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The application of x-ray powder diffraction for the analysis of synthetic organic pigments. Part 2: artists' paints.

Abstract Over 50 commercial paints were examined by x-ray powder diffraction in order to try and identify the synthetic organic pigments present. The binders included acrylic, oil, gum, and alkyd. Some pigments could be identified, though analysis is often complicated by the presence of large amounts of fillers and extenders in the paints relative to the small quantities of the pigment. A few of the paints did not have reflections due to fillers or extenders but the pigments could still not be identified. The best success in identifying the pigments was with acrylic binders, where the pigments could be identified in more than half of the samples examined, and with alkyds, where the pigment could frequently be identified. However, other binders, especially oil and gum, contain so many fillers that the pigment reflections are obscured. X-ray powder diffraction, therefore, is of limited utility in the identification of synthetic organic pigments in paints.

Keywords X-ray powder diffraction, Synthetic organic pigment, Paint binder

Introduction

The first paper in this series presented x-ray powder diffraction data for over 200 samples of synthetic organic pigments. The majority of these dry pigments diffracted, and they had characteristic powder diffraction patterns. Diffraction was very useful in distinguishing pigments within a class that are extremely similar in structure. In addition to being used to identify the pigments, the patterns were often useful in identifying polymorphic forms of a pigment.

Although powder diffraction is helpful for identification of pigments, occurrences of dry, isolated pigments are not generally found on works of art, but are rather part of a paint system. Therefore, the study was expanded to encompass artists' paints. Over 50 paint samples were collected from several artists' paint suppliers, including a variety of media such as drying oil, acrylic, alkyd, and plant gum. The x-ray powder diffraction spectra were obtained and compared to those of the expected pigments.

Experimental

X-ray diffraction patterns were obtained using a Bruker D8 DISCOVER with GADDS microdiffractometer equipped with a Hi-Star area detector. The paints were spread onto glass slides and allowed to dry. Flakes of dried paint were used directly without grinding. Each sample was placed onto the surface of a zero background plate that was subsequently centered on the stage. Correct placement of the sample in XYZ space was achieved using a video microscope with laser assist focus. Beam conditions included a Cu anode at 45 kV and 35 mA to produce Cu K[alpha] radiation ([lambda] = 1.542 [Angstrom]) through a 500 [micro]m collimator in air. Reflections were collected using a two-dimensional general area diffraction detection system (GADDS) set up for a single run, five frames, coupled (step) mode, with XY oscillation (0.1 mm amplitude). Theta 1 and 2 starting angles were 8 and 8 with a frame width of 6. Runtime for each frame was 45 s. Debye ring data were integrated over chi. Integrated frame data were combined and background corrected using EVA (a Bruker proprietary diffraction pattern evaluation program). Phase identification was accomplished using EVA through a link to the International Center for Diffraction Data (ICDD) database on CD (Powder Diffraction File Release 2000). Patterns of pigments not in the ICDD were compared to patterns collected of the dry pigments. Corundum was used to calibrate the instrument. Relative intensities were calculated from peak heights.

Results and discussion

The diffraction data are listed in Table 1. The data are arranged by binder in order to make it easier to interpret. A diffraction pattern was obtained with almost all of the paint samples. Binders listed as oil were either linseed or safflower oil. Gum binders were gum arabic.
Table 1: Diffraction data for paints containing synthetic organic
pigments

Manufacturer        Binder        Paint name           Pigments

Golden              Acrylic  Permanent Green       PY3, PG7
                             Light

                    Acrylic  Turquois              PG7, PB15:4

                    Acrylic  Phthalo Blue (Green   PB15:4
                             Shade)

                    Acrylic  Dioxazine Purple      PV23

                    Acrylic  Quinacridone Magenta  PR122

                    Acrylic  Jenkins Green         PG36, PY150, PBk9

                    Acrylic  Quinacridone Burnt    PR206
                             Orange

                    Acrylic  Cobalt Violet Hue     PV19, PR122, PV23

                    Acrylic  Hookers Green Hue     PB60, PY150,
                                                   PR122

