"Studying these layered strata is known as magnetostratigraphy and can be used to verify radioactive dating methods. Geologists used magnetostratigraphy to cross-check the dates of the Nakali Formation, where Nakalipithecus nakayamai’s jaw was found."
Because the location of the pole is constantly shifting, the TRM of ancient pottery points in a slightly different direction than that of pottery made today.
Did anyone else have a strong "WTF?" reaction upon reading that sentence? Yes, the pole has moved. No, it's completely irrelevant given that not every piece of pottery even manufactured at the same time is going to be fired in the exact same orientation relative to the pole.
The researchers determined that any sample older than a millennium had to be heated to at least 234 degrees Fahrenheit before its VRM was erased. New pottery samples’ VRMs, meanwhile, could be wiped at lower temperatures.
This was not about the magnetization during firing, but about the much weaker magnetization acquired later, while staying in the same position without being moved.
That weak magnetization happens below the Curie temperature. Actually, any magnetization happens below the Curie temperature. Heating above the Curie temperature just erases any previous magnetization. A material is easier to magnetize immediately below the Curie temperature, but it can be magnetized at any lower temperature. Thus the ceramics that contains iron oxides acquires an initial strong magnetization while cooling down, and a weaker superposed magnetization if it is not moved for a long time. The 2 magnetizations can be distinguished by having different directions, as the pottery stayed buried in a different orientation than when it had been fired.
Only pottery that has not been moved for a very long time acquires a strong enough secondary magnetization. So the test distinguished pottery that has been used recently from pottery that stayed buried for centuries, by the strengths of their secondary magnetizations, not by their primary magnetizations.
A weak magnetization can be erased by heating even below the Curie temperature (which completely erases any kind of magnetization), so this is how they tested. The ferromagnetic materials are divided into hard and soft. The former are used for things like permanent magnets, while the latter are used for things like transformers and inductors.
The difference between hard and soft magnetic materials is that the latter loose quickly their magnetization even at the ambient temperature. Most ferromagnetic materials, which have not been specifically designed to be as soft as possible or as hard as possible, have intermediate softness, i.e. they start to loose quickly their magnetizations at temperatures higher than the normal ambient temperatures, but still much lower than their Curie temperatures. The tested ceramics also behave like this.
Yes, you are right, once the VRM is erased it is gone for good.
Though it is possible to test only a fragment of an object, but I do not know if the instruments that were used were sensitive enough to work with only a very small chip, which could be glued back, afterwards.
Many such ancient pottery objects are already broken, so they must be glued anyway, if a restoration is desired.
We should probably assume that the authors are aware of ceramics, Curie points, and the earth magnetic field. While the article isn't very clear on what effect causes the change in temperature needed to erase the VRM. I suspect is has something to do with how long the object has spent stationary with respect to the earth's field. At least that's my takeaway from having to periodically rotate magnetometers to prevent them from acquiring a bias.
What prevents forgers from generating such magnetic fields? That might be worth while if people start to blindly believing that "pottery frozen magnetic fields are not possible to counterfeit". Note that "not possible to counterfeit" is not a claim in the article.
It's one of several techniques they can apply, at least. I for one don't even know why there's a forgery market for ancient pottery, or that there's a market for it.
There are two fields. The second very weak field is acquired over centuries, I assume when the piece is not moved. The primary stronger field is created when it's first formed. I think what you're talking about would orientate the primary field, not the weak field.
https://impacts.to/downloads/lowres/impacts.pdf#page=23
Quoting my book:
"Studying these layered strata is known as magnetostratigraphy and can be used to verify radioactive dating methods. Geologists used magnetostratigraphy to cross-check the dates of the Nakali Formation, where Nakalipithecus nakayamai’s jaw was found."
Because the location of the pole is constantly shifting, the TRM of ancient pottery points in a slightly different direction than that of pottery made today.
Did anyone else have a strong "WTF?" reaction upon reading that sentence? Yes, the pole has moved. No, it's completely irrelevant given that not every piece of pottery even manufactured at the same time is going to be fired in the exact same orientation relative to the pole.
The researchers determined that any sample older than a millennium had to be heated to at least 234 degrees Fahrenheit before its VRM was erased. New pottery samples’ VRMs, meanwhile, could be wiped at lower temperatures.
