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Archaeological Work

I thought it might be interesting to shed some light on how field researchers conduct their work. I am not an expert on this, but I learnt a thing or two out of interest - and personal experience at a dig many years ago.

The time-honoured sequence is:

  1. Excavation:
    Research relies mostly on field excavations (digs) even for comparatively recent periods, when city streets were not paved and dirt accumulated over time, concealing the traces of previous inhabitants. Orderly garbage disposal is also a very recent development, before it people simply dumped their refuse anywhere convenient unless authorities enforced some rules: e.g. up to the 3rd century Rome prohibited tombs within the pomerium so most were located along the Consular ways leading into it.

    An interesting case is the dump allotted for used wine and oil amphorae, which had to be discarded after few years of use because the residues in their porous clay walls would eventually rot and cause an unpleasant smell/taste: they were broken up and buried at a remote site that gradually became a tall mound, that Romans today still call Monte de' cocci (hill of shards) - possibly the first historical example of separate garbage disposal .

    Rome's pomerium - Monte Testaccio on map (small rectangle at extreme SW) - Photo showing pottery shards currently emerging from Monte Testaccio
  2. Stratigraphy
    As digging goes deeper, it uncovers different strata or layers corresponding to different historical periods, usually identifiable by a different soil colour. An example is the excavation of Troy at Hisarlik that revealed 12 levels, Homer's story probably referring to Troy VIIa (ca. 1300–1190 BC) and not to Schliemann's Troy II (2600–2250 BC).

    Stratigraphic map of Troy at Hisarlik

  3. Dating
    This is the hardest part of the game. It helps if levels contain some artifacts that survived decay like pottery or metal objects, showing similarities to others of known age.

    Rarer is the case of organic remains (bones, wood, cloth, etc.) which allow a more reliable estimate through dating up to about 60,000 years ago, although its accuracy decreases over long periods and may be affected by various historical and environmental factors.

Carbon and Cosmic Rays

The atom of Carbon constitutes a substantial part of living animal and vegetal tissue and has a molecular weight of 12.

Cosmic rays cause Carbon-12 to decay gradually into its unstable and weakly radioactive isotope Carbon-14 (Radiocarbon) at a known rate, but it is replaced by Carbon-12 atoms at a constant rate and therefore it is nearly constant at any given time - about 1 in a trillion Carbon atoms are Carbon-14.

However, when life of the hosting organism ceases its decay continues but the above replacement can no longer occur. Therefore, measuring the radioactivity of a sample of dead organic tissue gives an indication of how long ago its death occurred.

Carbon-14 decays with its half-life of 5,700 years - i.e. its percentage is halved - while the amount of Carbon-12 remains constant in the sample.

By determining the ratio of carbon-12 to carbon-14 in the sample and comparing it to that in a currently alive organism, it is possible to determine the age of a formerly living thing fairly precisely using the following formula:

Age = [ logn (Nf/No) / (-0.693) ] x 5,700 years

where Nf/No is the percentage of Carbon-14 in the sample to the amount in living tissue.

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