strengthening, and they embrittle with age. Unger cites a large-scale pro-
ject in Austria, in which an altar was consolidated in the 1950s using 1500 l
of shellac in ethanol (Unger 1988).
Drying oils, especially linseed oil, have also been used as consoli-
dants in the past, but they provide very little effective strengthening.
Cellulose derivatives (acetate or nitrate) did find use as consoli-
dants during the first half of the twentieth century, but today their applica-
tion has virtually ceased. It is difficult to achieve much penetration with
the cellulose derivatives, and the materials discolor and embrittle with age.
Natural materials are thus seen to have significant disadvantages.
Therefore, further discussion focuses primarily on synthetic polymers.
Synthetic polymers
Synthetic polymers can be divided into thermosetting and thermoplastic
types. This division is important in conservation, because thermosetting
resins that might find use in consolidation generally are not soluble in
organic solvents. Therefore, their use results in irreversible treatments.
Thermoplastic resins are generally soluble in organic solvents, although
they can become cross-linked, which leads to a loss of solubility (Ciabach
1983; Bockhoffet al. 1984).
Thermosetting resins
One class of thermosetting resins that might be considered for use as con-
solidants consists of the formaldehyde resins: phenol, resorcinol, urea, and
melamine. These are widely used as adhesives in the production ofwood-
based materials, because they are excellent adhesives for wood. Phenolic
and resorcinol resins are also waterproof and very resistant to weathering.
Unger cites some past uses of these resins in conservation; however, there
appearsto be little, if any, such use at present (Unger 1988). A particular
drawback of these resin systems is poor penetration, and all but the
melamine formaldehyde resins either are initially dark or darken with age.
Epoxy resins have found wide application in the rehabilitation and
repair of wood and concrete structures, and they are successfully used in
stone consolidation because of their excellent durability, adhesion, and
strength (Phillips and Selwyn 1978; Stumes 1979; Selwitz 1992). Unlike the
formaldehyde resins, which shrink upon hardening, epoxy resins in their
neat formulation do not change volume as they harden; consequently,
shrinkage stresses are avoided. However, the neat resins have relatively
high viscosity and therefore penetrate poorly. Penetration can be improved
by the addition of solvents to reduce viscosity. Unger gives a number of
examples where epoxy resins have been used in wood conservation proj-
ects, including some treatments of wooden panels. Their main application
lies in strengthening structural members in wooden buildings or in
strengthening museum objects that are exposed to the weather. According
to Unger, epoxy resins are suitable for consolidation of wooden panels
only if the wood is very severely deteriorated, because application into
wood that is only moderately deteriorated results in insufficient penetra-
tion (Unger 1988). This may not apply ifthe small-molecule epoxies
advocated by Munnikendam are used, however, because in their neat for-
mulation they have about the same viscosity as 15% Acryloid B72 in ace-
tone (Munnikendam 1973).
C W P 91