Studio preliminare sulla composizione di pitture commerciali ad olio, acriliche e viniliche e loro comportamento dopo invecchiamento accelerato
10.6092/issn.1973-9494/4753
application/pdf
1973-9494
4753
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Elisabetta Zendri
Conservation Science in Cultural Heritage
preventive conservation
A preliminary study of the composition of commercial oil, acrylic and vinyl paints and their behaviour after accelerated ageing conditions
Text.Serial.Journal
https://conservation-science.unibo.it/
14
Articles
2014-12-20
2015-02-25
2014-12-20
2015-11-02
353-369
CSCH
Elisabetta Zendri
2014/12/20
10.6092/issn.1973-9494/4753
353
1973-9494
preventive conservation
en
369
WHITMORE PM., COLALUCA VG., 1995, The natural and accelerated aging of an acrylic artists’ medium, Studies in Conservation, 40, pp. 51-64.
A preliminary study of the composition of commercial oil, acrylic and vinyl paints and their behaviour after accelerated ageing conditions
https://conservation-science.unibo.it/article/download/4753/4242
14
A    PRELIMINARY STUDY OF THE COMPOSITION OF
     COMMERCIAL OIL, ACRYLIC AND VINYL PAINTS
     AND THEIR BEHAVIOUR AFTER ACCELERATED AGEING
     CONDITIONS
   Francesca Caterina Izzo•, Eleonora Balliana, Federica Pinton, Elisabetta
   Zendri
   Dipartimento di Scienze Ambientali, Informatica e Statistica
   Università Ca’ Foscari di Venezia, Italy


   Keywords: contemporary paints, degradation, artificial ageing, preventive conser-
   vation


   1. Introduction

   Conservation, cleaning and general interventions on traditionally painted surfaces
have a long history of practice and research. During this time there have been occa-
sional setbacks, but also several exciting new developments. In contrast, diversity in
the stability and transformations that industrial paints used in contemporary works of
art are subject to with the passing of time have only recently been recognized as being
distinct processes from those occurring in traditional paints [1 - 2].
   Industrial development during the twentieth century has brought with it the rapid
production and diffusion of synthetic binding media in industrial paints, leading to a
radical change in the formulation of artists’ materials [3]. Because of the complexity of




                                                                                                  CONSERVATION SCIENCE IN CULTURAL HERITAGE
these products, whose composition is often not completely known, it is possible their
characteristics are affected over time, due to interactions between the media, pig-
ments, additives and all the different enhancers used. As yet, in-depth studies in the
behaviour of the resulting paint films have not yet been carried out [4 - 7].
   Moreover, many contemporary artists in search of new visual effects, have been
experimenting and mixing materials with different compositions and drying behaviour,
in some cases compromising the durability of the artefacts themselves [8].
   Recent studies and research have focused on studying the behaviour of these
materials when subjected to different environmental conditions. The information thus
obtained is typically related to degradation processes which have already affected
the artworks [2, 8-10]. With the aim of promoting effective preventive conservation of
contemporary paintings, we decided to test the stability of some industrial colours now
available on the market and commonly used by artists.
   Considering the huge choice of industrial paints available, a survey was launched
among young Venetian artists and students from the Academy of Fine Arts to learn
which are the most used. The survey found that nowadays the majority of artists prefer
to use oil, acrylic and vinyl-based paints. The most widely used manufactured oil paints
are Winton (Winsor & Newton, UK), for acrylic paints, Heavy Body (Liquitex, USA) and
for vinyl paints Flashe (Lefranc & Bourgeois, France).


• Corresponding author: fra.izzo@unive.it

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                                                                                                                                         For each product three commercial colours were selected based on their known re-
                                                                                                                                      activity and lightfastness: chromium oxide green (PG17), an inorganic pigment which
                                                                                                                                      appears to be quite stable in organic binding media; titanium white (PW6), the typi-
                                                                                                                                      cal white pigment used in contemporary paintings, usually forming spongy films in oil
                                                                                                                                      media; the yellow azo-dyestuff (PY74) which is known to rapidly undergo fading and
                                                                                                                                      changes in shade under the influence of light [11 - 12].
F.C. Izzo, E. Balliana, F. Pinton, E. Zendri - A preliminary study of the composition of commercial oil , acrylic and vinyl paintS




                                                                                                                                         2. Materials and methods

                                                                                                                                         2.1 Preparation of paint samples

                                                                                                                                         A full list of the selected paints is given in Table 1. The commercial name, brand, and
                                                                                                                                      product series, together with the declared composition in the manufacturer’s catalogue
                                                                                                                                      are reported for each paint.


