. Folgheraiter was the first to discover the value of kilns in archaeomagnetic studies.11 Kilns are immovable structures for the most part and the information stored in their walls can be invaluable for both intensity and direction of the magnetic field.12
These studies based on artifacts and kilns help us better understand the way the Earth’s magnetic field works, the ebbs and flows of its intensity, the cycle of reversal and their effects on human life. In case you were wondering, currently the magnetic field seems to be the strongest it has been in the last 100,000 years, even if it seems to be trending down.13
Memory and Earth
I think back to when Carl Sagan said that “we are a way for the cosmos to know itself”. Our bodies made of star-stuff, our minds, incapable of adequately comprehending the infinite universe. The product of our industriousness, our attempt to claim the title of masters of our world, are nonetheless intertwined with this planet. We are from here and what we make is from the Earth. The process through which we make clay objects permanent and strong - the passage through fire - leaves a signature of a moment in time in our world. Not merely a marking of a time and place, but information about the mysterious process of geodynamo, which takes place miles under our feet, ignited by molten metal, the same metal that is found in our pots. It seems as if our planet is leaving us pieces of a puzzle for us to put together. So, yes, we are a way for the cosmos to know itself: through clay we can get closer and closer to understanding the planet we live on.
Where you can find me or my work this month
Read my new diary entries for my 274 moons / 274 vases projects. I am making one vase for each one of Saturn’s moons and keeping a diary for each vase as I make it. Catch up on the diary entries here.
The dawn of archeomagnetic dating, Yves Gallet, in “Comptes Rendus Géoscience — Sciences de la Planète”, 2021, 353, n. 1, p.287
See Giuseppe Folgheraiter: the Italian Pioneer of Archeomagnetism, Claudia Principe and Jonas Malfatti, in “Earth Sciences History” (2020) 39 (2): 305–335, pp.312-314
North by northwest: The planet’s wandering magnetic poles help reveal history of Earth and humans, Sid Perkins, in “Science News”, December 22&29 2007, Vol 1/2, p.392
NASA, Earth’s Magnetosphere: Protecting Our Planet from Harmful Space Energy, https://science.nasa.gov/science-research/earth-science/earths-magnetosphere-protecting-our-planet-from-harmful-space-energy/ , accessed 20th April 2025
See Archaeomagnetism: the dating of archeological materials by their magnetic properties, D.H. Tarling, in “World Archeology” Volume 7 No.2, pp.185-186; see also Sid Perkins cited at (3)
Claudia Principe and Jonas Malfatti, p.321
See https://geomagia.gfz-potsdam.de, accessed 20th April 2025
The strength of the Earth’s magnetic field from Pre-Pottery to Pottery Neolithic, Jordan, Anita Di Chiara, Lisa Tauxe, Thomas E. Levy, Mohammad Najjar, Fabio Florindo and Erez Ben-Yosef, “Proceedings of the National Academy of Sciences of the United States of America”, August 24, 2021, Vol. 118, No. 34 (August 24, 2021), pp. 1-9
Ibid, p.7
Exploring geomagnetic variations in ancient Mesopotamia: Archaeomagnetic study of inscribed bricks from the 3rd–1st millennia BCE, Matthew D. Howlanda, Lisa Tauxec, Shai Gordind, Mark Altaweelf, Brendan Cychg, and Erez Ben-Yosef, “PNAS”, 2023 Vol. 120 No. 52, p.5
See Claudia Principe and Jonas Malfatti, p.323
See for instance, Ancient ceramic kilns: recorders of the earth’s magnetic field and firing technologies in Greek archaeological sites, Despina Kondopoulou and Christina Rathossi, “Cutting-edge Technologies in Ancient Greece”, Oxbow Books. (2020)
Flip Flop: Why Variations in Earth’s Magnetic Field Aren’t Causing Today’s Climate Change, NASA, https://science.nasa.gov/science-research/earth-science/flip-flop-why-variations-in-earths-magnetic-field-arent-causing-todays-climate-change/, accessed April 21st 2025
amazing. I loved this so much. There is so much more memory in our material world than people realize. It's alive and very much part of us.
I like to think our own cells hold a dim and distant memory of being Clay too