Consolidation of gouache

The described difficulties with thick, mat, opaque watercolor is
familiar. Like a sponge, the paint wants to absorb and hold the liquid
consolidant and so does the paper support.  Not much is left for the
area where the cleavage has occurred.  In fact, as drying takes place,
the methyl cellulose seems to follow the liquid's evaporative pathway to
the paint surface (and into the paper), drying there leaving
darkened/saturated paint and/or ringing.  Even if all of the MC stayed
at the site of separation, the 2% solution may not have enough holding
power to effect a bond once the water leaves.  More concentrated
solutions are too thick to flow adequately, which is why some
conservators prefer gelatin for the same task.  Gelatin has
significantly more flow capability and holding strength at comparable
concentrations.  Of course there is the concern about its propensity for
darkening over time, but if it can be applied to the paper paint
interface and held there until setting, then the darkening problem may
be avoided and bonding improved.

A first stage of prewetting with solvent or a solvent/water mixture (for
example, alcohol and water) temporarily fills the pores of the paint
(and paper) while the comparatively larger areas of cleavage remain
open.  This can be done delicately with an air brush. The second stage
involves a wicking application of the consolidating liquid into the
larger cracked spaces.  To some greater degree (compared to no
prewetting), the consolidant is held in place in the cracks (between the
paper and paint) as it begins to gel.  Over time, the water and alcohol
evaporates from the paint surface, but hopefully the consolidant has
thickened enough to remain in place.  To a greater extent, the paint
retains its unsaturated appearance.  There is a good deal of craft and
experimentation needed to make this treatment work.  Mock ups are useful
(school poster paint is great for experimenting because of the way it
cracks up when applied thickly) and thorough testing of individual cases
is essential in avoiding problems.

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                  Conservation DistList Instance 7:79
                    Distributed: Monday, May 2, 1994
                        Message Id: cdl-7-79-001
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