                                                                                                                                      Table 1. Commercial description of the investigated industrial paints


                                                                                                                                                                                            Commercial name and            catalogue
                                                                                                                                                                  Brand
                                                                                                                                       Paint abbreviation *                                  number
                                                                                                                                                                  Product series            (indicated formulation)

                                                                                                                                                                                            Oxide of Chromium 31
                                                                                                                                       OG                                                   (Anhydrous Chromium Oxide (PG17),
                                                                                                                                                                                             pure Linseed Oil)

                                                                                                                                                                                            Titanium White 40
                                                                                                                                                                  Winsor & Newton
                                                                                                                                       OW                                                   (Titanium Dioxide (PW6), Zinc Oxide
                                                                                                                                                                  Winton                     (PW4), Safflower Oil)

                                                                                                                                                                                            Cadmium Lemon Hue (Arylide yellows) 7
                                                                                                                                       OY                                                   (PY3-PY74 (Arylide Yellows), pure
                                                                                                                                                                                             Linseed Oil)

                                                                                                                                                                                            Chromium Oxide Green 166
                                                                                                                                       AG                                                   (Anhydrous     Chromium    Sesquioxide
                                                                                                                                                                                             (PG17), Acrylic Polymer Emulsion)

                                                                                                                                                                                            Titanium White 432
                                                                                                                                                                  Liquitex
                                                                                                                                       AW                                                   (Titanium Dioxide (PW6), Acrylic Polymer
                                                                                                                                                                  Heavy Body                 Emulsion)

                                                                                                                                                                                            Yellow Medium Azo 412
                                                                                                                                       AY                                                   (Arylide Yellow 5GX (PY74 LF), Acrylic
                                                                                                                                                                                             Polymer Emulsion)

                                                                                                                                                                                            Chromium Oxide Green 542
                                                                                                                                       VG                                                   (PG17, Vinyl colour)

                                                                                                                                                                  Lefranc & Bourgeois       White 001
                                                                                                                                       VW
                                                                                                                                                                  Flashe                    (PW5-PW6, Vinyl colour)

                                                                                                                                                                                            Senegal Yellow (primary) 195
                                                                                                                                       VY                                                   (PY74, Vinyl colour)

                                                                                                                                      * O=oil paint, A=acrylic paint, V=vinyl paint; G=green, W=white, Y=yellow


                                                                                                                                354
    For each commercial paint, four sets of colours/paints were applied to inert glass
slides 10×5 cm, coating them with a paint film 0.2-0.3 mm thick.
    All prepared samples were left to dry at laboratory conditions (25 ± 1°C and 60 ± 3
% RH), which were constantly monitored with an Escort RH iLog® digital datalogger.
    The curing period was fixed for all the paints at 80 days. This appears to be the ap-
propriate length of time to ensure oxygen uptake for oil paints and the beginning of the
drying process [11, 13].




                                                                                                  CONSERVATION SCIENCE IN CULTURAL HERITAGE
        Figure 1. Paint films with commercial formulation by W&N, Liquitex and Flashe



  2.2 Artificial ageing

   The artificial ageing treatments were performed using deliberately extreme ageing
conditions in order to speed up the chemical reactions eventually involved in the curing
of oil, acrylic and vinyl paints and their consequences on the morphological aspect of
the film [14].

  2.2.1. Exposure to ozone (O3)


   Ozone treatment was performed using a conventional industrial FISCHER ozone
generator (Labor-und Verfahrenstechnik Ozon-Generator, Germany), with water cool-
ing system, using oxygen gas as the raw material gas. The ozone is produced on
demand in response to the amount of O2 provided flue gas with a controlled ozone-
oxygen ratio (1:5). The low operating temperature allows stable and constant control


                                                                                            355
                                                                                                                                      regardless of variation in flow, load, or O2 content. The samples were placed in a glass
                                                                                                                                      chamber on Teflon plates. The ozone concentration within the chamber was monitored
                                                                                                                                      using an Aeroqual S500 Monitor®ozone meter (0-1 ppm) equipped with Aeroqual Moni-
                                                                                                                                      tor Software.
                                                                                                                                         The samples were exposed to O3 for 8 hours and the average concentration of
                                                                                                                                      ozone was about 100 ppb, approximately 100 times higher than the recommended
F.C. Izzo, E. Balliana, F. Pinton, E. Zendri - A preliminary study of the composition of commercial oil , acrylic and vinyl paintS




                                                                                                                                      threshold value for the indoor conservation of organic artefacts [15].

