of PVC, fixed at one end. A series of small cylindrical wood samples are
threaded onto the pole by means of holes drilled through their longitudinal
axis. Spacers of PVC tubing are placed between samples so that their position
on the pole corresponds with the depth of the artefacts, or the layers, which are
to be studied. Presently pine and balsa samples have been deployed on sites in
Denmark. These woods have been chosen because of their relative susceptibil-
ity to waterlogging, tendency to degrade and, certainly with pine, the relative
ease of understanding the wood ultrastructure and decay patterns produced by
microorganisms. Investigations using other wood types are planned. However,
it is not necessarily the rate of decay (which is influenced by wood species) that
is important in this instance but the types of decay that are seen.
The pole and samples are placed within a metal tube, the diameter of which
is slightly larger than the wooden samples yet smaller than the diameter of the
base of the spike. The metal tube and pole are then pressed or hammered into
the soil to the required depth. At least 30 cm of the pole are left above ground
to aid in locating and retrieval. The metal tube is thereafter pulled up leaving
the wood samples exposed to the soil conditions. The benefit of using many
small samples instead of one continuous wooden pole is that the surface area
of the longitudinal axis is greatly increased. Retrieval is achieved by pulling
up the carbon fibre pole using a tightly fitting clamp. In waterlogged soils,
placing and retrieving wood in this way is relatively straightforward, but the
system has yet to be tested on underwater sediments or in drained soils. The
extent of deterioration can be determined through changes in density and
the types of decay present assessed by various microscopic techniques.
For metals, individual “coupons” are tied to a thin polyamide thread and the
end of the coupon placed into a groove cut into the end of a metal rod. The metal
rod and coupon are then placed into a hollow steel tube, which has a sharpened
spike at one end with a thin slit cut into it to allow the coupons to pass through.
The whole assembly is pressed down through the soil to the desired depth, and
the thin rod is then pushed down forcing the coupon through the slit. The coupon
is released by a slight twist of the metal rod. The steel tube is removed and the
metal coupon is left in the soil still connected to the surface by the thread, which
is then used to retrieve it after the desired length of time. These coupons are used
for the study of weight loss and corrosion products. Other probes have been
designed to allow the measurement of corrosion potentials, electrical resistivity
(ER) and electrochemical impedance spectroscopy (EIS).
4 WHAT DOES ENVIRONMENTAL MONITORING
ALONE TELL US?
Environmental monitoring alone does not tell anything about the actual state
of preservation of artefacts preserved in situ, and should only represent one
In-situ Preservation of Waterlogged Archaeological Sites 317