THE LIGHT BLEACHING OF DISCOLORED FILMS OF AN ACRYLIC ARTISTS' MEDIUM
Paul M. Whitmore, Val G. Colaluca and Hannah R. Morris
Summary—Films of Liquitex gloss medium on gesso that had been discolored either by dark
storage of the original product or from addition of colored matter extracted from linen canvas were
bleached by brief exposure to high-intensity visible light. After 20 hours of exposure to a UVfiltered xenon lamp, an exposure dose equivalent to four days under UV-filtered daylight
fluorescent lights, the intrinsic yellow color of the naturally-aged films (initial Yellow Index of 23)
was found to bleach significantly (about 40% of the original color remained after bleaching). For
films that were discolored from material extracted from linen canvas (initial Yellow Index of 37),
only a small fraction of the yellow discoloration was bleached in the exposure (about 70% of the
original color remained after bleaching}. The bleaching of the discolored films was not linear with
light dose, and during the exposure of both the intrinsically discolored and canvas-discolored films
the color decreased rapidly at first and more slowly as the bleaching proceeded. In subsequent dark
storage of the bleached films, the original levels of discoloration returned. However, when
bleached films were subsequently exposed to periodic, moderate light levels simulating gallery
conditions, the films retained much of the improved appearance that had been gained from the
bleaching treatment.
Introduction
It has been observed in laboratory studies [1] and on artifacts [2, 3] that, unlike the solvent-based
acrylic polymers, films of unpigmented water-based acrylic paint media tend to discolor upon dark
storage. In his study of prepared white paints. Levison [4] demonstrated that the tendency to yellow
in the dark and to bleach during light exposure was common to both oil and acrylic media, although
this yellowing was much less severe for the acrylic paints. For unpigmented acrylic media films
having relatively large film thicknesses or for thin films having aged for many years, the yellow
color can become noticeable [1]. Furthermore, for films applied to canvas it has been found that the
discoloration of the acrylic media films can be quite severe, probably due to transfer of colored
matter from the underlying canvas to the coating [3].
Brief exposure to relatively intense light has long been a means of bleaching paper and textiles, and
it has been known for centuries that yellowed oil paints will bleach on exposure to light. Recently
this phenomenon has been studied in more detail and examined as a treatment of discolored paint
films. Levison's study [4] used modern instrumental methods to measure the extent and efficacy of
light bleaching of oil paints. In a study of very severely discolored oil medium, Tahk [5] examined
light bleaching as a treatment to reduce the darkening of a fire-damaged oil paint. More recently
Contompasis and Hamm [6] explored the possibility of using light bleaching as a remedy for the
discoloration of films of unpigmented acrylic media, using a similar method of light exposure to
that used for the light bleaching of paper. They reported a 'reasonable degree of success' in light
bleaching an unpigmented acrylic medium on a painting during a two-week treatment with fluorescent lights.
In this paper the authors report a study of the behavior of discolored films of Liquitex gloss medium
as they are bleached with light, and their subsequent color stability following bleaching. Films were
discolored in two different ways: by natural dark aging ('intrinsic' discoloration), and by
incorporation of water-soluble colored matter extracted from new linen canvas into the fluid
medium ('canvas-discolored'). While it has not been definitively established, it is likely that this
canvas-derived colored matter is a significant contributor to the discoloration observed in
applications of acrylic dispersion media on canvas. The bleaching of these films upon brief
exposure to high-intensity visible light has been measured. While near-ultraviolet light has been
reported to have greater efficacy in bleaching discolored oil paint, the authors chose to explore a
treatment that would avoid the known risks of damage to the acrylic medium from exposure to
ultraviolet (UV) wavelengths by bleaching the films using only visible light [1]. Since it has been
observed that light-bleached paper [7] and dammar resin [8] tend to discolor following light
exposure, the color of the bleached films was also
monitored for several months during dark storage. However, since it is unlikely that light-bleached
artifacts would be immediately stored in the dark but would more likely be exhibited, the color of
bleached films that were subsequently exposed to periodic low-intensity light that simulated gallery
conditions was also studied.
