Advantages and disadvantages of bricks as a material for archaeomagnetic study

  • Maria Kostadinova-Avramova National Institute of Geophysics, Geodesy and Geography, Bulgarian Academy of Sciences, Akad. G. Bonchev Str., bl.3, 1113 Sofia, Bulgaria
Keywords: archaeomagnetism, bricks, magnetic anisotropy, Diocletianopolis, Veliko Tarnovo


Three collections of bricks were archaeomagnetically studied – Roman bricks (taken from the Diocletianopolis city wall, present Hissarya), Turkish bricks (coming from a mosque excavated on the territory of „40 Saint Martyrs” church, Veliko Tarnovo) and bricks with inscription (found also on the territory of „40 Saint Martyrs” church). The bricks differ significantly according to their shape and size. The main aims of the study were: 1) to determine the geomagnetic field elements (inclination and intensity); 2) to evaluate the magnetic anisotropy effect; 3) to verity the reliability of previously obtained archaeomagnetic determinations for Hissarya bricks (involved in the Bulgarian archaeomagnetic database); 4) to demonstrate the reliability and capabilities of archaeomagnetic dating method using bricks with well-known manufacturing date.
The rock-magnetic experiments performed indicate that the magnetic properties of all bricks are suitable for archaeomagnetic study. This is a consequence of the fact that bricks are usually baked at high temperatures (above 700oC) so they always carry a stable full TRM.
The inclinational data obtained show unacceptable scatter for the Turkish bricks and mean inclinational value was calculated only for the Roman bricks and the bricks with inscription. The magnetic anisotropy corrections applied do not improve the dispersion observed for none of the collection studied. Therefore, it can be concluded that these bricks were not strictly arranged along their long narrow sides during their production and obviously, this dominates the magnetic anisotropy influence. It is suggested that the Turkish bricks were situated in the furnace much more randomly compared to the other ones. There is a good internal consistency among the archaeointensity results obtained for the different collections and the magnetic anisotropy does not affect significantly the final mean results. The repeated archaeomagnetic study of the referent site Hissarya confirms its old inclinational determination but changes the old intensity result. Based on all the archaeointensity data received for the Roman bricks it can be suggested that some of the bricks in the construction of Hissarya fortress were reused (they were baked at least in two different periods).
Archaeomagnetic dating was performed only by one geomagnetic field element (intensity) for the studied Turkish bricks and by two geomagnetic field elements (inclination and intensity) – for the bricks with inscription. The dating intervals obtained on 95 percent probability level (2σ) are: 1817 – 1894 AD (Turkish bricks) and 1833 – 1894 AD (bricks with inscription) as they ended with the last year of the Bulgarian archaeomagnetic dataset. The archaeomagnetic dating interval obtained for the Turkish bricks indicate that they were produced rather in the end of the Ottoman ruling than in the end of XVII – beginning of XVIII century. This does not contradict the archaeological data. The maximum of probability density distribution for the bricks with inscription is around 1880 AD and agree quite well with the year of their production – 1888 AD. It should be considered that dating by all geomagnetic field elements (impossible for bricks materials) in the most cases produces narrower and more precise archaeomagnetic dating intervals compared to dating by one or two geomagnetic field elements.
The Turkish bricks and the bricks with inscription are included as new referent points with their archaeological dates in the Bulgarian archaeomagnetic database. With the new study of the Roman bricks an important correction for the referent site Hissarya was done. This demonstrates the importance and requirement of regular revision of the available archaeomagnetic dataset especially with regard to the old results.

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