                    Acrylic  Diarylide Yellow      PY83

                    Acrylic  Hansa Yellow Light    PY3

Golden Open         Acrylic  Sap Green Hue         PY150, PG36, PBk7

                    Acrylic  Viridian Green Hue    PB15:4, PY150,
                                                   PBk7, PW4

                    Acrylic  Alizarin Crimson Hue  PR122, PR206, PG7

CAS                 Alkyd    Perylene Red          PR179

                    Alkyd    Quinacridone Burnt    PO48
                             Orange

                    Alkyd    Phthalo Green         PG7

                    Alkyd    Quinacridone Magenta  PR122

                    Alkyd    Pyrrol Red            PR254

Winsor & Newton     Alkyd    Scarlet Lake          PR188

                    Alkyd    Permanent Alizarin    PR177
                             Crimson

                    Alkyd    Magenta               PR122, PB15

Gamblin             Oil      Chromatic Black       PG36, PV19

Sennelier           Oil      Sennelier Red         PR255

                    Oil      Permanent Alizarin    PR209, PR179,
                             Crimson               PR202

                    Oil      Chinese Orange        PY13, PY83

                    Oil      Sap Green             PG36

                    Oil      Greenish Umber        PY83, PB60, PBk7

Old Holland         Oil      Scheveningen Green    PG36

                    Oil      Scheveningen Yellow   PY74
                             Lt.

                    Oil      Gamboge Lake Extra    PY95, PY129

                    Oil      Ruby Lake             PR209, PR168

                    Oil      Burgundy Wine Red     PR177

Gamblin             Oil      Quinacridone Magenta  PR122

Talens Van Gogh     Oil      Sap Green             PG7, PY17

Gamblin             Oil      Quinacridone Red      PV19

Sennelier           Oil      Turner Yellow         PY93

Liquitex            Oil      Hansa Yellow Light    PY3

Schmincke           Oil      Vermilion Red Tone    PR255

Talens              Oil      Cadmium Red Azo       PO43, PR57:1

Winsor & Newton     Oil      Bright Red, New       PR254
                             Formulation

                    Oil      Bright Red, Old       PR170
                             Formulation

                    Oil      Winsor Blue, Red      PB15
                             Shade

Permanent Pigments  Oil      Hansa Yellow Medium   PY1

Winsor & Newton     Oil (b)  Lemon Yellow          PY3

                    Gum      New Gamboge           PY153

                    Gum      Winsor Yellow         PY154

                    Gum      Perylene Maroon       PR179

                    Gum      Quinacridone Magenta  PR122

                    Gum      Winsor Blue (Green    PB15
                             shade)

Pebeo               Gum      Superfine Carmine     PR5, PR146

Schmicke Horadam    Gum      Ruby Red              PV19

Manufacturer        Binder             Pigments found

Golden              Acrylic  PY3, Ba[SO.sub.4]

                    Acrylic  PG7, PB15:4

                    Acrylic  PB15:4

                    Acrylic  PV23

                    Acrylic  PR122

                    Acrylic  -

                    Acrylic  PR206

                    Acrylic  ZnO

                    Acrylic  PB60

                    Acrylic  PY83

                    Acrylic  PY3

Golden Open         Acrylic  -

                    Acrylic  ZnO

                    Acrylic  PR122, PG7

CAS                 Alkyd    PR179

                    Alkyd    PO48

                    Alkyd    PG7

                    Alkyd    PR122

                    Alkyd    PR254

Winsor & Newton     Alkyd    CaMg[([CO.sub.3]).sub.2]

                    Alkyd    Ca[CO.sub.3]

                    Alkyd    CaMg[([CO.sub.3]).sub.2]

Gamblin             Oil      AI[(OH).sub.3]

Sennelier           Oil      [BaSO.sub.4]

                    Oil      -

                    Oil      Ba[SO.sub.4]

                    Oil      PG36, Ba[SO.sub.4]

                    Oil      Ba[SO.sub.4]

Old Holland         Oil      CaMg[([CO.sub.3]).sub.2]

                    Oil      CaMg[([CO.sub.3]).sub.2]