Experimental design
Details of the experimental procedures are given in the Appendix. The acrylic paint medium chosen
for this study was Liquitex gloss medium, because of its widespread use and because it was known
from earlier work to discolor during dark storage. For the films that discolored during natural aging,
termed here 'intrinsic' discoloration, the medium was used as supplied, and cast films were used that
had yellowed from dark storage for several years. For the canvas-discolored films, pieces of linen
canvas were steeped in the fluid dispersion in order to transfer colored matter to the medium, and
then films were cast. While these materials find use in a variety of art applications, it seemed likely
that the slight yellow discoloration would be most noticeable, and most likely to be judged in need
of bleaching, when applied over a light-colored substrate. To simulate this, dried films of discolored
acrylic medium were peeled from their supports (in this case, glass plates) and adhered to films of
acrylic gesso (also supported on glass) using a thin layer of acrylic medium as an adhesive. This
method of preparation was developed in order to create samples in which the films of acrylic
medium had controlled thicknesses and color.
Two sets of experiments were performed. In the first, the bleaching behavior of the films was studied, in order to determine bleaching rates during exposure to two different visible light sources: a
high-intensity xenon arc lamp and a bank of daylight fluorescent lamps. The fluorescent bleaching
test was performed for 10 days, a duration chosen to resemble that of a reasonably long bleaching
campaign, and approximating the two-week exposure of the reported successful treatment [6]. A
more intense xenon lamp was also used in a bleaching treatment in order to examine the dose
response under a source that better simulates the spectral distribution of sunlight. The second study
was done
Table 1
Color of intrinsically discolored Liquitex films on gesso
to examine the color stability of the bleached samples during subsequent dark storage and under
simulated gallery lighting (150 lux for 10 hours each day). For that study, three sample sets were
prepared: a control set that remained in dark storage and provided a measure of the additional
intrinsic yellowing that would occur during the storage time period, and two other sets that were
light bleached under the xenon lamp for 20 hours, providing a light dose that was found in the first
study to produce most of the bleaching effect (see 'Results and discussion, light bleaching'). One of
the bleached sets was subsequently stored in the dark and the other under simulated gallery lighting.
The chosen metric of yellow color in the films over gesso was the conventional Yellow Index [9].
which was calculated from the measured reflectance spectrum. It was discovered that the acrylic
gesso background was made less highly reflecting (a lower Munsell value) by the application of a
film of acrylic medium, presumably due to a reduction of optical scattering when wetted by the
medium: in addition, it had a very slight yellow color itself, probably arising from the slight yellow
color (absorption of blue wavelengths) of the titanium white it contains. Thus a completely
bleached acrylic film over the gesso, while seeming to be a clear film, would be measured as having
a small Yellow Index. In order to assess this residual unbleachable color and to establish the
minimum Yellow Index that can be reached by bleaching, films of fresh undiscolored acrylic
medium were prepared, transferred to gesso in the usual manner, and measured. Details of the
measurements and subsequent calculations are given in the Appendix.
Results and discussion
Light bleaching
A summary of the light bleaching results for the Liquitex films is shown in Tables 1 and 2. The
changes in Yellow Index, calculated from the reflectance spectra for the intrinsically discolored and
canvas-discolored films, respectively, during their bleaching with xenon and fluorescent lamps, are
shown in Figures 1 and 2. Measurements of an uncolored fresh film on gesso showed that a completely bleached film would have a Yellow Index of
Table 2
Color of canvas-discolored Liquitex films on gesso
Figure 1 The change in yellow discoloration of intrinsically discolored Liquitex gloss medium films
on gesso during exposure to visible light from (a) a filtered xenon lamp and (b) a bank of filtered
daylight fluorescent lamps. The dashed line is the measured Yellow Index for uncolored fresh films
on gesso (a value of 8), shown to represent the ultimate possible bleaching level for this type of
sample.
about 8 (indicated by a dashed line in the figure), due to the very slight inherent yellow color of the
gesso, mentioned above. The course of bleaching and the percent of bleaching of the discolored
films were followed in comparison to this ultimate limit. After 1-5 million lux hours of exposure,
the color of the intrinsically discolored films was reduced to about 40% of the original level of
color; despite the instrumentally measured Yellow7 Index values, the films on gesso were no longer
noticeably yellow: after the treatment. The films discolored with linen canvas extracts, however,
were only bleached to about 70% of the original color, and after the treatment they remained
significantly yellow in appearance. These results indicate that light bleaching of the yellow
discoloration may have different prospects for success depending on the origin of the discoloration.
