150 ARCHAEOLOGICALCONSERVATION ANDFIRST-AID FORFINDS
- conditions and nature of the immediate burial
context; - the juxtaposition of items and structures through-
out a site; - the potential for galvanic corrosion across a site,
potentially causing the enhanced preservation of
one metal to the detriment of other(s).
Underwater environments can be considered to be
solutions of potentially reactive chemicals, with high lev-
els of dissolved and partially dissolved salts (often held in
solution within voids and interstices in objects and also
in the outer surface layers). Seawater can be considered
to be one of the most concentrated solutions and there-
fore one of the most potentially reactive. Depth, temper-
ature and levels of acidity or alkalinity, oxygen, light and
pollution, amongst other factors, also all have an effect.
Thus the chemical and physical behaviour of each item
within one area can be considered to be ‘individual’
according to the local burial environment, even on a very
small scale. No two objects are likely, therefore, to be iden-
tical or to behave the same the way.
The effects of biological organisms, both microscopic
and macroscopic, are likely also to have an impact on
preservation. Microscopic organisms include bacteria,
which can survive in many different types of environment.
Macro-organisms include: wood-boring molluscs (such
as shipworm (Teredo navalis) – figure 16.1), crustacea
(gribble), seaweeds, barnacles and fungi, all of which will
often attack wood for food. Wood-boring molluscs are par-
ticularly voracious and thus wooden components lying
exposed above the sea-bed are vulnerable to destruction
within a matter of years rather than centuries. Some
organisms, such as barnacles, will use materials and
structures as a substrate for attachment rather than as
a food source, which may result in surface markings or
some degree of change. Physical deposition, such as in the
initial formation of a wreck-site, is often dependent on
factors such as the nature of wrecking, geographical and
topographical aspects of the location and the type of
sea-bed. The latter may be subject to the effects of con-
tinually shifting sand and/or strong tidal flow, potentially
causing scouring (erosion) or silting (deposition of sedi-
ments and sea-bed materials). Despite all these factors, wet
and waterlogged deposits have often yielded artefacts
in remarkably good states of preservation, particularly those
made of organic materials, such as wood, leather and textiles.
MATERIALS DEGRADATION AND POST-
EXCAVATION DETERIORATION
One of the most important aspects of finds conserva-
tion associated with materials recovered from underwater
burial environments relates to the presence of soluble
salts dissolved in surrounding solutions. Damage is likely
on drying because soluble salts will re-dissolve in solu-
tion in conditions when the air humidity is high and
re-crystallize when it falls, causing potentially destructive
physical pressures. Thus a continuous cycle of damage
may occur: even slight continuous fluctuations in relative
humidity (known as RH: a measure of the amount of water
vapour in the air, as a percentage, at a given temperature)
may cause considerable damage, sometimes in objects made
of materials that may appear to be robust. In the worst
cases, such processes may lead to the complete destruc-
tion of an object. Thus, the drying of an object, particu-
larly those from marine waters, even for seconds, may result
in the weakening of structures and surfaces, even if
not visible to the human eye. A vulnerable find, passed
around a team during the general excitement following
recovery is highly susceptible to such damage and a
better way of ensuring that such objects are not put at risk
entails arrangements for everyone involved to view the
objects in waterlogged storage soon after recovery.
In addition, burial in underwater environments may
lead to:- leaching of components from physical matrices –
leading to the weakening of structures that may
still appear robust while on or in the sea-bed; - the development of layers incorporating sediment
and attached debris – often known as ‘concre-
tions’ (chemically bonded to original surfaces)
Figure 16.1 Teredo navalis(shipworm): a large shell, evid-
ence for historical shipworm infestation of the timber, is
removed from the upper stem timber of the Mary Roseas
part of the recording and cleaning process, prior to active
conservation. (Photo: Doug McElvogue)