                                                                                                                                        2.2.2. Thermal ageing (T)

                                                                                                                                         The paint films were thermally treated in air through heating in a laboratory oven
                                                                                                                                      at 100°C and 55% relative humidity (RH%) for 24 hours in the absence of light. En-
                                                                                                                                      vironmental conditions (T and RH%) were monitored with an Escort RH iLog® digital
                                                                                                                                      datalogger.
                                                                                                                                         According to previous studies, these exposure conditions are strong enough to esti-
                                                                                                                                      mate degradation processes in close agreement with those obtained for naturally aged
                                                                                                                                      paint films [14, 16].

                                                                                                                                        2.2.3. Treatment in environment with high level of RH

                                                                                                                                          For the humidity treatment, all samples were exposed for 7 days at 20 ± 2°C in
                                                                                                                                      a sealed saturated salt chamber at 83% RH obtained using a saturated solution of
                                                                                                                                      potassium chloride as reported in the specific standards [17 - 18]. The relative humid-
                                                                                                                                      ity value inside the salt chamber was checked in conditions at equilibrium with an
                                                                                                                                      Escort RH iLog® datalogger and monitored during experimentation with conventional
                                                                                                                                      thermo-hygrometers.

                                                                                                                                        2.3.Analytical techniques and instrumentation

                                                                                                                                          Several complementary analytical techniques were used to investigate the compo-
                                                                                                                                      sition (media, additives, pigments, extenders, etc) of the selected commercial formu-
                                                                                                                                      lations and to study their behaviour during drying and curing time, and after artificial
                                                                                                                                      ageing treatments.
                                                                                                                                          Elemental analysis was performed using a Philips Minipal X-Ray Fluorescence
                                                                                                                                      spectrometer (XRF); the spectra collected were elaborated with Minipals software.
                                                                                                                                          Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FT-
                                                                                                                                      IR) analyses were performed with a Thermo Nicolet FTIR NEXUS 750 instrument
                                                                                                                                      equipped with a Smart Endurance ATR diamond accessory, from 4000 to 400 cm-1
                                                                                                                                      and for 32 scans with a resolution of 4 cm-1. Data were collected with Thermo Nicolet
                                                                                                                                      OMNIC 6.0a software and then elaborated with Origin 8 software [16, 19].
                                                                                                                                          The Pyrolysis Gas Chromatography Mass Spectrometry (Py-GC-MS) analysis was
                                                                                                                                      performed by using a Focus ISQ Thermo Quest Mass Spectrometer with a 20 me-
                                                                                                                                      ter SLB5ms column, id 0.18 mm, ft 0.18 μm; one-shot pyrolysis at 550°C. The inlet
                                                                                                                                      temperature was 300°C, while the MS interface was at 280°C. The temperature pro-
                                                                                                                                      gramme was set from 35°C to 300°C with a ramp of 10°C/min, held isothermally for 3
                                                                                                                                      minutes; total runtime 25 minutes. The MS was run in Full Scan mode (m/z 40-600)
                                                                                                                                      with a speed of 1.9 scans/sec. Samples were derivatised with TMAH (tetramethyl am-



                                                                                                                                356
monium hydroxide) 2.5%, using tridecanoic acid as an internal standard. Data were
processed using Xcalibur 1.4 software.
   To monitor the mass variations occurring during the drying processes and the film
formation of the paints, samples were weighed every day using a Mettler Toledo al204
analytical balance (precision 0.1 mg). All the investigated paint films were also weighed
before and after each single ageing treatment to check for possible mass variations
occurring with ozone, temperature and humidity cycles.
   Colorimetric measurements were carried out on the paint films before and after
ageing and performed with a Minolta CM-2600d/2500d colour spectrophotometer, in
the 360-720 nm region in SCI (Spectral Component Included) modality. The L*a*b*
colorimetric coordinates and DE (total colour difference) were calculated based on a
D65 source of illumination and a 10° observer, using CIEL*a*b* 1976 colour space.
Five measurements were carried out for each sample at different points (accurately
identified with a mask) and the mean value calculated.


   3. Results and discussion

   3.1 Characterisation of the commercial paints


   Table 2 reports the results obtained in terms of binders, pigments, additives, extend-
ers, and fillers detected in the paint formulations with the joint use of several analytical
techniques (XRF, ATR-FTIR and Py-GC-MS).

Table 2. Summary of the analytical results obtained for the analysed paints by XRF, ATR- FTIR
and Py-GC-MS.




