Series editors' preface - Wood Tools

that influences the choice of gelatin for a given
application is Bloom strength, sometimes called
gram weight strength (see section 4.7.4), which
correlates with molecular weight. The higher
the Bloom strength the faster the glue will gel,
the better its adhesive properties and the more
flexible the dried adhesive. Thus if consolidating
a powdery ground, a lower molecular
weight gelatin will penetrate more effectively
and, because low Bloom strength glues gel
more slowly, there is more time for the gelatin
to penetrate before it becomes an immobile gel.
If adhering flakes, however, slow gelling may
be undesirable and a higher Bloom strength
may be more appropriate.
Gelatin used in conservation should be
highly purified and can be obtained from suppliers
such as Sigma or Fisher in a variety of
Bloom or gram weight strengths. The longevity
of gelatin, and thus consolidation treatments
that utilize it, is dependent on good environmental
conditions. Gelatin may last for several
hundred years in an environment with minimal
exposure to light and a moderate, stable RH
and temperature. Excessively high, low or fluctuating
RH and temperature, and exposure to
high light levels and UV will result in rapid
degradation (Hansen et al., 1996).
Isinglass is a traditional consolidant for
painted surfaces and is utilized in both easel
paintings and polychrome sculpture conservation.
As with other glues derived from fish, isinglass
has a different amino acid composition
than mammalian collagen, and as a result does
not gel at room temperature. Isinglass and other
fish glues do not, therefore, have a Bloom
strength. The chain length of the collagen,
however, can be comparable to the best-quality
mammalian animal glues, and isinglass can
therefore be very strong. The viscosity of isinglass
and other fish glues is not as closely
related to chain length as is the case for mammalian
derived glues. The best isinglass was traditionally
derived from the swim bladder of the
Russian sturgeon fish (Acipenser stallatus and
A. güldenstädtii). Sturgeon glue has more tack
and less is needed to achieve a given viscosity
in comparison to many commonly available
types of isinglass.
Refined isinglass can be purchased through
suppliers of artists’ materials (e.g. in Europe,
Kremer Pigmente www.kremer-pigmente.de).
Isinglass may be prepared in the following way.
Principles of consolidation, aesthetic reintegration and coatings 569
Four grams of isinglass are soaked overnight in
deionized water. The water is then drained and
discarded. Eighty millilitres of fresh deionized
water is added to the isinglass, which is then
warmed (temperature not exceeding 55 °C), in
a double boiler for about an hour. It is then
strained through a double layer of fine silk or
nylon. If the isinglass has been prepared from
the dried bladder itself, the conservator may
wish to skim the detritus from the surface
before straining the mixture. When warmed,
the strained isinglass solution (c.5% w/v) is
ready to be used as a consolidant. Isinglass prepared
in this way can be brushed out onto
Melinex for later use. Once dry, the isinglass
can be broken up and stored. This thin prepared
isinglass will dissolve rapidly in warm
water and can be used to make up ‘instant’ isinglass
as required. Honey is added to the isinglass
as a plasticizer in some traditional recipes
but will produce a mix with a yellow-brown
colour (Petukhova and Bonadies, 1993). Long
periods of heating will denature collagen, so it
is better to pour off a small amount to use
rather than repeatedly heating a large quantity.
A few drops of alcohol may be added to collagen
mixtures to reduce surface tension or a surface
may be pre-wet with ethanol to increase
penetration.
Rabbit skin glue, when used as a consolidant
for gilding, should always be weaker than that
used for preparation of gesso. A proportion of
one part rabbit skin glue to fifteen parts water
usually provides a reasonable margin of safety.
Traditionally rabbit skin glue is prepared the
day before use, allowed to gel overnight and
the strength of the gel then assessed (see section
14.2.4). The grains are dissolved in warm
water, heated (c.55 °C) in a double boiler until
dissolved, then strained to remove any gritty
particles.
Collagen glues are an option where a waterbased
consolidant is safe for the object. They
can be applied drop-wise from a brush to an
affected area. Surface tension may be reduced
by pre-wetting with, or adding a drop of,
ethanol. Moderate heat (such that a heated
spatula can be held without discomfort in the
hand) can help to keep collagen glue warm as
it infuses into the decorative surface. The spatula
should be isolated from the surface by protective
tissue, blotting paper or Melinex/Mylar.
Gelatin and isinglass are compatible with many