borax is a well-known detergent, as well as a biocide, and scientists have
found that it increases the solubility of elemental sulfur in water. It may be
that the elemental sulfur, once solubilised in water, becomes more readily oxi-
disable.Unfortunately, in the case of the Vasa, this process was not carried to
a suitable endpoint, principally because the problem was not then recognised;
it was assumed that the acidity was caused by breakdown of the PEG (formation
of formic acid).
5.3 Polyethylene Glycol Method
By far the most commonly-used method for conserving small and large water-
logged wooden objects is the application of PEG. PEGs are polymers of ethyl-
ene oxide with the generalised formula of HOCH 2 (CH 2 OCH 2 )nCH 2 OH. All
PEGs are designated with a number that represents their average molecular
weight and these ranges from 200 to 20,000. Low molecular weight PEGs
(200–600) are clear viscous liquids at room temperature, and are readily mis-
cible with water. Higher molecular weight PEGs (1000) are white waxy
solids at room temperature, with decreasing solubility in water and solvents as
molecular weights increase.
PEGs are considered inert and safe. They are used in a great variety of
applications because of their chemical structure, low toxicity, solubility in water
and their lubricating properties. Since PEGs are considered to be so versatile,
conservators have for the past 4 decades recommended their use in the treat-
ment of waterlogged archaeological wood. The advantages of using PEGs out-
weigh their disadvantages.
It is well known that PEG-treatment methods require long treatment times
and increasing hygroscopicity with decreasing molecular weight can be a prob-
lem when treated objects are stored under high relative humidities (60%). This
has led to the frequent use of high molecular weight PEGs (1500 and 4000).
Unfortunately, these high molecular weight PEGs require very long treat-
ment times before sufficient levels can be detected inside the wood structure.
Although there are variations in the use of the PEG method between con-
servation laboratories, conservators usually follow the same basic procedure.
A wooden object is placed into a tank or container containing an aqueous
solution of low concentration PEG at an elevated temperature of 30–60°C.
The concentration is gradually increased by the evaporation of water or by the
addition of more PEG, until 70–100% PEG concentrations are reached for
highly degraded wood or around 15–25% concentrations for sound archaeo-
logical wood (followed by freeze drying). Excess PEG is removed and the PEG-
treated timbers are vacuum freeze dried or air dried. In some laboratories, the
process has been accelerated by dissolving PEG in solvents or methanol, again
with some level of success for relatively small objects. When considering the
Conservation of Ancient Timbers from the Sea 303