A new paper released by scientists at the Smithsonian's Museum Conservation Institute exposes long-term problems with zinc white in oil paint. The report "The Chemical and Mechanical Effects of Pigments on Drying Oils" describes the highlights of a 28-year study on the stability and strength of oil paint films. The results are revealing and have important implications for artwork made with artists' oil paints containing zinc white.

The paper by Marion Mecklenburg and Charles Tumosa examines the drying of different drying oils, traditional pigments and commercially prepared artists' oil paints. It also studies the differences of the effects of pigments on the long-term stiffening and film formation of paints. For the study, different pigments were ground in different oils, applied to polyester films and then stored in a controlled environment. Over the years, the paints were removed from the polyester film and tested mechanically as unsupported thin films.

Fig. 1 Detail of a painting by Henry Cliffe, painted in 1959, shows severe cracking and interlayer cleavage in paint containing lead white and zinc white.

Zinc White Causes Oil Paint to Become Brittle

Among the results, several white pigments were studied, including lead white (basic lead carbonate), titanium white (titanium dioxide) and zinc white (zinc oxide). What is of interest about the paints made with zinc oxide and drying oils is that they become extremely brittle in as little as three years. The report shows that a 14.5-year-old paint snaps like glass when bent. It compares a 14-year-old paint made with lead white and linseed oil, which can elongate 4.5% before breaking, to zinc white paint of the same age that can only elongate 0.3%.[1]

Another startling fact revealed by the report about paints made with zinc white -- they peel off. Paint layers containing zinc white delaminate. The photograph of a painting by Henry Cliffe (Fig. 1) shows this damage resulting from rolling the painting.[2]

The delamination is not limited to layers of oil paint. The paper describes the test of a large number of commercial acrylic emulsion grounds upon which both commercial and custom manufactured oil and alkyds paints were applied. The commercial lead white and titanium white oil paints containing zinc white became brittle after seven years of drying, and the paints with zinc white delaminated (Fig. 2). Paints containing zinc white were easily and cleanly separated from acrylic emulsion grounds.[3]

Fig. 2 Commercial acrylic ground underneath a layer of commercial lead white oil paint containing zinc oxide. With little effort the paint could be peeled away from the acrylic ground.

The brittle paint film and delamination observed in the tests by Mecklenburg and Tumosa are not confined to zinc white by itself in oil. The authors of the paper tested zinc oxide combined with other white pigments, such as lead white and titanium white.[4]

Should Artists Using White Oil Paint Be Concerned?

The results of the study highlight a major concern for artists today. The majority of white oil paint available today includes zinc white. This may point to the early failure of many artworks in the future. Table 1 lists the pigments and drying oils included in commercial oil paint when these components were identified on the label by the manufacturer. (Manufacturers complying with ASTM D 4302 Standard Specification for Artists' Oil, Resin-Oil, and Alkyd Paints list the pigments on the label. To identify the pigments the common name and Colour Index designation is used.[5] Not all artists' materials manufacturers comply with this standard.) For manufacturers complying with the standard, the table reveals that many white colors contain amounts of zinc white. What the table does not reveal is the zinc that may be included in white oil colors by manufacturers not conforming to the standard. ASTM does not enforce its voluntary standards. Some companies may copy ASTM terminology without actually conforming with ASTM D 4302.

