Series editors' preface - Wood Tools

There are four stages in the dissolution of a
polymeric coating in a solvent. First the
solvent molecules diffuse between the
polymer chains. If there is a strong interaction
between the solvent and the surface coating,
the solvent will be absorbed into the outer
layer of the polymer, causing it to swell. With
continued solvent exposure, the outer layer of
the polymer will continue to swell and
become a rubbery gel. If the coating is not
crosslinked it will dissolve, forming a solution
that can be removed with a swab. If the
varnish is substantially crosslinked it will reach
a region of peak swelling and may be
removed mechanically. The rate at which
solvent molecules are lost from the surface is
dependent on temperature, the vapour
Principles of cleaning 517
Figure 11.9 Comparison of varnish removal from an oil paint substrate by fast-penetrating, fast-dissolving, fastevaporating
solvent and a medium-penetrating, slow-dissolving, slow-evaporating solvent
The two graphs illustrate varnish removal with a fast-penetrating, fast-dissolving (4 μm/s) solvent, for example
acetone (a) and a medium-penetrating, slow-dissolving (0.4 μm/s) solvent, for example propan-2-ol (b) Fast-,
medium- and slow-diffusing (and swelling) solvents diffuse into (and swell) varnish and paint at different rates and,
within a given time, different amounts. During the process of varnish dissolution, solvent interacts with the varnish.
The longer it takes the solvent to penetrate, swell and dissolve the varnish, the longer the solvent is in contact with
the paint film and the greater the potential for the swelling of the paint layer(s). The effect of the solvent on the
substrate is a result of the combination of the diffusion rate of the solvent into the paint layer, the rate of the
swelling of the paint layer and the rate at which the solvent evaporates from the surface. Thus, during varnish
removal, an ideal solvent is one that rapidly diffuses and dissolves the varnish layer, while not diffusing into or
swelling the paint layer, and that rapidly evaporates from the paint film
Figure 11.9a
Time = 0 seconds: the initial thickness of each layer are oil paint = 40 μm, glaze = 5 μm and varnish = 10 μm. The
paint thickness is arbitrary and was chosen for illustrative convenience. In this bar the total varnish thickness
(10 μm) includes both the varnish that is dissolved after solvent application (3 μm) and the varnish that remains
(7 μm)
Time = 0.9 seconds: once applied, the fast penetrating solvent front diffuses into the varnish layer within 0.9
seconds; 3 μm of the original varnish is dissolved and the remaining 7 μm is swollen to a thickness of 14 μm
Time = 2.5 seconds: within 2.5 seconds the fast-penetrating solvent dissolves the varnish layer completely and
swabbing is stopped. The paint and glaze have been exposed to the solvent for 1.6 seconds resulting in the
penetration of the solvent into the paint layer to a depth of 12 μm. The swab leaves a 1 μm thick layer of high
volatility solvent on the paint surface; 50% evaporates, and the solvent front advances a further 2 μm (a total of
14 μm) into the paint layer. The glaze layer may exhibit some swelling as a result of this penetration. Because the
thickness of the paint layer is arbitrary, this bar does not represent the percentage of penetration of the solvent into
the paint layer, only the distance
Figure 11.9b
Time = 0 seconds: the initial thickness of each layer are oil paint = 40 μm, glaze = 5 μm and varnish = 10 μm. The
paint thickness is arbitrary and was chosen for illustrative convenience. In this bar the total varnish thickness
(10 μm) includes both the varnish that is dissolved after solvent application (3 μm) and the varnish that remains
(7 μm)
Time = 7 seconds: once applied, the medium-penetrating solvent front penetrates the varnish layer within 7
seconds; 3 μm of the original varnish is dissolved and the remaining 7 μm is swollen to a thickness of 14 μm
Time = 25 seconds: within 25 seconds the medium-penetrating solvent dissolves the varnish layer completely and
swabbing is stopped. The paint and glaze have been exposed to the solvent for 18 seconds, resulting in the
penetration of the solvent into the paint layer to a depth of 17 μm. The swab leaves a 1 μm thick layer of low
volatility solvent on the paint surface; most enters the paint, and the solvent front advances a further 4 μm (a total
of 21 μm) into the paint layer. The glaze layer may exhibit some swelling as a result of this penetration. Because
the thickness of the paint layer is arbitrary, this bar does not represent the percentage of penetration of the solvent
into the paint layer, only the distance
pressure of the solvent, and the rate of air flow
over the surface. The surface/volume ratio of
solvent over the surface as a whole will
increase evaporation time, but solvent trapped
in pores may take substantially longer to
evaporate than that on the surface. Secondary
bonding between solvent and substrate will
also affect the evaporation rate. The evaporation
rate of a given quantity of water from
cotton or wood, which are hydrophilic, is
slower than that from polyester, which is
hydrophobic.
Ueberreiter (1968) has described the process
of dissolution of polymers. Michalski (1990)
has modelled the physical process of diffusion
of a solvent into an oil paint layer. Solvent
penetration of, and swelling interaction with,