New Insights into the Cleaning of Paintings
SCMC-0003 SCMC-0003
128 • smithsonian contributions to museum conservation FIGURE 2. Stress- strain plots of (top) the acrylic Liquitex HB burnt umber paint film and (bottom) phthalocyanine blue paint film for the control test and after immersion in water or solvent for different times. The behavior of the burnt umber paint reflects that of the acrylic and PVAc paints in general. All paint films tested, i.e., acrylic and PVAc, except for the phthalocyanine blue, showed an increase in stiffness and a decrease in the elongation- at- break values after 20 minute and longer immersion times in water, ethanol, or acetone. As shown in Figure 2, acetone dissolves the polymer chains much faster than ethanol, making the paint film far more brittle, as shown above for the 20 minute and 12 hour immersions in acetone. The 12 hour exposure to ethanol embrittles the paint film far more than the equivalent water immersion. Interestingly, the phthalocyanine blue films showed an increased plastic deformation capacity for the case of water. Bar charts illustrating the weight loss for both these paints as a function of immersion time in water are shown in Figure 3. FIGURE 3. Weight loss as a function of water immersion time for the acrylic Liquitex HD (top) burnt umber and (bottom) phtalocyanine blue paint films. The extraction of minor compounds present in the bulk films can be readily measured in the first few minutes of immersion, particularly for the phthalocyanine blue, explaining the different mechanical properties of these films when exposed to water immersion longer than 20 minutes. The examination of both acrylic and PVAc paints by SEM clearly showed that water immersion creates micropores in the structure of the film (Figure 4). Complementary data collected with a microtensile strength tester coupled to a light microscope showed that these micropores were the starting points for cracking of these films when subjected to stress. Mechanical Properties of Acrylic Films after Water Immersion and Swabbing Tests The mechanical properties of acrylic Talens raw sienna films were evaluated after 5 and 20 minute water swabbing or water
number 3 • 129 FIGURE 4. The SEM photomicrographs of acrylic Liquitex HB pthalocyanine blue films before and after immersion in water (12 hours), ethanol (12 hours), and acetone (20 minutes). The photos below show light microphotographs taken during microtensile testing, where the starting points for the mechanical failure are the micropores. immersion tests. The stress- strain curves are presented in Figure 5 and show that these samples, similar to most acrylic ones, underwent an increase in stiffness and elongation- at- break values within 5 minutes for both immersion and swabbing. It is evident that in the first few minutes, the immersions affect the paint’s mechanical properties more dramatically. However, after 20 minutes of water exposure, both immersion and swabbing present similar stress- strain curves, indicating that water swabbing for longer periods of time may have mechanical effects on paints similar to those obtained with water immersion. Effect of Solvent Immersion on Test Films In general, the exposure of acrylics and PVAc paints to solvents resulted in the dissolution of polymer chains and extraction of additives. The AFM images presented in Figure 6 show a PVAc Flashe Senegal yellow paint film before and after immersion in acetone. The exposure of these films to solvents with lower polarity than water, such as acetone and ethanol, resulted in an increase in the roughness of the surface at micro- and nanoscales as the grains of pigment become more evident. This shows not only that additives are being removed but also that large amounts of polymer chains are being extracted, which affects directly the overall properties of the films. As mentioned, acetone affects the paint films by dissolving the polymer chains much faster than ethanol. As a consequence, the samples readily became stiff and brittle and almost impossible to test. Ethanol, on the other hand, affects the mechanical properties of the film in the first 20 minutes after immersion, but it takes a longer exposure time to embrittle the sample. On the other hand, both acrylic and PVAc paints showed less susceptibility to nonpolar solvents such as ligroin and white
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number 3 • 129<br />
FIGURE 4. The SEM photomicrographs <strong>of</strong> acrylic Liquitex HB pthalocyanine blue films before and after immersion in water<br />
(12 hours), ethanol (12 hours), and acetone (20 minutes). The photos below show light microphotographs taken during microtensile<br />
testing, where <strong>the</strong> starting points for <strong>the</strong> mechanical failure are <strong>the</strong> micropores.<br />
immersion tests. The stress- strain curves are presented in Figure<br />
5 and show that <strong>the</strong>se samples, similar to most acrylic ones, underwent<br />
an increase in stiffness and elongation- at- break values<br />
within 5 minutes for both immersion and swabbing. It is evident<br />
that in <strong>the</strong> first few minutes, <strong>the</strong> immersions affect <strong>the</strong> paint’s<br />
mechanical properties more dramatically. However, after 20 minutes<br />
<strong>of</strong> water exposure, both immersion and swabbing present<br />
similar stress- strain curves, indicating that water swabbing for<br />
longer periods <strong>of</strong> time may have mechanical effects on paints<br />
similar to those obtained with water immersion.<br />
Effect <strong>of</strong> Solvent Immersion on Test Films<br />
In general, <strong>the</strong> exposure <strong>of</strong> acrylics and PVAc paints to solvents<br />
resulted in <strong>the</strong> dissolution <strong>of</strong> polymer chains and extraction<br />
<strong>of</strong> additives. The AFM images presented in Figure 6 show a PVAc<br />
Flashe Senegal yellow paint film before and after immersion in<br />
acetone. The exposure <strong>of</strong> <strong>the</strong>se films to solvents with lower polarity<br />
than water, such as acetone and ethanol, resulted in an increase<br />
in <strong>the</strong> roughness <strong>of</strong> <strong>the</strong> surface at micro- and nanoscales as<br />
<strong>the</strong> grains <strong>of</strong> pigment become more evident. This shows not only<br />
that additives are being removed but also that large amounts <strong>of</strong><br />
polymer chains are being extracted, which affects directly <strong>the</strong><br />
overall properties <strong>of</strong> <strong>the</strong> films.<br />
As mentioned, acetone affects <strong>the</strong> paint films by dissolving<br />
<strong>the</strong> polymer chains much faster than ethanol. As a consequence,<br />
<strong>the</strong> samples readily became stiff and brittle and almost impossible<br />
to test. Ethanol, on <strong>the</strong> o<strong>the</strong>r hand, affects <strong>the</strong> mechanical<br />
properties <strong>of</strong> <strong>the</strong> film in <strong>the</strong> first 20 minutes after immersion, but<br />
it takes a longer exposure time to embrittle <strong>the</strong> sample.<br />
On <strong>the</strong> o<strong>the</strong>r hand, both acrylic and PVAc paints showed<br />
less susceptibility to nonpolar solvents such as ligroin and white
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