                                                                                                              CONSERVATION SCIENCE IN CULTURAL HERITAGE
   Paint samples           Composition of the paints

   Name        Brand        Pigment(s)      Extender(s)    Binder(s)                Additives


             Winsor &
                         chromium oxide     calcium                       metal soaps (likely
   OG        Newton                                       linseed oil
                         green              carbonate                     aluminium stearate),
             (Winton)

                                                                          plasticizers
                                                          acrylic         (dimethylphthalate),
             Liquitex
                         chromium oxide                   emulsion        defoamers (silicon
   AG        (Heavy                           /
                         green                            (n-BA/          oils), stabilizers (dibutyl
             Body)
                                                          MMA)            thiocarbammate,
                                                                          dimethylformamide)
                                                          vinyl
             Lefranc &                                                    dispersing and/or
                         chromium oxide     calcium       emulsion
   VG        Bourgeois                                                    sequestering agents
                         green              carbonate     (PVA-
             (Flashe)                                                     (polyphosphates)
                                                          VeoVA)

             Winsor &                                     linseed         metal soaps (likely
                         titanium oxide     calcium
   OW        Newton                                       oil and         aluminium stearate),
                         and zinc oxide     carbonate
             (Winton)                                     safflower oil   stabilizers (castor wax)




                                                                                                        357
                                                                                                                                                                                                            plasticizers
                                                                                                                                                                                              acrylic       (dimethylphthalate),
                                                                                                                                                  Liquitex
                                                                                                                                                                                              emulsion      defoamers (silicon
                                                                                                                                         AW       (Heavy       titanium oxide      /
                                                                                                                                                                                              (n-BA/        oils), stabilizers (dibutyl
                                                                                                                                                  Body)
                                                                                                                                                                                              MMA)          thiocarbammate,
                                                                                                                                                                                                            dimethylformamide)
F.C. Izzo, E. Balliana, F. Pinton, E. Zendri - A preliminary study of the composition of commercial oil , acrylic and vinyl paintS




                                                                                                                                                                                              vinyl
                                                                                                                                                  Lefranc &                                                 dispersing and/or
                                                                                                                                                                                 calcium      emulsion
                                                                                                                                         VW       Bourgeois    titanium oxide                               sequestering agents
                                                                                                                                                                                 carbonate    (PVA-
                                                                                                                                                  (Flashe)                                                  (polyphosphates)
                                                                                                                                                                                              VeoVA)
                                                                                                                                                                                                            wetting agents
                                                                                                                                                                                                            (sulphonates), anionic
                                                                                                                                                  Winsor &
                                                                                                                                                               Mixture of PY3-   calcium                    surfactants, metal
                                                                                                                                         OY       Newton                                      linseed oil
                                                                                                                                                               PY74              carbonate                  soaps (likely aluminium
                                                                                                                                                  (Winton)
                                                                                                                                                                                                            stearate), stabilizers
                                                                                                                                                                                                            (castor wax)

                                                                                                                                                                                                            metal soaps (Cu, Ca, Fe
                                                                                                                                                                                                            metal soaps), plasticizers
                                                                                                                                                                                              acrylic
                                                                                                                                                  Liquitex                                                  (dimethyl phthalate),
                                                                                                                                                                                              emulsion
                                                                                                                                         AY       (Heavy       PY74                /                        defoamers (silicon
                                                                                                                                                                                              (n-BA/
                                                                                                                                                  Body)                                                     oils), stabilizers (likely
                                                                                                                                                                                              MMA)
                                                                                                                                                                                                            dibutyl thiocarbammate,
                                                                                                                                                                                                            dimethylformamide)

                                                                                                                                                                                                            dispersing and/or
                                                                                                                                                                                              vinyl         sequestering agents
                                                                                                                                                  Lefranc &
                                                                                                                                                                                 calcium      emulsion      (likely polyphosphates),
                                                                                                                                         VY       Bourgeois      PY74
                                                                                                                                                                                 carbonate    (PVA-         plasticizers (diisooctyl
                                                                                                                                                  (Flashe)
                                                                                                                                                                                              VeoVA)        phthalate)
                                                                                                                                                                                                            thickeners(colophony)
                                                                                                                                      /=not detected

                                                                                                                                          The Py-GC-MS analyses allowed the correct identification of the paint binding me-
                                                                                                                                      dia [20 - 22]. Liquitex acrylic paints were analysed as co-polymeric emulsions of n-
                                                                                                                                      butyl acrylate and methyl methacrylate (n-BA/MMA), while a polyvinyl acetate/vinyl
                                                                                                                                      versatate (PVA/VeoVA) co-polymer is the main constituent of Flashe vinyl paints [23].
                                                                                                                                          Based on the ratio palmitic acid to stearic acid (P/S) and the relative amount of spe-
                                                                                                                                      cific fatty acids [9], Py-GC-MS results suggest that Winton oil paints are characterized
                                                                                                                                      by the presence of linseed oil in the OG and OY paints while a mixture of linseed and
                                                                                                                                      safflower oil is present as binder in the formulation of the OW paint.
                                                                                                                                          Although no additives are indicated by manufacturers, chromatographic analysis
                                                                                                                                      underlined the presence of several organic additives, such as dispersing, sequester-
                                                                                                                                      ing, wetting, stabilizing agents and plasticizers (see Table 2). As an example, Figure 2
                                                                                                                                      reports the m/z spectra of dibutyl thiocarbamate and diisooctyl phthalate respectively
                                                                                                                                      introduced as a stabilizer in the acrylic emulsion and as a plasticizer in the vinyl disper-
                                                                                                                                      sion.




