The application of x-ray powder diffraction for the analysis of synthetic organic pigments. Part 2: artists' paints.
Keywords X-ray powder diffraction, Synthetic organic pigment, Paint binder
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.
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.
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
J.Coat. Technol. Res., 7 (3) 325-330-2010
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|Author:||Lomax, Suzanne Quillen|
|Date:||May 1, 2010|
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