Table 1 Commercial Artists' White Oil Paint

White Oil Color Brand Names	Pigment Common Names and Colour Index Names*	Drying Oil
Art Spectrum Artists' Oil Colors
Titanium White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Safflower oil
Titanium White No. 2	Titanium dioxide (PW 6), zinc oxide (PW 4)	Safflower oil
Underpainting White
Zinc White	Zinc oxide (PW 4)	Safflower oil
Blockx Artists' Oil Colors
Flake White	Lead white (PW 1)	Poppyseed oil
Mixed White	Lead white (PW 1), zinc oxide (PW 4)	Poppyseed oil
Titanium White	Titanium dioxide (PW 6)	Poppyseed oil
Titanium-Zinc White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Poppyseed oil
Zinc White	Zinc oxide (PW 4)	Poppyseed oil
Daler-Rowney Artists' Oil Colour for Artists
Flake White	Lead white (PW 1), zinc oxide (PW 4)	Safflower oil
Titanium White	Titanium dioxide (PW 6)	Safflower oil
Zinc White	Zinc oxide (PW 4)	Safflower oil
Daniel Smith Artists' Oil Colors
Titanium White	Titanium dioxide (PW 6)	Safflower oil
Zinc White	Zinc oxide (PW 4)	Safflower oil
Da Vinci Paint Co. First Quality Artists' Oil Colors
Titanium White	Titanium dioxide (PW 6)	Safflower oil
Zinc White	Zinc oxide (PW 4)	Safflower oil
Gamblin Artists' Grade Oil Colors
Flake White Replacement	Titanium dioxide (PW 6), zinc oxide (PW 4)	Alkali refined linseed oil
Quick Dry White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Oil modified alkyd resin
Radiant White	Titanium dioxide (PW 6)	Poppyseed oil
Titanium White	Titanium dioxide (PW 6), zinc oxide (PW 4)**	Alkali refined linseed oil
Titanium-Zinc White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Alkali refined linseed oil
Zinc White	Zinc oxide (PW 4)	Alkali refined linseed oil
M. Graham & Co. Artists' Oil Colors
Titanium White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Walnut oil
Grumbacher Pre-Tested Artists' Oil Colors
Flake White	Lead white (PW 1), zinc oxide (PW 4)	Alkali refined linseed oil
MG® Underpainting White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Oil modified alkyd resin
Suberba™ (Titanium) White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Walnut and poppy oils
Titanium White (original formula)	Titanium dioxide (PW 6), zinc oxide (PW 4)
Titanium White (soft formula)	Titanium dioxide (PW 6), zinc oxide (PW 4)	Poppyseed and sunflower oils
Zinc White	Zinc oxide (PW 4)
Holbein Artists' Oil Colors
Ceramic White	Strontium titanate	Poppyseed oil
Foundation Color	Lead white (PW 1)	Linseed oil
Neo-Quick Base	Titanium dioxide (PW 6)	linseed oil, alkyd resin and petroleum solvent
Neo-Zinc White	Zinc oxide (PW 4)	Poppyseed and safflower oils
Permanent White	Titanium dioxide (PW 6)	Poppyseed and safflower oils
Quick Drying White	Titanium dioxide (PW 6), calcium carbonate (PW 18)	Poppyseed oil and alkyd resin
Silver White	Lead white (PW 1)	Linseed oil
Titanium White	Titanium dioxide (PW 6)	Poppyseed oil
Lefranc & Bourgeois Artists' Oil Colors
Flake White Hue	Titanium dioxide (PW 6), zinc oxide (PW 4)	Safflower oil
Silver White	Lead white (PW 1)	Safflower oil
Titanium White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Safflower oil
Titanium-Zinc White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Safflower oil
Zinc White	Zinc oxide (PW 4)	Safflower oil
Maimeri Puro Oil Colors
Titanium White	Titanium dioxide (PW 6)	Poppyseed and safflower oils
Zinc White	Zinc oxide (PW 4)	Poppyseed and safflower oils
