Pigments through the Ages - Intro to the whites

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Symbolism of the Color White

White objects such as clouds, snow, and flowers often appear in nature, creating many references within our human culture to the color white. In some cultures, like China&#;s, the color white represents death and illness. In many cultures, however, white represents freedom, purity, and innocence. This is why, for example, white is worn by brides in Western countries.

In ancient Egypt, white suggested omnipotence and purity. The name of the holy city of Memphis meant "White Walls." White sandals were worn at holy ceremonies. Ritual objects, such as small ceremonial bowls, were often white.

The high contrast between white and black is often used to represent opposite concepts, such as day and night, and good and evil. In Taoism, which has great influence in Eastern culture, yin and yang are usually depicted in black and white.

Toxic lead white was used by artists for hundreds of years before it was widely banned in the late 20th century. Lead white was commonly used not only as a canvas primer, but also for creating tints of various colors as well as highlights. Lead white was also regularly used in cosmetics, often with fatal consequences.

Wassily Kandinsky, a Russian painter and art theorist, describes his perception of the color white: &#;... white, although often considered as no color (a theory largely due to the Impressionists, who saw no white in nature), is a symbol of a world from which all color as a definite attribute has disappeared. This world is too far above us for its harmony to touch our souls. A great silence, like an impenetrable wall, shrouds its life from our understanding. White, therefore, has its harmony of silence, which works upon us negatively, like many pauses in music that break temporarily the melody. It is not a dead silence, but one pregnant with possibilities. White has the appeal of the nothingness that is before birth, of the world in the ice age.&#;

Short History of White Pigments

Lime powder and gesso where the first whites available in prehistoric times. The most important contribution to art materials from Greece was lead white, a pigment that would become ubiquitous in Western art. Modern whites are zinc white and titanium white. Thanks to its excellent qualities, titanium white has largely replaced lead white in both art and industry.

The perception of white is due to light that stimulates all three types of color sensitive cone cells in the human eye in nearly equal amounts and with high brightness.

Timeline of white pigments.

White pigments consist of natural or synthetic inorganic pigments. Inorganic pigments are easier to disperse in most paint vehicles than organic pigments. Nevertheless, many white pigments undergo treatment to improve their dispersibility, lightfastness, and weather resistance. The treatment involves coating individual pigment particles with inorganic and organic substances. Complete dispersion of any pigment in a paint vehicle is essential because only then can its optimum tinting strength be exploited.

White pigments are used for white colors, tinting colors, and covering tones in paint. For opaque coverage, titanium dioxide is preferred because of its excellent hiding power. All substances with a refractive index above 1.7 are classified as white pigments. If the refractive index is lower than 1.7, they are classified as fillers (see Table 1).

Strictly considered, the limiting value of 1.7 is not a constant but depends on the paint vehicle because every vehicle has its specific refractive index. The refractive indices of pigments vary from painting medium to painting medium and are consequently different for materials such as vegetable drying oils, alkyd resins, natural resins, and wax.

Good optical properties are expected from white pigments, for example, high light scattering power, a high degree of hiding power, good tinting strength, a high degree of brightness, an insignificant undertone (preferably none), and a high degree of whiteness. The most important property is the light scattering power, which depends on the refractive index, particle size and distribution, and the degree of dispersion in the paint vehicle.

Because of these dependencies, the light scattering power is a relative value and not an absolute value. The other parameters, such as high hiding power, brightness, undertone, and whiteness, depend more or less on the scattering power of the white pigment.

Hiding Power

Hiding power is the property of a paint that enables it to obliterate beyond recognition any background over which it may be spread. It is generally accepted that complete hiding has been reached when the paint applied over a back background has a reflectance of 0.98 of that applied in equal thickness over a white background. Thus, hiding power is a function of the background&#;s contrast ratio and the paint&#;s thickness required to reduce the contrast difference to 0.02. The value of 0.02 is based on the Weber-Fechner law, which states that differences of less than 2 percent for moderate illumination are indistinguishable to the average eye.

When light falls on a paint film, some is reflected from the surface, and the remainder enters the film. That which enters either is absorbed or emerges from the various faces (including the top). If all the light is absorbed, the film is black or dark in color and hides the substrate very well. The pigment produces hiding power in the film through its light-absorbing properties.