These results also demonstrate that the bleaching of the discolored films is not linear with light
dose.
During the exposure of both the intrinsically discolored and the canvas-discolored films, the color
decreased rapidly at first and then more slowly as the bleaching proceeded. While this could be evidence of a mixture of colored species that vary in their resistance to bleaching, this kinetic behavior
also resembles the loss of colorant in a fading transparent glaze [10]. As in the fading glaze, the
photochemical bleaching reaction rate depends on the amount of light absorbed by the yellow film:
as the bleaching progresses and the amount of absorbing chromophore decreases, the reaction rate
also decreases. By the end of the 1-5 million lux-hour exposure in this test, the rate of bleaching for
both types of discolored films had slowed to an extent that substantially more bleaching would only
be achieved after a considerably larger exposure dose.
The changes in the reflectance spectra as a result of bleaching are shown in Figures 3 and 4 for the
intrinsically discolored and canvas-discolored films,
Figure 2 The change in yellow discoloration of canvas-discolored Liquitex gloss medium films on
gesso during exposure to visible light from (a) a filtered xenon lamp and (b) a bank of filtered
daylight fluorescent lamps. The dashed line is the measured Yellow Index for uncolored fresh films
on gesso (a value of 8), shown to represent the ultimate possible bleaching level for this type of
sample.
Figure 3 Reflectance spectra of a typical intrinsically discolored Liquitex gloss medium film on
gesso, before and after exposure to visible light from a filtered xenon lamp, and following dark
storage of the bleached film for 524 days. Spectra shown are averages of three repeat
measurements on the film, and standard deviations of the average reflectance values are about 03%.
respectively. While the reflectance increases for the intrinsically discolored films (Figure 3) occur at
wavelengths from 400 to about 550nm, bleaching of the canvas-discolored samples (Figure 4)
seems to produce reflectance increases across the entire visible spectrum. These changes in the
long-wavelength reflectances for the bleaching canvas-discolored films could be evidence of the
decrease of absorbers in this spectral region, or it could be evidence for other optical changes, such
as a slight increase in optical scattering for the film.
Post-bleaching dark storage
Bleached films that were subsequently stored in darkness yellowed rapidly. After 524 days in the
dark, both the intrinsically discolored and the canvas-discolored films had reached essentially the
same level of color of the samples before light bleaching (Figures 5 and 6). Following this dark
storage, the samples that had been bleached retained a very" slight degree of improvement (4YI
units for the intrinsically discolored, 2YI units for the canvas-discolored) compared to the films that
had not been bleached but had been stored in the dark, although the visual appearance difference
would probably not be considered a significant improvement. The mechanism of this color
reversion of the bleached samples is unknown. However, the very small additional discoloration of
the never-bleached control samples during their two-year storage indicates that further intrinsic
discoloration of the bleached films is only a very minor contributor to the post-bleaching color
reversion.
The reflectance spectrum of the bleached intrinsically discolored film eventually resembled that of
the unbleached film (Figure 3). However, for the canvas-discolored film, dark storage of the
bleached
Figure 4 Reflectance spectra of a typical canvas-discolored Liquitex gloss medium film on gesso,
before and after exposure to visible light from a filtered xenon lamp, and following dark storage of
the bleached film for 524 days. Spectra shown are averages of three repeat measurements on the
film, and standard deviations of the average reflectance values are about 0-6%.
film resulted in decreased reflectance (increased absorbance) at the low wavelengths only: the
slightly greater reflectance level at the higher wavelengths remained even after prolonged dark
storage (Figure 4). This suggests that some of the changes that occurred in the canvas-discolored
films that caused the increase in reflectance at the longer wavelengths—the bleaching of
chromophores that absorb at long wavelengths, or an increase in overall optical scattering—were
not reversed upon dark storage.