Michael Harding's Artist Oil Colors
Cremnitz White	Lead white (PW 1)	Linseed oil
Cremnitz White	Lead white (PW 1)	Walnut oil
Flake White	Lead white (PW 1)	Walnut oil
Flake White No. 1	Lead white (PW 1)	Poppyseed oil
Foundation White	Lead white (PW 1) 50%, titanium dioxide (PW 6) 50%	Acid refined linseed oil
Titanium White	Titanium dioxide (PW 6)	Safflower oil
Titanium White No. 1	Titanium dioxide (PW 6)	Poppyseed oil
Titanium White No. 2	Titanium dioxide (PW 6), zinc oxide (PW 4) 25%	Acid refined linseed oil
Zinc White	Zinc oxide (PW 4)
Natural Pigments Rublev Oil Colors
Dutch White	Lead white (PW 1), barium sulfate (baryte, PW 21)	Alkali refined linseed oil
Flemish White	Lead sulfate (PW 2)	Alkali refined linseed oil
Lead White No. 1	Lead white (PW 1)	Alkali refined linseed oil
Lead White No. 2	Lead white (PW 1)	Walnut oil
Lead White (Lootwit)	Lead white (PW 1), calcium carbonate (PW 18)	Alkali refined linseed oil
Venetian White	Lead white (PW 1), barium sulfate (baryte, PW 21)	Alkali refined linseed oil
Old Holland Classic Oil Colors
Cremnitz White	Lead white (PW 1)	Cold-pressed linseed oil
Flake White No. 1	Lead white (PW 1), zinc oxide (PW 4)	Cold-pressed linseed oil
Mixed White No. 2	Titanium dioxide (PW 6), zinc oxide (PW 4)	Cold-pressed linseed oil
Titanium White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Cold-pressed linseed oil
Zinc White	Zinc oxide (PW 4)	Cold-pressed linseed oil
Pēbēo Fragonard Couleurs Extra-Fines Oil Colors
Titanium White	Titanium dioxide (PW 6)
Zinc and Titanium White	Titanium dioxide (PW 6), zinc oxide (PW 4)
Zinc White	Zinc oxide (PW 4)
Royal Talens Rembrandt Oil Colors
Mixed White	Titanium dioxide (PW 6), zinc oxide (PW 4)
Titanium White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Linseed oil
Titanium White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Safflower oil
Transparent White	Titanium dioxide (PW 6), zinc oxide (PW 4)
Zinc White	Zinc oxide (PW 4)	Linseed oil
Zinc White	Zinc oxide (PW 4)	Safflower oil
Vasari Classic Artists' Oil Colors
Flake White	Lead white (PW 1)	Linseed oil
Titanium White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Linseed oil
Titanium-Zinc White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Linseed oil
Zinc White	Zinc oxide (PW 4)	Linseed oil
Martin/F. Weber Professional Permalba® Oil Colors
Permalba White (Original)	Titanium dioxide (PW 6), zinc oxide (PW 4)	Alkali refined safflower oil
Titanium White	Titanium dioxide (PW 6)
Zinc White	Zinc oxide (PW 4)
Williamsburg Handmade Artists' Oil Colors
Flake White	Lead white (PW 1)	Alkali refined linseed oil
Silver White	Lead white (PW 1), zinc oxide (PW 4)	Alkali refined linseed oil
Titanium White	Titanium dioxide (PW 6)	Alkali refined linseed oil
Titanium-Zinc White	Titanium dioxide (PW 6), zinc oxide (PW 4)	Alkali refined linseed oil
Zinc White	Zinc oxide (PW 4)	Alkali refined linseed oil
Winsor & Newton Artists' Oil Colors
Cremnitz White	Lead white (PW 1)
Flake White No. 1	Zinc oxide (PW 4), lead white (PW 1)	Safflower oil
Flake White No. 2	Lead white (PW 1), zinc oxide (PW 4)	Safflower oil
Flake White Hue	Titanium dioxide (PW 6)
Titanium White	Titanium dioxide (PW 6), zinc oxide (PW 4)**
Transparent White	Titanium dioxide (PW 6)
Foundation White	Lead white (PW 1), zinc oxide (PW 4)	Linseed oil
Underpainting White	Titanium dioxide (PW 6), zinc oxide (PW 4)
Zinc White	Zinc oxide (PW 4)

