Conservation of Ancient Timbers from the Sea 291
Advice to Conservators. Gamma irradiation, is suitable for the passive
treatment of both slightly- and heavily-degraded waterlogged wood. An absorbed
dose of 100 KGy is acceptable for any one timber, and this dose level should
not be exceeded in a single or repeated treatment, since higher irradiation
dose may adversely affect the physical and chemical nature of the wood. One
drawback to this process is that gamma-irradiated timbers are unsuitable for
radiocarbon dating and any analytical procedure is invalidated by an exposure
to irradiation.
Gamma irradiation treatment involves wrapping the waterlogged wooden
artefacts in a metallised polyester barrier film and transporting them to a
commercial irradiation facility. For Mary Rosetimbers, inactivation of wood
biodeteriogens was achieved by a single dose of 20 KGy. Following irradi-
ation treatment, it is important to maintain the integrity of the wrapping mate-
rial surrounding the sterile wooden artefact. Any breach will allow contamination
of the sterile environment. One of the advantages of gamma irradiation is that
there is no residual radioactivity in the treated artefact, which is completely safe
to handle. This presents a clear advantage over biocide treated objects that often
present toxicity problems. Gamma irradiation is also a low-cost alternative to
cold storage, with far greater efficacy in preventing any on-going decay. Finally,
gamma irradiation may be compared favourably to the criteria for an ideal
passive conservation treatment method (see Table 3).
3.5 Storage of Large Wooden Artefacts
Wooden artefacts too large to enclose in a wrapping material have to be pas-
sively held without suffering serious drying effects. Tanking large items of arte-
facts, suchas boats and individual ships’ timbers, is a practical way of storing
large waterlogged archaeological wood. The size of individual artefacts will
dictate the size of water storage tanks used. Plastic tanks with lids to exclude
Table 3An evaluation of gamma irradiation as an ideal passive holding method
Criteria for ideal storage Evaluation of gamma irradiation
Prevent or minimise further decay Complete sterility of artefacts
Compatibility with future treatment At recommended dose levels (below 100 KGy), no
adverse effects
Ease of maintenance No specialised environment needed but wrapping
material must remain intact
Cost effectiveness Wrapping and irradiation cost, low storage and
maintenance costs
Health and safety Do not result in any residual activity in the
exposed wood