Post-bleaching exposure to periodic low-intensity light
In contrast to the bleached films stored in the dark, exposing the bleached films to gallery lighting
(periodic 150 lux visible illumination) seemed to maintain the bleached appearance for a
considerable period of time. Exposure of the intrinsically discolored film to the light bleaching
treatment with a xenon lamp reduced the Yellow Index from 23 to 14 (roughly 40% of the original
yellow color of the film remained after bleaching), and under simulated gallery lighting the
discoloration returned slightly, to a Yellow Index of about 16 after a month (Figure 5). During the
next 500 days this level of color remained essentially constant. Similarly, the canvas-discolored
film was bleached with a xenon lamp from a Yellow Index of 36 to 27 (about 70% of the original
color of the film remained after bleaching), and during subsequent exposure to simulated gallery
lighting the bleached film recovered to a Yellow Index of 30 in a month of simulated exhibition
(Figure 6). Subsequent light exposure decreased that level slightly, to a Yellow Index of about 28
after 524 days. These results suggest that periodic low-level light exposure is sufficient to maintain
much of the benefit gained from the light bleaching treatment.
Conclusions
The results of this study confirm the earlier observations of Levison [4] and those reported in prior
work by the authors [1] that, like oil media, films of acrylic artists" paint medium tend to yellow in
the dark and to bleach from light exposure. The light exposure conditions studied here were
intended to probe the effects of a conservation treatment, and the results indicated that such a
treatment promised significant improvement only for films that were suffering a moderate level of
intrinsic yellowing (an initial Yellow Index of 23). Those films that were stained yellow due to their
incorporation of colored matter from a support such as linen canvas (to a
Figure 5 The change in yellow discoloration of unbleached and light-bleached intrinsically
discolored Liquitex gloss medium films on gesso during subsequent storage in the dark and during
exposure to simulated gallery lighting.
Yellow Index of 37) were more resistant to light bleaching, and the slight improvement in appearance for such canvas-induced discoloration may be deemed inadequate to warrant such treatment.
Because of the kinetic behavior of the bleaching process, which sees progressively smaller changes
for increasing doses, significant improvements on these results might require substantially greater
light exposure times or more intense light sources, both of which incur greater difficulty and
attendant risks. If light bleaching is considered as a treatment, the display of the bleached artifact
seems the only sensible means to maintain the bleached appearance: the acrylic medium, like other
materials such as paper and oil paints, returns to nearly its original yellowed condition upon dark
storage. Of course, no light bleaching treatment should be performed without due consideration of
the risks to colorants or other light-sensitive materials that might be adversely affected by exposure
to these doses of visible light.
It remains to be seen whether acrylic media films on artifacts that are on constant display will
yellow
Figure 6 The change in yellow discoloration of unbleached and light-bleached canvas-discolored
Liquitex gloss medium films on gesso during subsequent storage in the dark and during exposure to
simulated gallery lighting.
to a level that warrants consideration of bleaching treatments. The results observed in this study,
coupled with Levison's observations for high-intensity light exposures [4], suggest that ambient
lighting in a gallery setting might be sufficient to prevent significant intrinsic discoloration of
acrylic media films. The prospects for this will be examined in future work.