* Only pigments identified on the manufacturer's labels or literature are listed in the table. The absence of zinc oxide in the list does not mean zinc white is not included in the paint, unless the manufacturer complied with standard ASTM D 4302 and listed all pigments used in the paint.

** The manufacturer notes a "small amount of zinc white" is added.

The table clearly identifies the pervasiveness of zinc in artists' oil paints not only labeled "zinc white," but also in paints labeled "flake white" (basic lead carbonate) and "titanium white" (titanium dioxide). The prevalence of zinc in paintings may present problems in the near future. Mark Gottsegen, author of The Painter's Handbook, wrote, "We think artists who use oil paints, and thus any white oil paint, are going to have problems with their paintings. Maybe not today or next week, or even next year, but within ten years, certainly."[6]

Physical Properties of Zinc White

Fig. 3 Transmission electron microscopic (TEM) image showing typical particles of zinc oxide.

To help us understand why zinc white is so commonly found in white oil paint today, let's examine its properties and history. Zinc oxide is a fine white powder that absorbs ultraviolet light at wavelengths below 366 nm. The particles of zinc oxide may be granular or nodular (0.1-5 µm) or acicular (needle-shaped, 0.5-10 µm). Particles produced by the wet process have an amorphous character (sponge form, particles up to 50 µm).[7]

A unique property of zinc oxide is that it is excitonic, that is it absorbs ultraviolet radiation and becomes defective, molecularly speaking.[8] Ultraviolet radiation causes aging in most things on earth, including human skin. Zinc oxide is used in many types of coatings whereby absorbing ultraviolet radiation, it can extend the life of the coating.

Many standard specifications have been established for the more important uses of zinc oxide. Typical classifications include indirect (French) process, direct (American ) process, wet process, pharmaceutical grade, chemical pure grade, etc. Manufacturers have their own standards, too. The chemical pure grade made by the indirect process contains over 99% zinc oxide. Formerly, zinc white was sold in various qualities and degrees of whiteness. "White seal" and "green seal" zinc white contain over 99% zinc oxide. "Red seal" and "gold seal" grades were slightly less pure and "gray seal" zinc white contains metallic zinc.[9] The terms gold seal, silver seal, red seal, green seal and white seal are still common in Europe.

The most common grade of zinc oxide used in artists' oil paint is the indirect process, chemical pure grade. In Europe, it is designated as either white seal or green seal. Table 2 shows the typical physical specifications for French process, green seal zinc oxide.

Table 2 Physical Properties of Zinc Oxide

Zinc Oxide Purity	99.5%
Mean Particle Size	0.33 μm
Particle Surface Area	3.2 m2/g
Passing Through 325 Mesh	99.99%
Oil Absorption	12 lbs oil for 100 lbs zinc oxide
Refractive Index	1.95-2.1
Crystal Structure	Hexagonal
Mohs Hardness	4-4.5
Density	5.65–5.68 g/cm3
Apparent Density	40 lb/ft3
Bulk Density	1 lb = 0.0214 gallon
Pounds per Solid Gallon	46.7 lbs

Brief History of Zinc White

Zinc has long been known as a byproduct of copper smelting. The Romans called it "cadmia" and used it in the production of brass. In the mid-18th century, the German chemist Cramer discovered that cadmia could be obtained by the combustion of metallic zinc. Courtois de Dijon began to produce zinc white in 1781 in France. In 1782, zinc oxide was suggested as a white pigment. In that year, Guyton de Morveau at L'Académie de Dijon, France, reported on white pigments and the raw materials that might serve as white pigments, including zinc oxide. He suggested zinc oxide as a substitute for lead white.[10]

It took about 50 years before zinc white was adopted by artists. As early as 1834, according to Church, a dense form of zinc white was introduced by the firm Winsor & Newton under the name "Chinese white."[11] The major obstacle in the way of its use among artists was its poor drying qualities in linseed oil. In the years 1835-1844, Leclaire in France showed that this difficulty could be overcome by using zinc oxide in oil that had been made faster drying. By 1850, it was regularly made as an oil paint.

In 1845, Leclaire began industrial production of zinc oxide in Paris with the indirect or French process. Around 1850, S. Wetherill of the New Jersey Zinc Company perfected a furnace in which the zinc was reduced by the partial combustion of coal and reoxidized at the furnace exit (direct or American process). This process was afterwards improved but is now no longer used in Europe and America but is still used in China. A third industrial production process exists but this wet process is less widely used.[12]

The only significant white pigment up until that time was lead white. Zinc oxide began to replace lead white in many types of paint, because it had the advantages of being nontoxic, of not darkening in the presence of sulfurous gases, and offering a brilliant white. Lead white had served the old masters well, but changes in color preferences during the 19th century created the demand for more brilliant colors. Artists found that zinc oxide was not as opaque as lead white was and provided cool, brilliant whites in oil paint. The transparency of zinc white makes it valuable for making tints with other colors. Tints made with transparent whites show the nuances of a color's undertone to a greater degree than tints made with whites that are more opaque.