                                                                                                                                358
Figure 2. m/z spectra of (a) dibutyl thiocarbamate in the acrylic emulsions and (b) diisooctyl
phthalate in the vinyl emulsions (as dymethyl esters after derivatisation and Py-GC-MS analysis)


    Furthermore, the presence of hydrogenated castor oil (also called castor wax or
12-hydroxy stearic acid) was identified in the Winton yellow paint (OY), thanks to the
identification of a specific marker (namely 12-hydroxy-octadecanoic acid). Castor wax
is frequently added by manufacturers in commercial oil paint formulations as a rheol-




                                                                                                         CONSERVATION SCIENCE IN CULTURAL HERITAGE
ogy modifier and stabilising agent [24].
    Elemental, inorganic and organic analyses of pigments confirmed what was de-
clared by the paint manufacturers. All green paints contain anhydrous chromium ses-
quioxide, commercially known as Chrome Green (PG17). Analysis outlined the use of
a mixture of titanium white (TiO2) and zinc oxide (ZnO) in the Winton white oil paint,
while a mix of PY74 and PY3 was detected in the Winton yellow oil paint. Calcium car-
bonate was identified in all paints with the exception of the Liquitex acrylic paints, and
was probably used as a filler in the paint formulations.
    Besides the chemical composition, the mass variation percentage of the paint films
caused by chemical-physical changes occurring during the curing and drying process-
es, were also monitored for the first 80 days after sample preparation (Figure 3).




                                                                                                   359
F.C. Izzo, E. Balliana, F. Pinton, E. Zendri - A preliminary study of the composition of commercial oil , acrylic and vinyl paintS




                                                                                                                                      Figure 3. Winton oil (OG, OW, OY), Liquitex acrylic (AG, AW, AY) and Flashe vinyl (VG, VW, VY)
                                                                                                                                      paint films mass variation percentage (ΔM%) monitored during the first 80 days of natural curing


                                                                                                                                         The resulting data are consistent with the known film-forming and drying properties
                                                                                                                                      of acrylic and vinyl paint [22, 25]. Vinyl and acrylic casting dispersion involves differ-
                                                                                                                                      ent steps: evaporation of the water fraction, deformation of the paint particles and the
                                                                                                                                      coalescence process with the final formation of a solid dried film [26]. As depicted in
                                                                                                                                      Figure 3, the Liquitex and Flashe colours showed a variation in mass during all the
                                                                                                                                      monitored curing period, primarily registered in correspondence to the very first days:
                                                                                                                                      this is mainly due to water evaporation from the emulsions. These variations were
                                                                                                                                      rather significant for the paints containing organic yellow dyestuff. Right after the first
                                                                                                                                      day, the Liquitex yellow paint (AY) presented a mass loss of 48%, while the Flashe yel-
                                                                                                                                      low colour (VY) a loss of 43%.
                                                                                                                                         Traditional oil-based paints usually exhibit an initial weight increase from oxygen
                                                                                                                                      uptake and then a mass loss due to the evaporation of volatile substances [27 - 28].
                                                                                                                                      This trend, normally typical of unpigmented oil media, was not observed in the ana-
                                                                                                                                      lyzed Winton oil paints since only a minor mass increase was recorded in the first days
                                                                                                                                      (Figure 2). The different behaviour in OW formulation might be explained due to the
                                                                                                                                      presence of safflower oil, whose drying behaviour differs with respect to traditional lin-
                                                                                                                                      seed oil. Another possible explanation might be linked to the addition of wetting agents,
                                                                                                                                      stabilizers, fillers, etc in the manufactured compositions [29 - 30], that might modify
                                                                                                                                      induction time and oxygen absorption.




                                                                                                                                360
   3.2. Analysis of the paint films after ageing treatments

   3.2.1 Exposure to ozone

   Ozone treatment results in a general softening of all the paints, in particular for
Winton oil paints.
   After ozone treatment, the acrylic paints did not show any relevant chemical varia-
tions, while spectroscopic modifications were detected in varying degrees both for the
vinyl and oil paints, mainly in correspondence with the binder absorption bands. Figure
4 shows the ATR-FTIR spectra obtained for the yellow paints OY, AY and VY subjected
to ozone treatment.