Appendix: experimental
Films of Liquitex gloss medium were prepared by drawing down the commercial product onto glass
plates. For the intrinsically discolored films, the medium was used as supplied and films on glass
plates were prepared by drawing down with a doctor's blade. The dry films ranged from 100 to
300μm thick. When dry to the touch, the films were peeled from the glass and left lying on the plate
in a dark cabinet at ambient laboratory temperature and humidity for three to six years. For the
canvasdiscolored films, the Liquitex medium was added to completely fill a 473ml jar containing a 10 X
60cm piece (approx. 40g) of medium-weight linen artists' canvas. The filled jar was capped,
wrapped with aluminum foil, and rotated on a roller mill for 24 hours. The medium containing the
linen extract was then decanted into another container and used to prepare draw downs on glass
plates. When dry, these films ranged from 200 to 740μm in thickness, and they were stored at
ambient conditions for approximately two years prior to use. Films of Liquitex acrylic gesso were
prepared by drawing down on 25 X 75mm glass microscope slides. These gesso films were
approximately 100-150μm in thickness when dry. Prior to the bleaching experiments, 15cm squares
of the acrylic medium films were removed from their glass substrates and adhered to the dried
acrylic gesso films, using small amounts of acrylic medium as an adhesive and applying slight
pressure to eliminate air bubbles. These composite film samples were then allowed to dry for 24
hours before initial reflectance measurements were made.
Reflectance spectra of the samples were measured on a diode array spectrophotometer with a pulsed
xenon lamp and integrating sphere (Macbeth Color Eye 7000). The samples were backed with white
Millipore filters to ensure optical opacity and placed against the port of the integrating sphere.
Spectra of small areas of the samples (3 X 8mm) were measured from 400 to 700nm, with the
specular beam excluded, and these results were used to calculate the tristimulus values (X, Y, Z) for
CIE Illuminant C. The Yellow Index was calculated from the standard formula [9] and the Munsell
color calculated according to the standard method [11] using a commercial computer program.
Individual film samples were measured in three to five different areas, and reported Yellow Index
values are averages of measurements from two to six films, depending on the number of prepared
films that were found to be similar in initial color.
Light bleaching was performed in two separate experiments. In each, the films were exposed to
visible wavelengths of light, to minimize the potential risk of photochemical damage to the acrylic
polymer. The discolored acrylic films on gesso were bleached under filtered daylight fluorescent
lamps (Sylvania F48T12D filtered with UF-3 Plexiglas, 17,000 lux visible intensity, 23°C, 50%
RH) or "in a xenon-arc exposure apparatus (Atlas Suntest CPS filtered with an IR filter and UF-3
Plexiglas. 74,000 lux visible intensity, 27°C on a water-cooled sample tray). Light intensities were
measured with a radiometer (International Light Model IL1700). The study of the color stability of
bleached films was performed with samples that had been bleached in the xenon-arc exposure
cabinet. After light
bleaching, one set of bleached films was stored in the dark at laboratory ambient conditions (2025°C and 30-60% RH), and a second set of light-bleached films was exposed in a ventilated, lighttight enclosure to daylight fluorescent lamps (General Electric F20T12D. filtered with UF-3
Plexiglas and attenuated with wire screen. 150 lux visible intensity) that were on for 10 hours each
day (controlled with an automatic timer).
Acknowledgements
This work was performed at the Research Center on the Materials of the Artist and Conservator at
Carnegie Mellon University. The financial support of the Andrew W. Mellon Foundation and the
Kettering Family Foundation is gratefully acknowledged.
Materials and suppliers
Belgique Belgian linen A: New York Central Art
Supply. Inc.. 62 3rd Avenue. New York. NY
10003! USA. Daylight fluorescent lamps (Sylvania F48T12D and
General Electric F20T12D): Northern Light
and Supply Company, 901 Allegheny Avenue.
Pittsburgh." PA 15212. USA. Light exposure cabinet (Suntest CPS): Atlas Electric
Devices. Inc., 4114 North Ravenswood
Avenue, Chicago. IL 60613. USA. Liquitex acrylic gloss medium and Liquitex acrylic
gesso: Binney and Smith. Inc.. PO Box 431.
1100 Church Lane. Easton. PA 18044-0431.
USA. Radiometer (IL1700): International Light. 17 Graf
Road. Newburvport. MA 01950. USA. UF-3 Plexiglas (Rohm & Haas): Talas. 568
Broadway. New York. NY 10012. USA. Spectrophotometer (Color Eye Model 7000):
Kollmorgen Instrument Corporation, Macbeth
Division. 405 Little Road. New^ Windsor. NY
12553, USA. Munsell conversion software: downloaded from
www.munsell.com
References
1 WHITMORE. P.M.. and COLALUCA. V.. The natural and accelerated aging of an acrylic artists'
medium'. Studies in Conservation 40 (1995) 51-64.
