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guesswork, but more recently conservators have experimented by nuclear
magnetic imaging to help solve these problems.
Lumen-filling chemicals used by conservators include high molecular weight,
solvent-based polymers such as styrene, methyl methacrylate and butyl
methacrylate. These are usually polymerised by heat or gamma irradiation.
The problem with these chemicals is that they produce a composite object in
which a significant part is the polymer. This often results in a considerable
increase in object density.
A more suitable lumen-filling treatment is high molecular weight polyethyl-
ene glycols (PEG) (1500–4000). This polymer is often used to treat highly
degraded waterlogged archaeological wood. A major drawback with the use
of high grade PEG is that the object tends to look waxy and wet, due to a reduc-
tion in the intensity of internally-reflected light from cells near the surface
of the object. Furthermore, unlike the solvent-based polymers, PEG-treated
objects must be kept in controlled environments (18–20°C, 55% RH).
4.3 Bulking Treatments
Bulking treatments are used to reduce cell wall shrinkage of well-preserved
wood cells (Figure 13). Only small molecules are able to penetrate intact cells
and are not suitable for the stabilisation of highly degraded archaeological
wood. These treatments also act indirectly by reducing the vapour pressure of
the water present in the microcapillary system of the cell walls, and so retain
water in these areas. This helps to keep the cells turgid, and so provides hydraulic
support as the cell dries.
Bulking chemicals can be divided into non-reactive and reactive. Non-reactive
chemicals enter into the molecular structure of the cell walls, replacing water,
and they help maintain cell size and shape. These chemicals react with remain-
ing cellulose by means of hydrogen bonding. Examples of non-reactive chem-
icals include low molecular weight PEGs (200–600) and sugars. Non-reactive
chemicals also reduce the stiffness of the cell wall layers. In theory, these
can be removed from the wood matrix by warm water extraction.
Reactive bulking chemicals form covalent bonding in the cell wall. Examples
include alkylene oxides and low molecular weight thermosetting resins.
Unlike non-reactive chemicals, treatment with these chemicals is considered
non-reversible.
4.4 Surface Coatings
Small amounts of chemicals are often applied to the surface of dry archaeo-
logical wood. Coatings are used to improve the surface quality and integrity
of very fragile objects. Waxes (beeswax) and PEG 6000 are frequently used
to coat dried wooden objects.