                    Oil      a

                    Oil      AI[(OH).sub.3]

                    Oil      CaMg[([CO.sub.3]).sub.2]

Gamblin             Oil      Ca[CO.sub.3] + AI[(OH).sub.3]

Talens Van Gogh     Oil      -

Gamblin             Oil      AI[(OH).sub.3]

Sennelier           Oil      Ba[SO.sub.4]

Liquitex            Oil      Ba[SO.sub.4]

Schmincke           Oil      Ba[SO.sub.4]

Talens              Oil      Ca[CO.sub.3]

Winsor & Newton     Oil      Ba[SO.sub.4]

                    Oil      Ba[SO.sub.4]

                    Oil      PB15 + Ba[SO.sub.4]

Permanent Pigments  Oil      PY1 + Ba[SO.sub.4]

Winsor & Newton     Oil (b)  PY3

                    Gum      PY153 + Ca[SO.sub.4] * 2[H.sub.2]O

                    Gum      PY154 + Ca[SO.sub.4] * 2[H.sub.2]O

                    Gum      Ca[SO.sub.4] * 2[H.sub.2]O

                    Gum      Ca[SO.sub.4] * 2[H.sub.2]O

                    Gum      Ca[SO.sub.4] * 2[H.sub.2]O

Pebeo               Gum      Ba[SO.sub.4]

Schmicke Horadam    Gum      Ba[SO.sub.4]

(a) Pattern did not match PY129, sample of PY95 not available for
comparison
(b) Water mixable oil


Artists' paints are made up of a number of components that may include, but are not limited to, a binder, solvents, resins, plasticizers, pigments, and extenders. Pigments are responsible for the color of the paint while extenders contribute to the rheological properties, adhesion, and film strength of the paint. In general, extenders are less expensive that the primary pigments. Their inclusion in paint makes it possible to use less of the more costly colorant. The amount of synthetic organic pigment in an artists' paint varies depending on the binder, tinting strength, and the grade of paint (student vs artists' grade) but is usually between 10% and 25% by weight. *

Artists' paints that conform to ASTM D 4302 (standard specification for artists' oil, resin-oil and alkyd paints), ASTM D 5098 (standard specification for artists' acrylic emulsion paints), ASTM D 5724 (standard specification for artists' gouache paints) or ASTM D 5067 (standard specification for artists' watercolor paints) list the Colour Index name for the pigment(s) present, but do not list the extenders. Therefore, as the paints examined in this study conformed to the ASTM labeling, the manufacturer's label was used to determine which pigments were present in a given artists' paint.

Pigments could usually be identified in the acrylic paints examined. For example, Fig. 1 shows the diffraction pattern for Golden Quinacridone Magenta. ** The pigment, PR122, is clearly visible in the sample, and there are no fillers and extenders that contribute to the diffraction pattern. However, in some cases, in spite of not having fillers and extenders, the pigment is not observed. In these cases, perhaps the high tinting strength of the pigments means that the amount of pigment present is not detected by diffraction. For example, the x-ray diffraction pattern of Golden Jenkins Green, supposed to contain PG36, PY150, and PBk9, does not have any reflections attributed to the pigments, but none due to fillers, either.

[FIGURE 1 OMITTED]

In general, fewer types of filler were found in the acrylic binder samples. Nevertheless, occasionally other materials obscure the reflections of the pigments. As shown in Fig. 2, Golden Acrylic Cobalt Violet hue contains PV19, PV23, and PR122. However, none of the pigments could be observed, but rather zinc oxide is the predominant reflecting species in the sample. One acrylic sample, Golden Permanent Green Light, contain barite as a filler.

[FIGURE 2 OMITTED]

In addition, with acrylic paints, there is sometimes difficulty in seeing all of the components when there is a mixture of pigments. For example, in the case of the Golden Open Acrylic Alizarin Crimson hue, PG7 and PR122 were found, but not the PR206 expected as well. The sample of Golden Acrylic Hookers Green Hue, containing PB60, PY150, and PR122, only showed PB60 in the powder diffraction pattern.

