Microstructural Characteristics of Paint Pigments


Kevin Kenific
kk005j@mail.rochester.edu

University of Rochester
Department of Optics

Introduction:

The art of painting is essentially the organization of colors on a surface in order to depict or convey some situation or idea. Like cooking, painting is both an art and a science. Aside from the large number of questions regarding form and content, the physical constituency of a painting must be considered at length. Choice of surface, surface preparation, the color application process, and most importantly, the colors themselves are all aspects of the mechanics of painting that deserve much attention because they are inextricable from ideas of form and content as they are the means by which these things are conveyed.

The chemistry of painting has long been a subject of investigation in order to determine the most permanent pigments and most permanent pigment combinations. In this project, artists' pigments are compared to student grade pigments in order to determine differences in chemical composition and differences in the physical structure of pigment particles. Comparison was also done between pigments used for under painting and over painting.

 

Methods:

Pigments were obtained from oil paint by washing the paint in turpentine to strip the oil from the pigment. The residual turpentine and oil were removed with acetone. Samples of each pigment were then attached to sample stubs with carbon tape and sputtered with gold to increase their conductivity. Each sample was then imaged and compositional analysis was conducted using EDAX and ZAF correction. Imaging conditions varied so as to reduce charging, which proved to be the main obstacle in obtaining images of the pigments. The most common imaging conditions consisted of acceleration voltages less than 5 kV, aperture setting 1, and a slow scan rate, or the use of TV integration mode. EDAX analysis proved far easier. The working distance was set to 20 mm with a magnification of 10,000X or 20,000X. The aperture setting was 2 so as to increase the x-ray count. Each pigment analysis was done three times in different regions of the specimen so as to reduce the effects of anomalies.

 

Titanium White vs. Zinc White

Figure 1. Titanium white (left) and zinc white (right) both at 20,000x.

Figure 2. EDAX spectra for titanium and zinc white.

 

Titanium white is intended for under painting while zinc white is used for over painting. or glazes. This is determined by the oil absorption and opacity of these pigments. Pigments suited for under painting are characterized by a low oil absorption and high opacity. Oil paintings traditionally start with layers with minimal oil content with each successive layer having increasing oil content. This arrangement prevents the paint surface from cracking as the layers oxidize. As oil oxidizes it expands. If a paint with high oil content is used beneath one with a lower oil content it will expand more than the the surface layer causing it to crack. Oil absorption is a function of particle size. As particle size decreases, the total surface area of a volume of pigment increases. Oil bonds to the surface of the pigments and more is required as surface area is increased. The degree of opacity is determined by a pigment's ability to scatter light, which is really just the pigment's refractive index. Titanium dioxide has a refractive index of 2.72 [1] while that of ZnO is 2.01.

Figure 2 shows that titanium white is composed of much more than titanium dioxide crystals. Calcium, zinc, and magnesium are added to decrease the tendency of the paint to yellow with age and exposure. Aluminum is added to modify the surface polarity of the crystals so that they disperse more uniformly in a nonpolar vehicle-linseed oil.The carbon detected is likely the result of residual oil and the carbon tape.[2]

On the other hand, zinc white is a very pure, untreated pigment. This is basically because ZnO disperses well in oil in accord with its high oil absorption and because it has a minimal tendency to yellow.

 

Cadmium Red vs. Cadmium Red Hue

 

Figure 3. Cadmium red (left) at 10,000x and cadmium red hue (right) at 3,000x.

Figure 4. EDX spectra for cadmium red and cadmium red hue.

 

Cadmium red hue is a student grade pigment and on average costs about $7 for a 75 mL tube while artists' grade cadmium red is approximately $17 for 75 mL from Windsor and Newton, the largest oil paint manufacturer. Figures 3 and 4 indicate stark differences between these pigments. Cadmium red is an inorganic pigment consisting mostly of the mineral cadmium sulfide, CdS. However, selenium, a member of the same group in the periodic table as sulfur, may serve as a substitute for sulfur in the crystal lattice roughly maintaining the physical characteristics of the crystal while simultaneously altering its hue. The variation of the ratio of cadmium to selenium allows a range of colors from orange to dark maroon. Cadmium sulfoselenide crystals measure .5 microns on average. [3]

Cadmium red hue is an organic pigment. This is indicated by the large amounts of carbon and oxygen present. Windsor and Newton lists this organic pigment as beta naphthol. [4]Typically calcium carbonate (chalk) is added to this cadmium red hue as an extender to reduce production costs. However, in America pure chalk is uncommon, so a variation of limestone, CaC03.MgCaCO3, is used instead The image of cadmium red hue shows the limestone crystals which measure approximately 1 micron across.[5]

Each imaged was colorized using the Windsor and Newton color charts to obtain the actual color of each pigment.