Figure 4. Comparison of yellow oil (OY), acrylic (AY) and vinyl (VY) paint films ATR-FTIR spectra




                                                                                                          CONSERVATION SCIENCE IN CULTURAL HERITAGE
before and after ozone treatments (OY_O3, AY_O3, VY_O3)


    Being a strong oxidizing agent, ozone promotes the radical chain polymerization of
oil, leading to the reaction of double C=C bonds and the cleavage of C=O bonds. This
is evident in the Winton yellow paint (OY), whose FTIR profile shows a decrease in the
intensity of the carbonyl absorption band at about 1735 cm-1 and of the CH stretching
bands at 2920 cm-1 and 2850 cm-1 [16, 19, 31]. Moreover, after ozone treatment all the
oil paints exhibit an FTIR absorption at around 1600-1700 cm-1 most likely due to the
formation of oxidation products.
    A higher degree of oxidation in oil paints after exposure to ozone was detected also
by Py-GC-MS, as visible in Figure 5 which displays the pyrogram of Winton yellow oil
(OY) paint after transesterification and chromatographic analysis. Besides the charac-
teristic drying oil components (saturated fatty acids such as palmitic, stearic, myristic,
lauric acids; unsaturated fatty acids such as oleic acid), several oxidation compounds
due to the oxidative degradation of the original triglycerides were identified. They are
mainly dicarboxylic acids (such as azelaic, suberic, sebacic, pimelic acids) and oxi-
dised octadecanoic acid compounds (oxo-, epoxy-, dimethoxy-octadecanoic acids)
[32].




                                                                                                    361
      F.C. Izzo, E. Balliana, F. Pinton, E. Zendri - A preliminary study of the composition of commercial oil , acrylic and vinyl paintS




362
           Py-GC-MS analysis.
           Figure 5. Total ion current pyrogram of Winton yellow oil (OY) paint after transesterification and
   For all the yellow paints, variations in the FTIR absorption bands were also ob-
served in the region between 1600-1000 cm-1 corresponding to the ‘finger print’ region
of yellow azo pigments, possibly due to the oxidation of the organic dyestuff as can
be seen also in the Py-GC-MS [2]. This trend was less evident for the vinyl and acryl
paints probably due to the greater stability of the polymer binders compared to the oil
when exposed to a strong oxidizing agent.
   The ozone treatment does not seem to have any effect on the inorganic green and
white pigments.

   3.2.2 Thermal ageing (T)


    Thermal treatment (100°C for 24 hours) clearly affects the visual appearance of the
paint films, in particular that of the oil paints. Partial melting of the films and separation
of the binders from the paint bulk was in fact observed for all the oil paints.
    Acrylic and vinyl films are less affected by the thermal treatment compared with
untreated samples.
    Table 3 reports the mass variation of the treated paint films after thermal treatment.
All samples exhibited a common mass loss associated with the evaporation of volatile
products and water.
    The mass variations were generally higher for the yellow paints, since organic
dyestuff is more sensitive to heat treatment than titanium and chromium oxide based
paints [33, 34].
    Of all the analyzed paints, acrylic emulsions clearly present the highest variation in
mass up to -7.6 % due to their higher water content.

Table 3. Mass variations (%) of paint films after thermal ageing at 100°C for 24 hours and after
treatment with relative humidity at 83% for 7 days.




                                                                                                         CONSERVATION SCIENCE IN CULTURAL HERITAGE
                                               Mass variations (%)
     Samples                  After T ageing                         After RH ageing
       OG                          -2.15                                   +0.25
       OW                          -1.73                                   +0.56
       OY                          -7.50                                   +1.34
       AG                          -3.86                                   +3.64
       AW                          -3.01                                   +3.01
       AY                          -7.62                                   +7.21
       VG                          -2.08                                   +2.22
       VW                          -2.92                                   +2.25
       VY                          -3.53                                   +2.91




                                                                                                   363
                                                                                                                                         Figure 6 illustrates the comparison of the FTIR spectra obtained before and after
                                                                                                                                      thermal treatment for the different white paint formulations (OW, AW, VW), here re-
                                                                                                                                      ported as a representative example.
F.C. Izzo, E. Balliana, F. Pinton, E. Zendri - A preliminary study of the composition of commercial oil , acrylic and vinyl paintS




                                                                                                                                      Figure 6. ATR-FTIR spectra of Winton (OW), Liquitex (AW) and Flashe (VW) white paints before
                                                                                                                                      and after T treatment