That's a more useful difference which I would attribute to the ceramics having different Curie points in their compositions, and of course it's not something too difficult to change: https://en.wikipedia.org/wiki/Curie_temperature#Changing_a_m...
Incidentally, this effect of temperature on magnetism is how the https://en.wikipedia.org/wiki/Magneto-optical_drive media records data.
This was not about the magnetization during firing, but about the much weaker magnetization acquired later, while staying in the same position without being moved.
That weak magnetization happens below the Curie temperature. Actually, any magnetization happens below the Curie temperature. Heating above the Curie temperature just erases any previous magnetization. A material is easier to magnetize immediately below the Curie temperature, but it can be magnetized at any lower temperature. Thus the ceramics that contains iron oxides acquires an initial strong magnetization while cooling down, and a weaker superposed magnetization if it is not moved for a long time. The 2 magnetizations can be distinguished by having different directions, as the pottery stayed buried in a different orientation than when it had been fired.
Only pottery that has not been moved for a very long time acquires a strong enough secondary magnetization. So the test distinguished pottery that has been used recently from pottery that stayed buried for centuries, by the strengths of their secondary magnetizations, not by their primary magnetizations.
A weak magnetization can be erased by heating even below the Curie temperature (which completely erases any kind of magnetization), so this is how they tested. The ferromagnetic materials are divided into hard and soft. The former are used for things like permanent magnets, while the latter are used for things like transformers and inductors.
The difference between hard and soft magnetic materials is that the latter loose quickly their magnetization even at the ambient temperature. Most ferromagnetic materials, which have not been specifically designed to be as soft as possible or as hard as possible, have intermediate softness, i.e. they start to loose quickly their magnetizations at temperatures higher than the normal ambient temperatures, but still much lower than their Curie temperatures. The tested ceramics also behave like this.
Just curious, the heating to remove the VRM is that permanent so you can only check the authenticity once?
Yes, you are right, once the VRM is erased it is gone for good.
Though it is possible to test only a fragment of an object, but I do not know if the instruments that were used were sensitive enough to work with only a very small chip, which could be glued back, afterwards.
Many such ancient pottery objects are already broken, so they must be glued anyway, if a restoration is desired.
It strikes me as risky, if the item is in any way delicate or has absorbed water at all. But I'm no ceramics expert!
Even when buried wouldn't their orientation shift over time from geological forces?
We should probably assume that the authors are aware of ceramics, Curie points, and the earth magnetic field. While the article isn't very clear on what effect causes the change in temperature needed to erase the VRM. I suspect is has something to do with how long the object has spent stationary with respect to the earth's field. At least that's my takeaway from having to periodically rotate magnetometers to prevent them from acquiring a bias.
I'm reminded of thermoluminescence for dating objects. Radiation creates defects in the material over time. When heated the defects release light.
Imprecise but will fink on ceramics that were fired 30 years ago vs 3000 years ago. Difficulty you need to heat a fair sized sample.
"Reads article"
Impression, author didn't understand any of what he was told.
But yes similar, over time the object picks up a faint magnetic field superimposed on the one locked in when the ceramic was made.
What prevents forgers from generating such magnetic fields? That might be worth while if people start to blindly believing that "pottery frozen magnetic fields are not possible to counterfeit". Note that "not possible to counterfeit" is not a claim in the article.
From TFA:
> “So it’s hard for us to be ahead of the forgers since they can read our papers and figure out ways to trick our methods.”
I would love to see such counterfeiting. The gear they would need to get and the scientists.
Some wire and DC? and Claude et al
It's one of several techniques they can apply, at least. I for one don't even know why there's a forgery market for ancient pottery, or that there's a market for it.
The method of seems to involve hearing the artifacts which seems plausibly risky for an ancient artifact.
can this clue be circumvented by using magnets oriented in a certain way around the workshop while constructing the pottery?
There are two fields. The second very weak field is acquired over centuries, I assume when the piece is not moved. The primary stronger field is created when it's first formed. I think what you're talking about would orientate the primary field, not the weak field.
The second field can probably be created by changing the first field while the piece is still cooling down.