Series editors' preface - Wood Tools

774 Conservation of Furniture
between layers of gesso and internal adhesive
failure (Von Reventlow, 1991). Delamination
from the substrate can be localized and take the
form of bulges or blisters or may be more widespread,
in which case it can be detected by
tapping the surface. A well-adhered surface will
sound dull when tapped lightly whereas loose
gesso will produce a more resonant and
sharper sound with a slightly higher pitch.
Adhesive failure between substrate and gesso
that results in delamination may be a result of
substrate shrinkage, stress around localized
faults such as knots, or weak or uneven size
application. Delamination between layers of
gesso can be caused by the use of a strong size
over a weak size or by overgilding in a previous
restoration. Internal adhesive failure, which
results in a friable powdery gesso, may be
caused by the use of a weak glue size or long
exposure to excessive high humidity, sometimes
in combination with fungal attack. This
last is usually a difficult and time-consuming
problem to address and in some cases consolidation
may prove impossible. Consolidant may
be delivered through facing tissue if a surface
is very flaky or unstable and this technique is
described on p. 574 and in Figure 12.4.
As is the case with other decorative surfaces,
a decision must be made about whether to use
traditional or modern materials for consolidation.
This issue is discussed in section 12.2.4 in
the general context of decorative surfaces.
Opinions on the subject are divided and the
conservator should consider the long-term
needs of the object within an ethical framework.
Whether a traditional or modern adhesive
is used, when carrying out localized
treatment such as laying flakes or cupped
areas, it is important to avoid leaving excess
adhesive on the surface. A heated spatula can
be used in conjunction with traditional and
many modern adhesives.
Consolidation of gesso layers on shaped
areas such as intricate carving may require the
use of a collagen consolidant that will gel and
may remove the need for the lengthy application
of pressure to the surface. Alternatives
such as flexible cauls, or the use of cling
film/Saran wrap can allow the use of consolidants
with a longer drying time. The use of low
vacuum pressure can be appropriate for treating
large areas of friable gilding. Water-based
adhesives will soften the gesso and may allow
flakes to be gently repositioned and flattened.
A small amount of alcohol, which acts as a surfactant,
can be added to the size to aid penetration
of the gesso. Powdery, friable gesso,
however, can become an amorphous slurry
and if this occurs the need for a non-aqueous
consolidant is indicated. If applying a waterbased
consolidant to a water-gilded surface, it
may be necessary to protect the surface by
applying a temporary hydrocarbon-soluble
protective coating (see section 12.4.7). Such
coatings can be tinted with pigment to identify
where they have been used and ensure their
removal after treatment is complete.
Rabbit skin glue or gelatin, when used as a
consolidant for gilding, should always be
weaker than that used for preparation of gesso.
A solution of one part dry glue granules to
twenty parts water (5% v/v) may provide a reasonable
margin of safety. Paraloid B72, PVAC
and acrylic dispersions or emulsions can also
be used as consolidants for gesso. The primary
issue with these materials is retreatability, since
removal of a consolidant from a porous surface
is rarely practicable. Some conservators have
expressed concern that the use of such materials
may inhibit the future penetration of aqueous
consolidants and Thornton (1991b) alludes
to this problem when discussing the use of isolating
layers. The use of Aquazol, poly(2-ethyl-
2-oxazoline) as a consolidant for gilded
surfaces has been examined by Friend (1996),
Shelton (1996) and Wolbers et al. (1998). The
use of wax-based materials for the consolidation
of porous gesso surfaces is not recommended
because they cannot be completely
removed and can inhibit or prevent the future
use of most other consolidants.
16.9.7 Reintegration
The material selected to replace losses to the
wooden substrate will depend on the size and
position of the damage. Wood may be the
material of choice where repairs must be
weight-bearing or where the loss has occurred
on an edge and wood movement related to
fluctuations in relative humidity will not add to
the instability of the surface. Hanlon (1992)
utilized a carvable epoxy resin (Araldite 1253),
isolated from the original wood with animal
glue, to fill losses from a wooden substrate
(Figure 16.28).