                                                                                                                                         Thermal ageing seems to have a greater effect in the case of vinyl and oil paints
                                                                                                                                      even though evident changes can also be observed in acrylic paints. The Flashe paint
                                                                                                                                      (VW) exhibits a decrease in the intensity of the binder absorption peaks (CH stretching
                                                                                                                                      at around 2900 cm-1, C=O stretching at 1730 cm-1, 1225 cm-1).
                                                                                                                                         In addition to the already observed variations in the binder peaks [31], the formation
                                                                                                                                      of metal carboxylates was observed in the oil paints, shown by the typical absorption
                                                                                                                                      peak at around 1590 cm-1. As evidenced by XRF analysis, ZnO is present together with
                                                                                                                                      TiO2 in the composition of the Winton white paint (OW). The formation of zinc carboxy-
                                                                                                                                      lates in modern and contemporary oil paints seems to stem from a common process
                                                                                                                                      of degradation as previously stated, especially when metal soaps and extra fatty acid
                                                                                                                                      sources are added as dispersing agents [35].

                                                                                                                                         3.2.3 Humidity treatment

                                                                                                                                         Changes in the visual appearance of the paint films were recorded in particular
                                                                                                                                      for acrylic and vinyl formulations after exposure for 7 days in sealed saturated salt
                                                                                                                                      chambers at 83% RH. Acrylic films visually increased their glossiness while vinyl films
                                                                                                                                      increased their matte appearance.
                                                                                                                                         All the paints changed in weight as a consequence of the humidity treatment (see
                                                                                                                                      Table 3). In general, these changes were more marked for the acrylic and vinyl films
                                                                                                                                      as they are more sensitive to humidity than oils [18, 23, 25]. As expected, at 83% RH
                                                                                                                                      acrylic films show the highest mass increase up to 7.2 % (Liquitex yellow paint, AY)
                                                                                                                                      compared to the untreated ones.
                                                                                                                                         Any relevant chemical variations occurred after humidity treatments for acrylic and
                                                                                                                                      vinyl paints. However, as already seen for the thermal treatments, the high level of
                                                                                                                                      humidity seems to promote the formation of metal carboxylates for all analysed oil
                                                                                                                                      formulations [19, 28].



                                                                                                                                364
   3.2.4 Total colour changes

   Figure 7 shows the overall colour changes of the paint films, calculated by compar-
ing the colorimetric parameters before and after ageing treatment and expressed as
∆E.
   Chromatic variations are clearly visible in the case of the Winton white and yellow oil
paints which were respectively yellowed and more saturated due to the red component
(a*).
   Minor chromatic variations were detected for ozone treatments. On the other hand,
when the paints were subjected to T and RH treatments, the high ∆E values recorded
were significant, in particular for the Winton white (OW) and yellow (OY) paints.
   All the white paints show an increase of the yellow coordinate (b*>0) while the a*
coordinate becomes higher corresponding to a decrease of the green component in
the paint colour. The Winton white paint films (OW) present the most significant vari-
ations for all the chromatic coordinates: L*, a* e b*. Finally, all the yellow paint films
show a similar trend with a decrease of the yellow coordinate (b*>0) and more positive
a* values corresponding to a shift towards red.




                                                                                                   CONSERVATION SCIENCE IN CULTURAL HERITAGE

Figure 7. total colour changes (ΔE) of the paint films after ageing treatments



   4. Conclusions and future perspectives

   This preliminary study provides important information on the composition and be-
haviour of some commercially available paints, among those most frequently used by
young Venetian artists and students from the Academy of Fine Arts: Winton oil paints
(Winsor & Newton, UK), Heavy Body acrylic paints (Liquitex, USA) and Flashe vinyl
paints (Lefranc &Bourgeois, France).
   As expected, the commercial paints investigated show quite complex formulations,
where- in addition to the binder and pigments declared in the producers’ technical
datasheets- several other products were detected:



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                                                                                                                                      - Liquitex acrylic paints contain n-butyl acrylate and methyl-methacrylate (n-BA/
                                                                                                                                        MMA); Flashe vinyl paints are emulsions of poly-vinyl acetate/vinyl versatate (PVA/
                                                                                                                                        VeoVA); Winton oil paints contain linseed oil, but in the case of the white paint (OW)
                                                                                                                                        the binding medium is composed of a mixture of linseed oil and safflower oil, the
                                                                                                                                        latter being a semi-drying oil;
                                                                                                                                      - additives such as castor wax in the oil paints, dibutylthiocarbamate as a stabilizer
F.C. Izzo, E. Balliana, F. Pinton, E. Zendri - A preliminary study of the composition of commercial oil , acrylic and vinyl paintS




                                                                                                                                        in the acrylic emulsions and diisooctyl phthalate as a plasticiser in the vinyl disper-
                                                                                                                                        sions;
                                                                                                                                      - calcium carbonate was found in the Winton and Liquitex paints as an extender.


