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Subject: Wood filling materials

Wood filling materials

From: Ray Marchant <raymarhki>
Date: Saturday, March 2, 2002
Antonia Iliopoulou <iliotonia [at] hotmail__com> writes

>I am a Conservation student and I am doing my Major Project on a
>Greek Icon. The Icon has insect damage on the wooden support which
>has caused some loss of paint and loss of wooden material(holes,
>tunnels). The paint layer is affected locally and some flaking and
>cracking is obvious. I would like to know which material is best to
>use as a filler  for the wood.

Having had to deal with this type of problem many times on oak,
pine, and poplar, I would comment as follows. The first thing to
consider, before you even think of using a filler, is how to
consolidate the weak structure.

This can best be done using a fluid, introduced through the existing
cracks and flight holes, from both surfaces. To begin by blocking
this access with a filler would only make life difficult. The best
consolidant to use would probably be Paraloid B72, in a vehicle such
as xylene, or toluene, at a concentration of about 25% w/v.

The choice of solvent needs to be made taking into account factors
such as, the susceptibility of the paint film, when it is applied
from the front, so do some tests first if unsure. The rate of
evaporation, because if it dries too fast it can actually trap
solvent within the structure for longer, and personal preferences
relating to health and safety for working with aromatic solvents.
Sometimes because of these considerations I opt to use Paraloid B67
which is soluble in White spirit again at about 25%. The
concentration of either depends on achieving a balance between the
viscosity required for high penetration, which may be influenced by
the amount of damage and the type of timber, and the need to
introduce an adequate amount of resin solids to be effective, in the
lowest volume of solvent. Hardening of the structure will not be
complete until all the free solvent has dispersed, which may take up
to 3 weeks. If adequate impregnation of the honeycomb structure can
be achieved to a point of saturation, such that the frass left in
the galleries is bound together, that alone will make a stronger
support for the ground and paint film, and will also provide a
better foundation for filling. A wood filler was developed in our
London studio for use with PVA wood working adhesive as a gap filler
for rejoins, but we have also used it with Paraloid, after
consolidation, with quite good results. It is a mixture of Phenolic
resin micro-balloons and coconut shell flour mixed 1:1 by weight,
and made into a paste with the chosen consolidant. It does not
produce the type of hard bullets which are often found when fillers
such as chalk and animal glue are used, but is far more compatible
with timber, and retains a degree of flexibility. When made up with
the same consolidant, it provides a weak adhesion with the prepared
surface, and is not unlike the frass which it is often required to
bond with, except the particles are far smaller. It can be tamped
into the holes using dental tools, or we have made some of our own
tools using blunt needles bent to a suitable shape, so that it can
be fed some way into the voids below the paint film.

The handling characteristics are quite good when it is made up to
the consistency of soft fudge, which requires that some of the
solvent is allowed to evaporate off, and it can then be kept wrapped
in Melinex.

What we normally do is to compact it to a level just below the
surface and when dry use a putty over it in the conventional manner.
It should be noted that this filler does not have any significant
bonding or adhesion power, when used with a paraloid resin, and will
add very little strength to the fractures or structure as a whole,
that question would need addressing as a separate issue. What it
does, is to provide a firm stable foundation for the fills which is
reasonably reversible.

Ray Marchant
Simon Bobak conservation studio,
and Hamilton Kerr Institute
+44 20 7730 7874

                  Conservation DistList Instance 15:60
                  Distributed: Tuesday, March 5, 2002
                       Message Id: cdl-15-60-006
Received on Saturday, 2 March, 2002

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