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Iron Meteorites and Heat

Vagn Buchwald: Handbook of Iron Meteorites, Vol. 1, pp. 40-43

On the Use of Meteoritic Iron by Our Ancestors (excerpts only):

It is quite interesting to note that only a few iron meteorites are
known from the Old World, while they are common in North America,
Mexico, Chile, South Africa and Australia. See, for example, the World
Map, published by Buchwald (1968a). The reason is not difficult to find.
The countries of the Old World which have for countless generations been
tilled and exploited, have already yielded the majority of the
meteorites accumulated through the ages. The iron meteorites have no
doubt been utilized by prehistoric man in Europe, either by chipping and
cold hammering, by forging or by remelting, and most of this activity
has gone unrecorded. An example of the fate of an iron meteorite is
revealed by the story of Hraschina in 1751, where one of the two
recorded fragments was entirely lost soon after the fall because of the
activity of blacksmiths. Elbogen was apparently exposed to the fire of a
medieval blast furnace, but miraculously survived, while Bitburg, Magura
and Netschaevo were almost entirely lost.
In Appendix 5, 94 (B.Pauli: there are actually 99 entries listed) iron
meteorites which have been exposed to artificial reheating and working
are listed. It is important to note that such reheating may have taken
place (Buchwald 1965a; 1968d). Particularly when estimating the effect
of cratering impact and of collisional shocks in general, it is
necessary to bear in mind that man's curiosity has often led him to
reheat the meteorites, without leaving any record of this experiment for
future generations. Since the structures which are produced by
artificial reheating are deceptively similar to some genuine structures
from shock reheating, one has to be extremely careful. Sometimes the
artificial reheating effectively destroyed the original cosmic
structures, causing great confusion. The classifying term "nickel-poor
ataxite" which has been used for generations (Cohen 1903d: I; Hey 1966:
xix) is, thus, superfluous since the class is very heterogeneous and
mainly based upon the erroneous interpretation of damaged hexahedrites
(e.g., Chesterville, Cincinnati, Locust Grove and San Francisco del
Mezquital) and coarse octahedrites (e.g., Campo del Cielo).
The 94 reheated meteorites constitute no less than 18% of all iron
meteorites. This is a surprisingly large percentage and indicates the
eagerness and curiosity with which our forefathers utilized the costly
material. However, it is not generally accepted that the percentage is
so high. Perry (1944: 71), an authority on iron meteorites, thus stated
after a thorough examination of many meteorites: "We may fairly conclude
that artificial metabolites, though perhaps sometimes produced, are
rare."
Often, only a fragment of the whole mass has been reheated. In other
cases, one or two masses of a shower have been maltreated, while the
remainder has escaped. It is unusual for museum labels to state that a
particular sample has been artificially reheated, so one normally has to
detect the damage oneself. In my experience, the damage is best revealed
on a near-surface section with corrosion products. The sample is
initially examined in the polished state, and the interfaces between the
corrosion products and the meteoritic minerals are closely studied. Even
brief reheatings above 550-600 degrees Celsius will alter the limonitic
products; usually tiny ½ - 1 mü (B.Pauli: the Greek letter mü) metallic
particles will precipitate in them, and the kamacite-limonite interfaces
will acquire a lace-like structure. At 800 degrees Celsius and above,
oxidation will create an intercrystalline attack along the austenite
grain boundaries. At 900 degrees Celsius and above, the sulfides will
react with oxygen and form low-melting quaternary eutectics, such as
Fe-Ni-S-O. At 1000 degrees Celsius and above, the phosphides will melt.
On the etched sections, additional information is to be gained. The
kamacite matrix will, if heated above 750 degrees Celsius, transform to
fine-grained austenite, which upon cooling ...

P.S.: My database METCAT yields different results concerning the total
of artificially reheated samples depending on the search criterion I
enter. I will work on this and check the keywords to get unambiguous
results. If I enter:

1) ‘artificially’ - the result is 30
2) ‘artificially heated’ - the result is only 7
3) ‘artificially reheated’ - the result is 12
4) ‘heated’ yields 80
5) ‘reheated’ gives 34
6) ‘heat-affected’ = 5
7) ‘heat affected’ = 0
8) ‘heat’ even yields 114

This shows how important it is to clearly and unambiguously define terms
(Remember our ‘fireball-bolide’ discussion!). If the interest in that
subject persists, I’ll inform you about my results.

Regards, Bernd