Series editors' preface - Wood Tools

584 Conservation of Furniture
saturating layers must be as thin as possible,
and the best results are often achieved using
spray application. It is essential to avoid building
up excessive thickness in these layers.
Different binding media lend themselves to
particular methods of application. Some synthetics
may require fine hatching whilst others
may be used to lay down a smooth base of
solid colour. As a result, different binding
media may be employed by conservators for
different purposes within the same retouch,
such as base coats and glazes.
Paraloid B72 Given the photochemical and
thermal stability of Paraloid B72, it is hardly surprising
that conservators have used it as a
retouching medium. Although in the past it was
often used with xylene or toluene, a less harmful
aromatic hydrocarbon solvent or 1methoxypropan-2-ol
can be substituted (Phenix,
1992a,b). B72 can have a ‘rubbery’ feel if the solvent
level drops too far before application is
complete. As solvent content drops the resin
begins to form a gel phase and thus it may be
necessary to add diluent to maintain good handling
properties. Inexperienced retouchers may
be tempted to reduce the concentration of
medium to help prevent this problem, but this
will produce a matte retouch with a high pigment
volume concentration. As a general rule, underbound
or matte paint makes refined judgement
about the degree of retouching necessary more
difficult, because underbound paint compromises
the colour and saturation of the retouching.
Although it may be possible to locally
introduce a thin varnish or resin solution to ‘feed’
a retouch (and in some cases this will be necessary),
this often makes the task of producing a
final surface with even gloss more difficult.
PVAC Conservation grade PVAC is widely
used as a retouch medium because it is photochemically
stable, soluble in xylene and mixtures
of IMS and water, and the base resin can
be selected for the desired characteristics (e.g.
Tg, molecular weight). It is very versatile and
easy to use. PVAC can be used in a viscous
solution for texturing or in very thin solutions
for wash-like glazes. Some conservators find it
easier to control gloss with PVAC in comparison
to Paraloid B72. PVAC resins with a Tg
around room temperature have a tendency to
cold flow (creep).
As a general rule, pure PVAC resin is used as
a retouching medium. Commercial emulsions
and dispersions may not be appropriate
because they contain a wide range of additives,
including plasticizers and stabilizers, and vary
widely in their photochemical stability. Union
Carbide PVAC resins (AYAA etc.) are widely
used in North America. Other pure PVAC resins
are listed in Down et al. (1996). Although there
are variations in the PVAC resins used, a standard
recipe may utilize 1:1 or 2:1 AYAA:AYAC
(see, e.g., Berger, 1990). 1-methoxypropan-2-ol
or an aromatic hydrocarbon can be used as the
diluent.
Low molecular weight synthetics MS2A and
ketone resins may be used as binding media for
retouching. MS2A may be stabilized against
photochemical degradation using Tinuvin 292
(see section 12.4.5). Although MS2A and ketone
resins have a longer induction period (the
period before photochemically induced
changes occur) than natural resins, polar solvent
mixtures similar to those necessary for the
removal of aged natural resins are required for
the removal of unstabilized aged formulations
of MS2A and ketone resins. It may be necessary
to isolate unstabilized MS2A and ketone
retouches from the original surface. MS2A and
ketone resins are soluble in aliphatic and aromatic
hydrocarbons. They flow out well and
their behaviour is closer to that of a natural
resin than a paint. They may be used for scumbles
and are very effective for glazes. They can
be applied over a base coat of Paraloid B72 to
increase optical complexity.
Though Regalrez 1094 and Arkon P-90 stabilized
with Tinuvin 292 offer the advantages of
solubility in aliphatic hydrocarbons and longterm
photochemical stability, their ready solubility
makes the application of multiple layers
extremely difficult without the use of a less soluble
intermediate varnish such as Paraloid B72.
Laropal A81 (BASF) is a LMW resin that is a
condensation product of formaldehyde and
urea. It has good photochemical stability and
has been developed as a commercially available
medium for inpainting (de la Rie et al.,
2000; Leonard et al., 2000).
Water colours Water colours or gouache tend
to be used when a surface is incompatible with
solvent-based retouching systems. Water