Series editors' preface - Wood Tools

522 Conservation of Furniture
Figure 11.15 Using the Teas’ chart to select a solvent
for varnish removal. In a situation where it is desirable
to remove an upper layer such as a varnish from a
solvent sensitive layer, such as a painted surface, the
solubility regions of both materials can be plotted on a
Teas’ chart. In this hypothetical example, the substrate
is represented by the outlined area, whilst the varnish is
represented by the dotted area. The area where the
solubility region of the varnish does not overlap the
solubility region of the lower layer can be used to
formulate the least potentially damaging solvent blend
for varnish removal. In this case, alcohols could be
used to remove the varnish without damaging the
substrate
or crosslinked, although the polymer may
swell and soften enough to allow mechanical
removal.
One of the main advantages of the Teas’
chart is that it readily allows the solubility
effect of mixing solvents to be predicted. The
contribution of an individual solvent to the
overall solubility parameter is equal to its
proportion in the mix. The position of a
solvent blend can be predicted mathematically
or geometrically. The volume-wise contribution
of the solubility parameters of the individual
solvents is used for the mathematical
method (see Box 11.1). The geometric method
utilizes the Teas’ chart itself. The solubility
parameters of a mixture of three parts acetone
and one part white spirit, for example, can be
plotted geometrically by drawing a straight
acetone
white spirit
Figure 11.16 Method for calculating the Teas’
parameters of a two-part solvent blend. A mixture of
three parts mineral spirits and one part acetone can be
plotted on a Teas’ chart by drawing a straight line
between the two pure solvents and marking off a point
1 ⁄4 of the way from mineral spirits. This represents the
shift in polarity the addition of 25% acetone causes in
the mixture. The positioning of the point that marks the
new blend can be a little confusing, but it should be
closest to the solvent present in the greatest proportion
(in this case, mineral spirits)
line between the two pure solvents and
marking off a point 1 ⁄4 of the way from white
spirit (Figure 11.16). Mixtures of more than
two solvents can also be plotted using a Teas’
chart (Fig. 11.17). In many cases the Teas’
chart can allow the conservator to substitute a
mixture of less harmful solvents for a more
toxic one but still achieve the required solubility
parameter.
It should be noted that whilst this approach
can be useful as a rule of thumb, it does not
take into account differences in evaporation
rates or interactions between solvents. Since
IMS evaporates faster than white spirit, the
solubility parameters of the mix will be shifting
back towards white spirit as the mix is
used. In this way it is possible for a solvent
mix that is on the edge of a solubility region