bronze age environment and economy in the troad - Universität ...

chapter 2: methods
in both its completeness of recovery and in the time needed for
processing. Wagner (1988) estimates that flotation is about 50
times more effective than wet-sieving in regard to the recovery
of plant remains in quantity. Only with clayey sediments is
flotation problematic. Many seeds will stay in the heavy
fraction because of clay sticking on them. Hand sorting may
not be enough and reflotation will eventually result in the loss
of material.
At Troy, the results from 1991 demonstrated that wet sieving
causes much more destruction to the remains than flotation
does. Also, flotation (either hand or mechanical flotation) is
less time consuming than other methods. The processing time
strongly depends on the type of sediment and is the longest for
clayey sediments. At Troy the amount of sediment that could
be floated in one hour was not more than 30 litres, including
the emptying and cleaning of the sieves and preparation of the
flots for drying, which took most of the time. The sediment
often contained fine roots which clogged the sieves.
The recovery rates of different flotation systems depend mainly
on the mesh size. The smallest mesh size should not be larger
than 0.5 mm. Schaaf (1981) found that a complete recovery is
given with a mesh size of 0.25 mm. The smallest objects in the
Troy material measured about 0.4 mm (Characeae). With the
use of a mesh size of 0.125 mm a complete recovery of the
remains was guaranteed. In the third year a 0.25 mm mesh
made the processing less time consuming. The use of a second,
more powerful flotation machine in Troy from 1994, which
resulted in problems of clogging of the sieve, made it necessary
to use a 0.5 mm mesh with this machine (water regulation at
the inlet could not prevent the repeated clogging of the sieves).
Higher water pressure also results in a higher destruction rate
of fragile remains. A comparison of the efficiency of the two
flotation machines with differing water pressure, and different
processing speed, was conducted with some of the big samples.
A second flotation machine was built in 1994, because the old
system was thought to be no longer powerful enough to deal
with large samples in a sensible amount of time. The main
change was an increase in water pressure. This was only
possible after 1993, when the site was connected to the local
water supply of the village. The stronger water pressure
resulted in a faster flotation, but the clogging of the sieves by
tiny roots and fine sediment particles could not be stopped with
a reduction of the water pressure. The 0.125 mm sieve was
replaced by a 0.5 mm sieve. Additionally the strong water flow
from the spout through the sieves, was assumed to have a
strong destructive influence on the remains in the sieves.
Some of the samples were split and processed with both
machines. Unfortunately the question of efficiency was
approached quite late in the sampling campaign of this
excavation, and not enough samples were processed like this to
get a final conclusion about the efficiency of the two flotation
machines. However, at least some tendencies can be outlined
here.
The assumed general loss in material was not verified, but
there is a tendency to lose very small seeds, such as Trifolium
sp. or Cyperus longus, because of the bigger mesh sizes used
with the second flotation machine. The only sample with high
numbers in crops, which were discussed above in relation with
the buoyancy of macroremains, showed more remains with the
use of the second, more powerful flotation system. It is likely
that the stronger water pressure increased the buoyancy of
chaff remains and cereal grains. Because of the differences in
efficiency, it was decided from 1996 to use the second flotation
system only when large samples were split into two halves, to
accelerate the processing.
The flots or ‛light fractions’ were sorted for identification in
the laboratories in Sheffield and Tübingen. Fully sorting large
samples or fine fractions (