If much of the light emerges from the film&#;s top surface and the substrate is not obscured, then the film hides poorly. An example is a transparent linseed oil film containing no pigment over a white substrate. Conversely, if much of the light emerges from a paint film and the substrate is obscured, then the film hides well. Such a film is white or a light tint. Here the light has undergone multiple reflections, refractions, and diffractions by the pigment such that the light emerges from the top without allowing the substrate to show. The pigment produces hiding power because it scatters the light.

If the amount of light emerging from the top is a function of its wavelength, the film may hide well or poorly depending on how well the substrate is obscured. The paint film, in this case, is colored; that is, it is blue, red, or other colors.

In all cases, the light-absorbing properties of the pigment and vehicle and the light-scattering properties of the pigment combine to produce the hiding power of the film.

Refractive Index

For white or near-white paints, the magnitude of the hiding power is a function of the difference between the refractive indexes of the pigment and vehicle. Thus, the same pigment will hide better in water than in linseed oil and, better yet, in the air (&#;high dry hiding&#;) because of the increasing difference in refractive indexes.

For maximum hiding power with a given type of pigment, the particle size of the pigment and its degree of dispersion in the vehicle is also essential.

The refractive indexes for most white opaque pigments and extender pigments used in paint are given in Table 1.

Most pigments are crystalline and usually have different refractive indexes along the crystal axes. The values given in the table are mean values for all directions. The refractive indices also vary somewhat, with the wavelength of the light generally being higher for blue light than for red.

Table 1&#;Refractive Indices of White Pigments and Extender Pigments

Colour Index Name

Pigment

Formula

CAS No.

Refractive Index

Pigment White 1 Lead white Pb(OH)2·2PBCO3 -46-6 1.94&#;2.09 Pigment White 3 Basic lead sulfate PbSO4 -14-2 1.93 Pigment White 4 Zinc oxide ZnO -13-2 2.02 Pigment White 5 Lithopone ZnS/BaSO4 -05-7 1.84 Pigment White 6 Titanium dioxide, anatase TiO2 -67-7 2.55 Pigment White 6 Titanium dioxide, rutile TiO2 -67-7 2.76 Pigment White 7 Zinc sulfide ZnS -98-3 2.37 Pigment White 8 Strontium sulfide SrS -96-1 2.1 Pigment White 10 Barium carbonate BaCO3 513-77-9 1.67 Pigment White 11 Antimony oxide Sb2O3 -64-4 2.09&#;2.29 Pigment White 12 Zirconium oxide ZrO2 -23-4 2.4 Pigment White 13 Bismuth tungstate Bi2(WO4)3 -87-4 2.17 Pigment White 14 Bismuth oxychloride BiOCl -59-9 2.15 Pigment White 15 Tin oxide SnO2 -10-5 2 Pigment White 17 Bismuth subnitrate Bi5O(OH)9(NO3)4 -85-4   Pigment White 18 Calcium carbonate (chalk) CaCO3 471-34-1 1.58 Pigment White 18:1 Calcium magnesium carbonate (dolomite) [Ca,Mg][CO2]2 -88-1 1.68 Pigment White 19 Aluminum silicate (kaolin) Al2Si2H4O9 -58-7 1.55 Pigment White 20 Mica (muscovite) KAl2(Si3Al)O10(OH,F)2 -26-2 na=1.560, nb=1.594, nc=1.598 Pigment White 21 Barium sulfate (blanc fixe) BaSO4 -43-7 1.64 Pigment White 22 Barite (baryte) BaSO4 -86-7 1.64 Pigment White 23 Alumina blanc fixe (blancopone) AlO2 BaSO4   1.64 Pigment White 24 Aluminum hydroxide AlO2 -51-2 1.57 Pigment White 25 Calcium sulfate (gypsum) CaSO4·2H2O -18-9 1.59 Pigment White 26 Talc Mg2[Si4O10][OH]2 -96-6 1.50&#;1.60 Pigment White 27 Silica (quartz) SiO2 -86-9 1.55 Pigment White 28 Calcium metasilicate (wollastonite) CaO3Si -39-0 1.65 Pigment White 32 Zinc sulfate (white copperas) ZnSO4 -02-0 1.65 Pigment Yellow 47 Lead titanate PbTiO3 -00-3 2.7

 

Mass Color

Mass color or mass tone is the color when viewed by the reflected light of a pigment and vehicle mixture of such thickness as to obscure the background entirely. Mass color encompasses lightness, hue, and saturation. Generally applied to colored pigments, it may also be applied to white pigments.