Early Problems with Zinc White

Despite its many advantages, it was soon found that zinc white oil color also has a drawback; it makes a rather brittle paint film. Zinc whites' brittleness can cause cracks in paintings after only a few years, if it is used by itself and in excess. During the latter half of the 19th century, some artists used zinc white as a ground for their oil paintings. They wanted the brilliant white to obtain highly saturated colors in their works, but did not realize its long-term disadvantage. After a period of years, many of the paintings developed cracks where older works painted on traditional grounds remained free from cracks.[13]

The affects of zinc oxide in paint have been investigated since the early part of the 20th century. For example, it was found that zinc oxide shows appreciable reactivity in drying oils.[14] Zinc oxide tends to saponify the fatty acid component of certain oils, with the degree of soap formation dependent upon the particle size of the pigment, with the finest particles resulting in more rapid soap formation.[15] The formation of soaps is beneficial in some vehicles and is one reason zinc white has long been used as an additive in titanium white paints. However, this reactivity can be a two-edged sword. Researchers also found it can rapidly form a hard and sometimes brittle film, especially when using zinc oxide as pigment.[16]

In paints and coatings, zinc oxide is no longer the principal white pigment, although, as noted above, its superb white color is used by artists. It is used as an additive in exterior paints for wood preservation, and in antifouling and anticorrosion paints.[17] It improves film formation in some paint vehicles, and resistance to mildew (having a synergistic effect with other fungicides) because it reacts with acidic products of oxidation and can absorb ultraviolet radiation.

Still Used by Artists Today

If problems were observed since the early part of the 20th century, why is zinc white still used in artists' oil paints today? The problems mentioned above were primarily observed when zinc white was used alone or in excessive amounts, especially in the grounds of paintings. It appeared that the cracking and embrittlement of dried paint films could be minimized or eliminated when zinc white was mixed with other white pigments or used sparingly. In addition, its brilliant white color is seductive to artists, hence manufacturers continue to supply the demand for paint with zinc white. Manufacturers add zinc white to artists' white colors to offset the shortcomings of other pigments.

In the first half of the 20th century, a new white pigment appeared on the scene -- titanium dioxide. This pigment was more opaque, less reactive than zinc white, and did not form brittle oil paint film. Titanium white by itself, however, has too much tinting strength for some use in artists' colors and makes stringy oil paint.[18] Manufacturers often combine titanium white with zinc oxide to produce a manageable white paint.

Shortly after the introduction of titanium white, reports appeared to confirm the advantages of zinc white in oil colors. White oil paint containing as little as 10 to 15% zinc oxide yellowed less, especially when ground in safflower oil.[19] In his book, Artists' Pigments, Lightfastness Tests and Ratings, Henry Levison favorably reports on white paint containing zinc oxide. Among the different white colors he studied, paint film composed of safflower oil and 15 to 20% zinc white to titanium white withstood his test. Levisons' tests, however, involved exposure to the sun for several months.[20] The study by the Smithsonian scientists was done over a period of 28 years.

There is no question of the advantages to the visual appearance of white oil paint incorporating zinc oxide, especially titanium white. This is why zinc oxide is commonly used in titanium white oil colors. The combination of titanium white and zinc white yellows less than other combinations of white pigments in oil. However, artists may be giving up the long-term integrity of the paint film for a short-term advantage of less yellowing, which Levison found is reversible in white oil paints.[21]

Other Potential Problems

If these problems were not enough, other problems began to surface. Earlier we learned that zinc is very reactive with certain fatty acids in oil. For example, zinc oxide reacts with stearic acid to form metallic soaps, containing zinc alkonate and zinc stearate. Soap formation occurs when zinc oxide is used in an oil medium with a high proportion of acid groups. These substances can cause efflorescence on the surface of paint film, often producing hazy white patches on the surface of a painting.[22] Because zinc is a divalent metal oil tends to cross-link, causing the paint to excessively thicken or increase viscosity upon storage.[23]

An advantage is the strong ultraviolet absorbency of zinc oxide, which protects lead and titanium pigments from photochemical changes. On the other hand, researchers have shown that near ultraviolet light can promote the formation of hydrogen peroxide, a process that causes the dehydration of other pigments and subsequent embrittlement.[24]

Discard Zinc White Artists' Oils?