2 WATHERSTON. M., 'Problems presented by-color field paintings: cleaning of color field
paintings' in Conservation of Paintings and the Graphic Arts. IIC. London (1972) 831-845.
3 HAMM. J.. GAVETT, B.. GOLDEN, M., HAYES, J.. KELLY, C, MESSINGER, J., COMTOMPASIS, M., and
SUFFIELD, B., 'The discoloration of acrylic dispersion media' in Saving the Twentieth Century: The
Conservation of Modern Materials, ed. D.W. GRATTAN. Canadian Conservation Institute. Ottawa
(1993) 381-392.
4 LEVISON, H., 'Yellowing and bleaching of paint films', Journal of the American Institute for
Conservation 24 (1985) 69-76.
5 TAHK. C.. 'The recovery of color in scorched oil paint films', Journal of the American Institute
for Conservation 19 (1980) 3-13.
6 CONTOMPASIS, M., and HAMM, J., 'Potential treatment for discolored acrylic emulsion medium'.
The Painting Specialty Group Annual (American Institute for Conservation) 3(1990)19.
7 LEE, S.B., BOGAARD, J.. and FELLER. R.L., 'Darkening of paper following exposure to visible and
near-ultraviolet radiation', Journal of the American Institute for Conservation 28 (1989) 1-18.
8 DE LA RIE. E.R.. 'Photochemical and thermal degradation of films of dammar resin'. Studies in
Conservation 33 (1988) 53-70.
9 ASTM D 1925-70: 'Standard test method for yellowness index of plastics" (1994).
10 WHITMORE. P.M.. and BAILIE, C., 'Further studies on transparent glaze fading: chemical and
appearance kinetics". Journal of the American Institute for Conservation 36 (1997) 207-230.
11 ASTM D 1535-89: 'Standard test method for specifying color by the Munsell system' (1994).
Authors
PALL M. WHITMORE received
a PhD in physical chemistry from the University of California at
Berkeley. Following an appointment at the Environmental Quality Laboratory at Caltech studying
the effects of photochemical smog on works of art. he joined the staff of the Center for
Conservation and Technical Studies at Harvard University Art Museums. Since 1988 he has been
director of the Research Center on the Materials of the Artist and Conservator at Carnegie Mellon
University, where his research has been directed towards the study of the permanence of modern art
and library materials. Address: Carnegie Mellon
University, 700 Technology Drive, Pittsburgh, PA 15219, USA.
HANNAH R. MORRIS received a PhD in analytical chemistry from the University of Pittsburgh, where
her research focused on materials characterization in complex polymer blends using spectroscopy
and chemical imaging techniques. Since 2000 she has been deputy director of the Research Center
on the Materials of the Artist and Conservator. Address: as for Whitmore. as for Whitmore.
VAL G. COLALUCA received a BS degree in biology from Point Park College in Pittsburgh. He is an
associate scientist at the Research Center on the
Materials of the Artist and Conservator. Address: as for Whitmore.
Resume—Des films de liant Liquitex brillant sur gesso qui avaienl ete jaunis soil en les
entreposant a I'obscu-rite ou en ajoutant au produit d'origine une matiere coloree extraite de la
toils de tin ont ete blanchis en les exposant brievement a une lumiere visible de forte intensite.
