Series editors' preface - Wood Tools

526 Conservation of Furniture
a polar molecular group dominates, the
solvent will mix with other polar solvents.
When a hydrocarbon, non-polar part of the
molecule dominates, the solvent will mix with
other non-polar solvents. Solvents may be
miscible, partly miscible or immiscible.
Miscible solvents will form a stable solution in
any proportion. Partly miscible solvents may
form a stable solution up to a maximum
proportion. Benzyl alcohol, for example is
miscible with water only up to around 4%.
Immiscible solvents will separate out into
distinct layers. They may be shaken to form
an emulsion, but will require the addition of
a surfactant if a stable emulsion is required.
Solvent blends may exhibit azeotropic
behaviour. An azeotrope is a mixture of two
or more liquids that has a constant boiling
point at a specific concentration. The boiling
point of the solution is different from the
boiling point of the individual constituents.
When two liquids that are capable of forming
an azeotrope are mixed, the liquid that is more
volatile will evaporate more quickly, until the
concentration reaches the azeotropic concentration.
At this point the proportion of the
solvents in the mixture will remain constant,
as both solvents evaporate at the same rate.
For example, a mixture of 95% ethanol and
5% water boils at a lower temperature than
either pure ethanol or pure water. The proportion
of ethanol to water remains constant
whilst the mixture is evaporating. Azeotropic
behaviour is particularly likely to occur with
aqueous solutions. Tables of azeotropic
mixtures can be found in Horsley (1952).
11.3.4 Proprietary paint strippers
Commercial paint strippers are familiar to
anyone who has tried paint or finish removal
projects. New products are regularly being
developed and may be useful in particular
layer removal treatments. In general, these
products are either based on solvents, di-basic
esters or alkaline solutions. They are designed
to remove a wide range of finishes in a nonspecific
manner, intentionally covering a wide
range of Teas’ solubility parameters. Thickening
agents, waxes, surfactants or chelating
agents are often included to increase the effectiveness
of the product. It is safer for both the
object and the conservator to use materials
specific to a given coating and that cause
minimal change or damage to the substrate.
Retaining earlier colourants and coatings in the
wood as well as remnants of old finish can be
of aesthetic, practical and monetary value.
Wollbrinck (1993) reviewed forty-seven
proprietary paint strippers and discussed the
principles behind their chemical activity.
Solvent-based strippers included methylene
chloride, acetone, alcohols, toluene or xylene
as the primary active ingredients. These
materials present significant health risks and
measures should be taken to reduce the risk
to the conservator. They can be very effective
for dissolving coatings quickly but have little
selectivity where the stratigraphy of a surface
is characterized by similar binding media. This
lack of selectivity can be problematic if a treatment
requires the removal of a single coating
whilst leaving others unaffected.
Dibasic (i.e. possessing two replaceable
hydrogen atoms, in this case on the carboxylic
acid groups) ester systems function by
swelling materials which, like themselves, are
characterized by ester linkage structures.
Modern synthetic coating materials that incorporate
such structures include urethanes and
those based on polyesters such as alkyds.
They are designed to work after prolonged
contact with the coating that is to be removed.
Product literature recommends a forty-five
minute minimum contact time for cellulose
nitrate and shellac, one to three hours for
some paints, varnishes and polyurethanes, and
overnight for multi-layered coatings. Other
sources suggest that the contact times needed
to swell or dissolve shellac and cellulose
nitrates are significantly longer (Flexner, 1994).
The reduced rate of activity in comparison to
solvent-based strippers may offer an advantage
for the conservator.
N-methyl-2-pyrrolidone will swell or dissolve
polar materials such as PVALs, PVACs,
carbohydrates or other materials that are easily
oxidized such as aged natural resins or aged
alkyds. It has been reported as an effective
solvent for polysaccharides (Horie, 1992). It
has been observed to swell collagen glue
based materials such as gesso.
Alkaline removal systems usually contain a
strong basic reagent such as sodium hydroxide
and their action is chemical rather than
solvent based. Strongly basic materials are very