Should artists toss out their tubes of white oil color containing zinc white? If artists are concerned about the long-term prospects of their paintings, the answer is affirmative. Does this mean that zinc white should be removed from the lists of pigments suitable for serious artistic use? Maybe. For the time being, it should only be used in paint where the objective is not archival.

Mecklenburg and Tumosa did not examine every grade of zinc oxide. Presumably, the grade of zinc oxide in the paint studied was of the French or indirect process, since this is the type most widely used in artists' oil paints. Generally, commercial zinc white particles have nodular or irregular shapes in sizes ranging from 0.1 micron to slightly greater than one micron. It is known that grades of zinc oxide with acicular particle shapes have greater covering ability or hiding strength and this shape may alter its reactivity in oil. However, acicular zinc oxide is more reactive in paint vehicle and not less, according to Robinson.[25] In addition, the acicular form of zinc oxide is very expensive and primarily used in specialty polymers, not paint.[26] Whereas larger particle-size zinc oxide is less reactive, the larger size makes it unsuitable for paint.

The initial condition of zinc white powder is important, especially the water content prior to mixing with drying oils and other additives. Zinc oxide is known to be hygroscopic, meaning it absorbs water when exposed to air and becomes sticky -- the particles of zinc white stick to one another. If "sticky" zinc oxide is used to make oil colors, some moisture can be trapped inside the paint, which may lead to cracking and peeling. To avoid this problem, paint manufacturers must heat (above 600° C.) the sticky zinc oxide in order to remove the water content. However, this is typically not done by artists' color manufacturers.

According to Mahmud, the hygroscopic property of zinc oxide causes it to react with the atmosphere after long exposure to moisture and sunlight. Zinc oxide can absorb moisture from the air, causing physical expansion of the paint film; and similarly, it can lose moisture if the air is dry, triggering contraction. This results in physical stress of the paint film that can lead to cracking.[27]

On a more positive note, it may be possible to mitigate the reactivity of zinc oxide in oil and reduce its effects of cracking and peeling. Titanium white is reactive to light and to certain substances in paint. Pigment manufacturers coat titanium dioxide particles with silica and alumina to reduce this photochemical reactivity. Several patents exist for coating or doping zinc oxide particles, such as that used for thermal coatings.[28] It may be possible to coat zinc oxide particles, such as the coating of titanium dioxide, to produce zinc white pigment that does not cause cracking and peeling in oil paint.

Conclusion

Paints made with zinc oxide and drying oils become extremely brittle in as little as three years. Paints containing zinc oxide exhibit severe delaminating problems after drying and especially zinc oxide paint on acrylic emulsion grounds. Even commercial lead white and titanium white oil paints containing zinc oxide become quite brittle after seven years of drying.

Of course, not every painting containing zinc white will crack and its paint fall off the support. Artists of the Pre-Raphaelite Brotherhood (19th century) often used zinc white in their paintings. One such painting, Ophelia (1852-97) by John Everett Millais has areas of zinc white priming.[29] Remarkably, the painting is in generally good condition and does not show problems due to the reserves of zinc white.[30] However, it is impossible to draw a parallel between the present condition of 19th century paintings to the future state of paintings made by today's artists. First, the materials used then are significantly different from those used today. Second, individual painting techniques may moderate the problems caused by the materials. Third, oil paintings are complex structures with many chemical and physical interactions that cannot always be predicted or explained with current scientific methods.

Fig. 4 Ophelia (1852-97) by John Everett Millais has areas of zinc white priming

Mecklenburg and Tumosa's paper forewarns us about the potential problems with oil paints containing zinc white commonly used by artists today. Artists may choose to ignore the report and continue using zinc in their paintings -- perhaps at their own peril. Whatever the choice, the paper helps artists gain a better understanding of the materials used in their work.