                                                                                                                                          The paints were subjected to different accelerated ageing treatments. The results
                                                                                                                                      indicate different behaviours in relation both to the type of binder and pigments present
                                                                                                                                      in the different formulations. The study has moreover underlined how not only the bind-
                                                                                                                                      ing media but the pigments too, are important factors for evaluating the drying behav-
                                                                                                                                      iour, stability and interactions with respect to the studied range of T, RH and O3 level.
                                                                                                                                          The results obtained from studying the paint films showed that ozone plays an im-
                                                                                                                                      portant role in the stability of oil paints, in particular those containing the yellow azo-
                                                                                                                                      pigment (OY); on the other hand, acryl and vinyl paints show minimum variations when
                                                                                                                                      subjected to ozone treatment. Temperature, as expected, significantly affected all the
                                                                                                                                      commercial paints resulting in the formation of degradation products and visible al-
                                                                                                                                      terations in the appearance of the paints’ aspects. On the other hand, humidity clearly
                                                                                                                                      affected acrylic and vinyl paints, commercially found as water emulsions, as was evi-
                                                                                                                                      dent in particular in the mass variations.
                                                                                                                                          The influence of the ageing treatments also resulted in clear changes in the paint
                                                                                                                                      colorimetric coordinates, in particular for the white and yellow oil paints.
                                                                                                                                          The results of this study can contribute to promoting the development of preventive
                                                                                                                                      plans and monitoring campaigns for the conservation of contemporary paintings, pay-
                                                                                                                                      ing particular attention to environmental parameters that might influence the stability
                                                                                                                                      of the different commercial colours in relation to their chemical composition (binding
                                                                                                                                      media, pigments, additives).


                                                                                                                                         Acknowledgements

                                                                                                                                         The authors express their thanks to Dr. Henk van Keulen of the Cultural Heritage
                                                                                                                                      Agency of The Netherlands (RCE) for assisting in the Py-GC-MS data interpretations,
                                                                                                                                      Dr. Laura Falchi of Ca’ Foscari University for her patient help in reviewing the final
                                                                                                                                      aspect for the graphs.




                                                                                                                                366
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  Biographical Notes

   Francesca Caterina Izzo graduated in Chemical Sciences and Technologies for
the Conservation of Cultural Heritage at Ca’ Foscari University of Venice in 2007, with
a master thesis related to Modern and Contemporary Italian Mural Painting. During her
European PhD in Chemical Sciences, she worked at RCE (The Netherlands Cultural
Heritage Agency), collaborating on the Modern Oil Project. From 2012 she has been
working as a researcher in Conservation Science for Contemporary Art in Venice, col-
laborating with several Italian and international organizations and institutions. She has
been involved in several European projects dealing with the Science, Education and
Conservation of Cultural Heritage.

   Eleonora Balliana obtained a Degree in Conservation of stone materials from the
International University of Art of Venice in 1997. She graduated in Chemical Sciences
and Technologies for Conservation and Restoration at Ca’ Foscari University of Venice
in 2007. Since 2009 she has been a researcher in Conservation Science at Ca Foscari
University. Since then she has been working in the Laboratory of Conservation Sci-
ences for Cultural Heritage. Her topics of interest include the study of art materials and
their interaction with the environment, and the development and characterization of
new low-impact and sustainable solutions for the cultural heritage sector.

   Federica Pinton graduated in Chemical Sciences for Conservation and Restora-
tion at Ca’ Foscari University of Venice in 2012, with a master thesis related to the
degradation of contemporary commercial paints. After her degree, she took part in an
internal internship in the group of Conservation Science for Cultural Heritage held by
Prof. Elizabetta Zendri regarding the degradation of contemporary commercial paints
and their behavior indoors, in collaboration with the Galleria Internazionale d’Arte Mod-




                                                                                                   CONSERVATION SCIENCE IN CULTURAL HERITAGE
erna of Venice.

   Elisabetta Zendri graduated in Industrial Chemistry in 1988 at the Ca’ Foscari Uni-
versity of Venice. Since 2001 she has been associate professor in the same University.
She has been in charge of several projects: MIUR-COFIN, two MIUR-PRIN projects on
materials and technologies for the conservation of modern painting and a JPI project
. She is the scientific coordinator for Ca’ Foscari of the MACC Modern Art Conserva-
tion Centre. Her research activities are developed in collaboration with national and
international Universities. She has published more than 150 publications in national
and international journals.




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