Cadmium Yellow vs. Cadmium Yellow Hue

Figure 5. Cadmium yellow (left) at 30,000x and cadmium yellow hue (right) at 10,000x

Figure 6. EDX spectra for cadmium yellow and cadmium yellow hue.

Cadmium yellow is compositionally similar to cadmium red as is cadmium yellow hue to cadmium red hue. Cadmium yellow is actually the pure form of cadmium sulfate. The addition of selenium to CdS is what creates cadmium red. As a result, no selenium appears in the EDX spectrum for cadmium yellow as it does for cadmium red. The images of each pigment are also nearly identical owing to the fact that selenium is in the same periodic group as sulfur and does not affect the cadmium sulfate crystal structure significantly.

A comparison of the cadmium red and yellow hues reveals many similarities as well. Cadmium yellow hue results from an organic pigment called arylide. Both beta napthol and arylide are molecules composed of carbon rings joined by nitrogen with chlorine atoms attached to them. Napthol has four carbon rings, and arylide has two. [6] Unfortunately, comparisons of the EDX spectra of the two hues do not reflect the difference in carbon or hydrogen content, nor was chlorine or nitrogen detected. This is likely a result of sample preparation. EDX spectra for each hue were also generated with the pigments in binder and show traces of both chlorine and nitrogen. However, a comparison of hydrogen and carbon content would prove inaccurate as oil, which is nothing but hydro-carbon chains, was present.

Figure 7. Napthol molecule (left) and an arylide molecule (right). [6]

All cadmium colors are best suited for under painting due to high refractive index and low oil absorption.

Cobalt Blue vs. Cobalt Blue Hue

Figure 8. Cobalt blue (left) at 10,000x and cobalt blue hue (right) at 5000x.

Figure 9. EDX spectra for cobalt blue and cobalt blue hue.

Cobalt blue is a mix of cobalt oxide and aluminum oxide (CoO.Al2O3).[7] Its refractive index is 1.74 [8] and has a high oil content. Cobalt blue hue is composed of copper phthalocyanine, zinc oxide and sodium alumino-silicate with sulfur. [4] Phthalocyanine is an organic compound composed of carbon rings bonded together with nitrogen. The EDX spectra for cobalt blue hue shows no nitrogen. This was perhaps stripped away during sample preparation. Copper is not evident either. This is likely the result of lack enough over voltage to give the 9 eV needed to stimulate x-ray emission from copper. The presence of magnesium and copper suggests that as with the cadmium yellow and red hues, limestone was used as an extender. The image of cobalt blue hue shows structural similarities to the cadmium red and yellow hues, both of which definitely contain limestone. Cobalt blue hue differs from cobalt blue in one important way. Unlike cobalt blue, cobalt blue hue is actually an opaque paint. This is a consequence of the presence of a limestone extender, zinc, silicon, and sulfur.

Conclusion:

The chemistry of paint pigments proves to be as vast and as varied as painting itself. The need for quality pigments as well as inexpensive ones has yielded unique solutions that at times maintain long held standards (many of these pigments have been in use for centuries) as well as creating new ones that may be just as useful. Many of these new substitutes, although not quite the same as the original colors, can be just as effective in the hands of a skilled colorist when manipulated properly. .

 

Works Cited:

[1] Feller, Robert. Artists' Pigments: A Handbook of their History and Characteristics. Volume 1. pp 172-174. Cambridge University Press, 1986.

[2] Feller, Robert. Artists' Pigments: A Handbook of their History and Characteristics. Volume 3. pp 302, 308-316. Cambridge University Press, 1997.

[3] Feller, Robert. Artists' Pigments: A Handbook of their History and Characteristics. Volume 1. pp 65-81. Cambridge University Press, 1986.

[4] Windsor and Newton, www.windsornewton.com

[5] Feller, Robert. Artists' Pigments: A Handbook of their History and Characteristics. Volume 2. pp 203-204. Cambridge University Press, 1992.

[6] Synthetic Organic Pigments, http://www.handprint.com/HP/WCL/pigmt1d.html

[7] http://webexhibits.org/pigments/indiv/history/coblue.html

[8] Feller, Robert. Artists' Pigments: A Handbook of their History and Characteristics. Volume 2. pg. 122. Cambridge University Press, 1992.

 

 

 

 

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