Alkyd paints gave patterns similar to those found with the acrylic paints. The CAS alkyd paints were all single pigment materials. All of the CAS alkyd paints produced powder diffraction patterns containing the expected pigments. However, this is not the case with the Winsor & Newton Griffin Alkyds. Although two of the three samples are one component paints, they all show only the presence of fillers. For example, Fig. 3 shows Winsor & Newton Griffin Alkyd Magenta, which contains PR122 and PB15. Only dolomite is found in the sample. The other Griffin samples also produced patterns of fillers, where the pigments were not observed.

[FIGURE 3 OMITTED]

Oil paints produced mixed results, but in general, it was much more difficult to identify the pigments than with alkyd and acrylic paints. Oil paints appear to contain more fillers that obscure pigment reflections. These fillers include barite, dolomite, calcite, gypsum, and alumina. For example, Fig. 4 shows the powder diffraction pattern of Sennelier Turner Yellow (bound in safflower oil), which only has reflections for barite. This was the case with almost all of the oil containing paints, and was the same whether the paint was bound in linseed or safflower oil. There were very few cases where reflections due the pigment could be observed. For example, Fig. 5 shows the diffraction pattern obtained from Permanent Pigments Oil Hansa Yellow Medium, which contains PY1. The pigment is present in very small quantities, with the major lines due to the barite filler.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Gum binders presented similar challenges to oil-containing binders. Fillers, including gypsum and barite, were found in all of the samples. In only two cases out of seven were the pigments able to be identified. Figure 6 shows the x-ray powder diffraction pattern for Winsor & Newton Winsor Yellow, containing PY154. The pigment can be observed in the presence of the gypsum filler.

[FIGURE 6 OMITTED]

Almost all of the commercial artists' paints examined were found to contain large amounts of fillers and extenders. Many of these contain heavy metals, such as barium and zinc, but also other inorganic materials containing aluminum and calcium. Due to their good x-ray scattering, and also the fact that they are present in substantial quantities, these extenders contribute to the difficulty in observing the expected pigments in the paints.

Spectral subtraction would be of limited utility in examining these paints. As Figs. 2-5 demonstrate, a subtraction of the diffraction pattern of the observed extender would lead essentially to a baseline. A better approach to examining these pigments in paint systems might be to perform an initial separation to try and remove the extenders. There has been some success with dissolving monoarylide yellow pigments, such as PY1 and PY3, leaving behind the extenders. However, this pigment class is, in general, quite soluble in most solvents except for aliphatic hydrocarbons. There has been less success with trying to remove the extenders with other pigment classes, although this work is continuing.

Conclusion

X-ray powder diffraction was employed to try and identify pigments in artists' commercial paints. Over 50 commercial paint samples were examined representing a variety of pigments and binding media. The technique proved to be of greatest utility with acrylic and alkyd binders, where the pigments could be identified in many of the samples. However, in the case of oil-containing binders, the x-ray powder diffraction patterns contain reflections due to fillers such as barite, calcite, dolomite, gypsum, and alumina. In general, it was very difficult to see reflections due to pigments. The same was true for gum-containing binders. X-ray powder diffraction would be of limited utility for the identification of synthetic organic pigments in paint systems where either oil or gum is used as binders.

* Personal communication with Bob Gamblin, Gamblin Artists Colors, Portland, Oregon. Also, information provided relating to a Golden Acrylic paint by Gregory Smith, Andrew W. Mellon Professor of Conservation Science, Buffalo State University, Buffalo, NY.

** Peaks labeled x in the figures correspond to the pigment or extender found. These assignments were based on comparisons with reference materials in the ICDD database.

S. Q. Lomax (*)

Scientific Research Department, National Gallery of Art, DCL-SR, 2000B South Club Drive, Landover, MD 20785, USA

e-mail: s-lomax@nga.gov

J.Coat. Technol. Res., 7 (3) 325-330-2010

DOI 10.1007/s11998-009-9205-1
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Author:Lomax, Suzanne Quillen
Publication:JCT Research
Date:May 1, 2010
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