Tinting Strength

Tinting strength is the power of a pigment to color a standard paint or pigment. When applied to white pigments, tinting strength is the ability to resist discoloration by colorants. Tinting strength is often used as a guide for estimating relative hiding power. Consequently, it is unsurprising that the same fundamental factors for hiding power apply to mass color and tinting strength. Thus, the refractive index of the pigment is the primary factor, followed by its particle size and degree of dispersion in the vehicle. For colored pigments, the inherent light-absorbing ability is also a primary factor.

Undertone

The undertone of a specific paint color or hue is the color you see when the paint is spread very thinly or by mixing with white. This can be done physically by brushing the color thinly, scraping with a painting knife, or diluting the color with a medium.

White Pigments

Aluminum Silicate

Chemical Formula: Al2Si2H4O9 | CAS No.
Colour Index: Pigment White 19 ()

Commonly referred to as clay or kaolinite, aluminum silicates are used as fillers in paint and as raw materials for manufacturing ultramarine blue and violet pigments.

Antimony(III) Oxide

Chemical Formula: Sb2O3 | CAS No.
Colour Index: Pigment White 11 ()

This white pigment is seldom used in artists&#; paint because of its toxicity and abrasiveness. In other types of paint, it is sometimes used as a flame retardant.

Barium Sulfate

Chemical Formula: BaSO4 | CAS No. -43-7
Colour Index: Pigment White 21 ()

Barium sulfate occurs in nature as the mineral barite (UK English, baryte). Its density is in the range of 4.3&#;4.6 g/cm3. Owing to its low Mohs hardness, it is not very abrasive compared to other inorganic pigments. Barium sulfate is practically inert to acids, alkalis, and organic solvents. Its lightfastness and weather resistance are excellent. It has a low tendency to flocculate and form aggregates, so it is easy to disperse.

Owing to its low refractive index of 1.64, barium sulfate is not considered a white pigment. Its main application in paint is as a filler and mineral base for the precipitation of lake pigments. Its low opacity can be advantageous&#;on the one hand, it is transparent enough to allow light to pass through a film of barium sulfate, and on the other hand, a certain amount of light is scattered. The result of this combined effect is diffuse light scattering.

Two types of barium sulfate are commercially available. One type is a finely ground, natural mineral barite, and the other is a precipitated, synthetic barium sulfate known as blanc fixe. The synthetic type is often more expensive; hence the natural mineral is more commonly used. Barium sulfate improves the flowing properties of organic pigments and can aid in dispersing pigments during paint production.

Health and Safety

Barium sulfate is not considered toxic, and its use is therefore permitted in many countries, including the United States and most European countries.

Barium sulfate is a component of lithopone, Pigment White 5.

Calcium Carbonate

Chemical Formula: CaCO3 | CAS No. -65-3
Colour Index: Pigment White 18 ()

Besides silicates, calcium carbonate is the most common mineral on earth. There are several types of calcium carbonate commonly used in paint:

Calcite is colorless, transparent, translucent, or opaque dense crystals with perfect rhombohedral cleavage. Some varieties are fluorescent in ultraviolet light.

Chalk is a very pure limestone formed during the cretaceous period of fine calcite crystals consisting mainly of fossil remains of the shells and skeletons of microscopic plankton. Many deposits throughout the earth are exploited commercially.

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Limestone is compact chalk formed from the accumulation of calcareous skeletons of marine organisms.

Marble is produced by the metamorphism of limestone around igneous intrusions Marble is essentially calcite but may contain greater or lesser amounts of dolomite (magnesium sulfate) and other minerals. It is a coarse compact mineral.

Calcium carbonate is semi-hard and not very abrasive. The Mohs hardness is 2 to 3, and the density is between 2.6 and 2.8 g/cm3. Calcium carbonate is soluble in weak acids and insoluble in alkali. Its properties are often adjusted after treatment to improve its dispersibility in different paint binders.

Calcium carbonate is not considered a white pigment because of its low refractive index of 1.58. Its main application in paint is as a filler to lower the cost of the paint by substituting more expensive pigments. Similar to barium sulfate, it improves the flowing properties of pigments, especially organic pigments, and the dispersion of pigments in paint vehicles during production.

Lead White (Basic Lead Carbonate)

Chemical Formula: Pb(OH)2·2PBCO3 | CAS No. -46-6
Colour Index: Pigment White 1

For more information on how Natural Pigments makes stack process flake white, how it was made throughout history, and how it differs from modern lead white, please read Stack Process White Lead: Historical Method of Manufacture. Another article, Notes About Stack Process Lead White, discusses differences between modern lead white and our production of stack process flake white pigment. A third article, Variations of Stack Flake White, examines the properties of the larger particle-size stack process flake white have in oil paint.