Apres 20 heures d'exposition a une lampe a xenon a filtre UV, equivalente a quatre jours sous des
tubes fluorescents de type lumiere du jour, on a observe que la couleur jaune des films vieillis
naturellement (indice de jaune initial 23) diminuait de facon significative (environ 40% de la
couleur d'origine subsistait apres blanchiment). Pour les films jaunis par le produit extrait de la
toile de lin (indice de jaune initial 37), seul un faible pourcentage du jaune a ete blanchi par
I'exposition (environ 70% de la couleur d'origine subsistait apres I'operation). Le blanchiment des
films jaunis ne variait pas lineairement avec la quantite de lumiere refue, la teinte jaune diminuant
rapidement au debut et de plus en plus lentement au fur et a mesure que le blanchiment se
poursuivait pendant I'exposition des deux categories de films. Lors de I'entreposage a I'obscurite
des films soumis au blanchiment qui a suivi, les niveaux originaux de jaunissement ont ete
retrouves. Cependant, lorsque des films blanchis ont ete ensuite exposes a des niveaux de lumiere
periodiques et moderes, semblables aux conditions qui prevalent dans des salles d'exposition, les
films gardaient en grande partie I aspect ameliore obtenu par le traitement de blanchiment.
Zusammenfassung—Liquitex-Filme auf Gesso, die sich entweder durch Lagerung im Dunklen
oder durch die Zugabe van aus Leinwand extrahierten Farbstoffen verfdrbt hatten, warden durch
eine kurze Inten-sivbestrahlung mil sichtbarem Licht gebleicht. Nach 20 Stunden unter einer
Xenon-Lampe mil UV-Filter, einer Expositionsdauer, die etwa 4 Tagen UV-gefiltertem Tageslicht
entspricht, hatte sich die gelbe Farbung des natiirlich gealterten Films (anfanglicher Yellow Index
23) deutlich vermindert (etwa 40%, der gelben Farbe verblieb nach dem Bleichen). Filme, die
durch die Zugabe van aus Leinwand extrahierten Farbstoffen verfdrbt waren (anfanglicher Yellow
Index 37), wurden nur in geringem Mafie gebleicht (etwa 70% der gelben Farbe verblieb nach dem
Bleichen). Das Ausmaft des Bleichen verlief nicht linear mit der Lichtdosis. Sowohl beim natiirlich
gealterten Film wie auch bei dem durch Leimwandextrakte verfdrbten, verminderte sich die
Gelbfarbung zundchst schnell und dann immer langsamer. Bei einer erneuten Lagerung der
gebleichten Proben im Dunklen erschien die Verfdrbung wieder. Wenn die Filme jedoch moderaten
Lichtmengen ausge-setzt wurden wie sie Galleriebedingungen entsprechen, erhielten sie viel van
dem durch die Bleichprozedur verbesserten Erscheinungsbild :uriick.
Resumen—Peliculas de media brillante de Liquitex sobre gesso que fueron decoloradas tanto par
almace-namiento en la oscuridad del producto original como par la adicion de materiales
coloreados extraidos de lien-zos de lino fueron blanqueadas con una breve exposition a luz visible
de aha intensidad. Despues de 20 horas de exposition a lampara de area de xenon con filtration UV
(una dosis de exposition equivalente a cuatro dias bajo luz fluorescent'e filtrada UV tipo 'luz dia')
el color amarillento intrinseco de las peliculas envejecidas naturalmente (indice de amarillo initial
de 23) se habia blanqueado significativamente: cerca de un 40%> del color original se mantenia
despues del blanqueamiento. Para las peliculas que fueron decoloradas partiendo del material
extraido del lienzo de lino (indice de amarillo initial de 37) solo una pequena portion de la decoloration amarilla fue blanqueada en la exposition (cerca del 70%, del color original se mantenia
despues del blanqueamiento). El blanqueamiento de las peliculas decoloradas no era lineal con las
dosis de luz, y durante la exposition de ambas (la intrinsecamente decolorada y la de lienzo
decolorado) el color decrecio rdpida-mente al principle y mas lentamente segun el proceso de
blanqueamiento avanzaba. En el subsiguiente almace-namiento en oscuridad de las peliculas
decoloradas se recuperaron los niveles originales de decoloration o amarilleamiento. Sin embargo
cuando las peliculas blanqueadas fueron subsiguientemente expuestas periodica-mente a niveles de
iluminacion moderados, simulando las condiciones tipicas de museo, las peliculas retuvieron la
mayor pane de la apariencia mejorada que habia sido obtenida con el tratamiento de
blanqueamiento.