Sources

1. Marion F. Mecklenburg and Charles S. Tumosa (2007). The chemical and mechanical effects of pigments on drying oils. Unpublished. p. 13-14.
2. Ibid, p. 15.
3. Ibid, p. 16.
4. Marion Mecklenburg, private communication.
5. ASTM D 4302 Standard Specification for Artists’ Oil, Resin-Oil, and Alkyd Paints, see section 5.1.7: "Mixed Pigments -- Artists' paints containing more than one pigment comply with this specification if all colored pigments included in the mixture are on the suitable pigment list (Table 1) and provided the mixture itself has passed all other test requirements in this specification. The Common Names for the pigments in the mixture, or the word "Mixture," must appear under the title in letters no less than the next type size smaller than the title. …If the word "Mixture" is used under the title, the Common Names of the pigments in the mixture, as given in Table 1, must be listed along with their Colour Index Names and the Lightfastness Category of the mixture somewhere on the label." ASTM does not enforce its voluntary standards. Some companies may copy ASTM terminology without actually conforming with ASTM D 4302.
6. Mark D. Gottsegen (2007). "Zinc oxide and its effects on drying oil." Discussion Forum. AMIEN: Art Materials Information and Education Network. http://www.amien.org/forums/showthread.php?t=366.
7. Gerhard Auer, Wolf-Dieter Griebler, Burkhardt Jahn. (2005) "White Pigments." Industrial Inorganic Pigments. 3rd Edition. p. 90.
8. For the definition of "excitonic," see the Wikipedia article, "Exciton." Wikipedia. http://en.wikipedia.org/wiki/Exciton
9. Rutherford J. Gettens, George L. Stout (1966). Painting materials: A short encyclopedia. New York: Dover Publications. p. 177.
10. D. B. Faloon (1925). Zinc oxide history, manufacture and properties as a pigment. New York.
11. A. H. Church. The chemistry of paints and painting. 3rd ed. London: Seely, Service and Company. p. 134.
12. Gerhard Auer (2005), pp. 89-90.
13. Michael Douma, ed. (2001). "Zinc white -- history" WebExhibits. http://webexhibits.org/pigments/indiv/history/zincwhite.html
14. L. A. O. O'Neill, R. A. Brett (1969) "Chemical reactions in paint films." J. Oil Chem. Assoc. 52 (11) (1969) pp. 1054-1074.
15. S. H. Bell (1970). "Controlling zinc oxide hazing in gloss paints." Paint and varnish production 60 No. 4. pp. 55-60. Payne, H. F. (1961). Organic coating technology. New York: John Wiley & Sons.
16. A. E. Jacobsen (1941) "Zinc soaps in paints." Ind. Engin. Industry 33 (10) (1941) 1254-1256.
17. G. Meyer (1986). New application for zinc. New York: Zinc Institute Inc.
18. Henry W. Levison (1976). Artists' pigments, lightfastness tests and ratings. Hallandale, Florida: Colorlab. p. 30.
19. Henry W. Levison (1985). "Yellowing and bleaching of paint films." Journal of the American Institute of Conservation, 1985, Volume 24, Number 2, Article 2. pp. 69-76. See also Levison (1976). Artists' pigments, lightfastness tests and ratings. pp. 31
20. Levison (1976). pp. 32-34.
21. Levison (1985). p. 70.
22. Eugena Ordonez, John Twilley (1988). "Clarifying the haze, efflorescence on works of art." Western Association for Art Conservation (WAAC) Newsletter. January 1998 Volume 20 Number 1.
23. G. P. A. Turner (1988). Introduction to paint chemistry and principles of paint technology. London: Chapman & Hall. p. 252.
24. Nicholas Eastaugh (2004). Pigment compendium: A dictionary of historical pigments. Elsevier. P. 407. John S. Mills, Raymond White (1994). The organic chemistry of museum objects. Elsevier p. 165.
25. Peter John Robinson, private communication. Dr. Robinson is a consultant to the zinc industry for the past 40 years.
26. Shahrom Mahmud, private communication. Mr. Mahmud is reviewer for the Elsevier journal, Materials Science Engineering, author of 11 papers on zinc oxide manufacturing and 20 publications, and a consultant to zinc oxide factories in several countries.
27. Shahrom Mahmud, private communication.
28. United States Patent 4331706, Composite zinc oxide coating on an inert pigment core product and process; United States Patent 5589274, Thermal control coating; United States Patent 5770269, Thermal control coating; United States Patent 7235298, Doped zinc oxide powder, process for its preparation, and its use.
29. Joyce H. Townsend, Jacqueline Ridge and Stephen Hackney (2004) Pre-Raphaelite painting techniques. London: Tate Publishing. p. 137.
30. Leslie Carlyle, private communication.

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