Are you confused about the difference between flake white and Cremnitz white? The article Flake White and Cremnitz White explains the origins of these names and resolves the confusion.

For a complete description of white pigments used in artists&#; paints, please read the article titled White Pigments.

Lithopone

Chemical Formula: ZnS/BaSO4 | CAS No. -05-7
Colour Index: Pigment White 5 ()

Lithopone is produced by co-precipitation and subsequent calcination of zinc sulfide and barium sulfate mixture. The ratio between the two components varies; for example, one type consists of a ratio of 60% zinc sulfide and 40% barium sulfate.

Health and Safety

Zinc sulfide and barium sulfate are not considered toxic in the United States and most European countries. Soluble zinc is toxic when present in large amounts, but the human body requires small quantities for metabolism. Owing to its low solubility, lithopone is not considered toxic to humans. Studies have shown no acute or chronic toxicity to human health in manufacturing this pigment despite exposure to dust while handling the finely ground pigment.

Magnesium Carbonate

Synonym: Magnesia alba
Chemical Formula: MgCO3 | CAS No. 546-93-0 (anhydrous)
Colour Index: Pigment White xx

Magnesium carbonate (archaic name magnesia alba) is an inorganic salt that is a white solid. Several hydrated and fundamental forms of magnesium carbonate also exist as minerals. Magnesite consists of white trigonal crystals. The anhydrous salt is practically insoluble in water, acetone, and ammonia. All forms of magnesium carbonate react in acids. Magnesium carbonate crystallizes in the calcite structure where six oxygen atoms surround Mg2+. The dihydrate one has a triclinic structure, while the trihydrate has a monoclinic structure.

CAS Numbers for different forms of magnesium carbonate:
546-93-0 (anhydrous)
-00-5 (monohydrate)
-48-2 (dihydrate)
-83-1 (trihydrate)
-72-6 (pentahydrate)

Magnesium carbonate is ordinarily obtained by mining the mineral magnesite. Seventy percent of the world&#;s supply is mined and prepared in China.

Because of its low solubility in water and hygroscopic properties, MgCO3 was first added to salt in to make it flow more freely. The Morton Salt company adopted the slogan &#;When it rains, it pours&#; because its magnesium carbonate-containing salt would not stick together in humid weather.

Mica

Chemical Formula: K/Na/Al2[(OH,F)2Si3O10] | CAS No. -26-2
Colour Index: Pigment White 20 ()

Mica is the collective term for an entire series of mineral chemicals known as hydrous alkaline aluminum silicates. Most members of this group of silicates are muscovite, phlogopite, biotite, and paragonite. They differ in their potassium, sodium, and fluorine content; their characteristic feature is their formation of very thin sheets called laminae.

Muscovite and phlogopite are the principal minerals coated with a thin layer of metal oxides for pearlescent effect pigments. Silvery pearlescent pigments result from titanium dioxide coating, and colored pearlescent pigments result from coatings of iron and chromium oxides.

Silica or Silicon Dioxide

Chemical Formula: SiO2 | CAS Nos. -86-9 and -52-5, respectively
Colour Index: Pigment White 27 ()

Owing to its low refractive index of 1.55, silica is not used as a white pigment but is an essential extender pigment and processing aid in paint. The main functions of silica in paint are:

• Thickener or anti-sedimentation agent and to adjust the viscosity of paint;
• Matting agent in paint and varnishes;
• Improvement of the flow properties of organic pigments;
• Extender in student grade artists&#; paint; and

The Colour Index name Pigment White 27 only applies to the naturally occurring silica and not synthetic grades.

Titanium Dioxide, Anatase

Chemical Formula: TiO2 | CAS No.
Colour Index: Pigment White 6 ()

Titanium Dioxide, Rutile

Chemical Formula: TiO2 | CAS No.
Colour Index: Pigment White 6 ()

Because of the high temperature of 900&#;º C. (&#;º F.) during the combustion of titanium tetrachloride, only the thermodynamically stable rutile modification is formed.

After treatment of titanium dioxide is essential for its durability in paint and other applications. Titanium dioxide adsorbs water onto the surfaces of its particles, and accordingly, its surface is saturated by coordinately bonded water that forms hydroxyl ions. The presence of hydroxyl groups makes photochemically-induced reactions possible. Treatment consists of coating the pigment particles of the low-refractive index and essentially colorless inorganic compounds by precipitating them onto the surface of the particles. Zirconium (Zr), tin (Sn), aluminum (Al), and silicon (Si) compounds are used to coat titanium dioxide, sometimes followed by additional coatings with organic compounds. Selecting a particular organic compound can make the final surface either hydrophobic or hydrophilic. The coating prevents direct contact of the environment with the reactive surface of titanium dioxide, consequently improving the lightfastness and weather resistance of the pigment.

Titanium dioxide is inert, insoluble except in concentrated sulfuric acid and hydrogen fluoride, and stable.

Health and Safety

Titanium dioxide is not considered toxic, which is underlined by its application in toothpaste and pharmaceutical pills.

Application

The rutile form of titanium dioxide is most often preferred in the paint for pure white colors or for creating tints of colors.

Zinc Oxide

Chemical Formula: ZnO | CAS No. -13-2
Colour Index: Pigment White 4

Zinc oxide is a synthetic inorganic pigment. Because zinc oxide is amphoteric, it reacts with acids and is soluble in alkaline solutions. It is known to cause embrittlement in vegetable drying oils and oil-modified alkyd paint.

Zinc oxide is manufactured by two different processes, either by the direct or American process or by the indirect or French process.

The direct or American process is a simple, low-cost one. Zinc-containing raw material or zinc ore is mixed with a reducing agent such as coal and heated to &#;º C. (&#;º F.). Under this condition, zinc oxide is reduced to metal, which evaporates. The zinc vapor is then oxidized to zinc oxide. The purity of the zinc oxide is determined by the composition of the raw materials used.

The indirect or French process starts with metallic zinc, which is heated, and the resulting vapor oxidized to zinc oxide.

Zinc ores often contain several other metals: lead, cadmium, iron, and aluminum. Various separation techniques are therefore necessary to purify the zinc vapor before oxidation.

Zinc oxides with different degrees of purity are commercially available; some types still contain a few percent lead. Before using the pigment, the manufacturer must check the purity of the zinc oxide to be sure it can be used in the specific application intended. Lead amounts must be negligible in the pigment before using zinc oxide that comes in contact with food and human skin.

Health and Safety

Zinc oxide is not considered toxic to humans, although a few early studies show some toxicity. The toxicity from the zinc oxide in these studies was most likely due to impurities, especially lead. Zinc is an essential trace element for humans, animals, and plants. Lack of sufficient zinc causes deficiency diseases; for example, it affects hair growth and human reproduction.

Zinc oxide is insoluble in water and can be separated easily from wastewater. Due to the toxicity of zinc to fish and other aquatic life, the concentration of zinc ions in wastewater must be limited.

Zinc Sulfide

Chemical Formula: ZnS | CAS No. -98-3
Colour Index: Pigment White 7 ()

Zinc sulfide is an essential white pigment in paint. The refractive index is 2.37. The Mohs hardness is 3, making it a soft, non-abrasive pigment. The lightfastness is good, but its weather resistance is fair and inadequate for outdoor paints. Ultraviolet radiation combined with humidity oxidizes zinc sulfide to the colorless zinc sulfate (ZnSO4) in paint. If mixed with lead compounds, zinc sulfide can react to form dark lead sulfide.

Health and Safety

Because of its low solubility, it is not considered toxic to humans. Studies have shown no acute or chronic toxicity to humans in manufacturing this pigment despite exposure to the dust while handling the finely divided pigment. Zinc sulfide is permitted in contact with food by the United States Food Drug Administration (FDA) and most European countries.

Applications

Zinc sulfide is the main component of lithopone, Pigment White 5, and the base of some luminescent pigments. One form of luminescent pigment is zinc sulfide doped with silver or copper.

The optical properties of zinc sulfide are inferior to those of titanium dioxide. Therefore its use as a pigment diminished during the past century. Like lithopone, zinc sulfide is used in paint that requires pigments with low abrasion qualities. This is perhaps its only advantage in comparison to titanium dioxide.

Zirconium Oxide

Chemical Formula: ZrO2 | CAS No.
Colour Index: Pigment White 12 ()

Zirconium dioxide has been entirely substituted by titanium dioxide. It is used, with other substances, for the adulteration of titanium dioxide to improve the fastness properties of rutile.

Bibliography

Albrecht Müller, Coloring of Plastics: Fundamentals, Colorants, Preparations. 68&#;77.

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