White Pigments for Paint

White pigments are derived from both natural and synthetic inorganic sources. In many cases, inorganic pigments are simpler to integrate into paint vehicles compared to their organic counterparts. Regardless of their origin, various white pigments are often treated to enhance properties such as dispersibility, lightfastness, and resistance to weather conditions. This process typically involves coating the individual pigment particles with both organic and inorganic substances. To make the most of a pigment's tinting strength, it's vital to achieve complete dispersion within the paint medium.

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White pigments are essential for achieving pure white colors as well as tinting and cover tones in paints. Among these, titanium dioxide is commonly favored for its superior hiding ability, making it ideal for opaque coverage. All materials with a refractive index above 1.7 are categorized as white pigments, and those below this threshold are classified as fillers (see Table 1).

It's important to note that the critical value of 1.7 is not universally applicable; it is influenced by the specific paint vehicle utilized, as each medium possesses its unique refractive index. Moreover, the refractive indices of different pigments vary based on the type of painting medium, including vegetable drying oils, alkyd resins, and natural resins.

High optical performance is expected from white pigments, including significant light scattering, hiding power, good tinting capabilities, brightness, minimal undertones, and high whiteness levels. The light scattering ability is particularly crucial, as it is influenced by the refractive index, particle size and distribution, and the degree of dispersion within the paint medium.

Considering these factors, light scattering power serves as a relative measurement rather than an absolute one. The other key metrics, such as high hiding power, brightness, and undertone, are, to varying extents, contingent on the scattered light from the pigment.

Hiding Power

Hiding power refers to the ability of paint to cover or obliterate the underlying surface completely when applied. Complete concealment is generally deemed achieved when a layer of paint applied over a darker background reflects 0.98 of the light compared to an equivalent film over a white background. As a result, hiding power depends on both the contrast ratio of the background and the thickness of the paint film, which must bring down the contrast difference to 0.02, based on the Weber-Fechner law.

When light strikes a paint film, a portion is reflected back while the remainder penetrates the film. This penetrating light may be absorbed or refract back out from various surfaces of the film (including the top). Complete absorption renders the film black or a dark shade, resulting in excellent coverage. The pigment imparts hiding power through its capacity to absorb light.

If light largely emerges from the surface without adequately hiding the substrate, the film's coverage will be poor, as seen with a clear linseed oil film on a white background. Alternatively, if light refracts and emerges from a film while still concealing the substrate, the paint will demonstrate good hiding quality and will generally appear white or with a light tint. In such cases, the light undergoes multiple reflections, refractions, and diffractions by the pigment, allowing it to exit without revealing the underlying substrate. The pigment therefore enhances hiding ability through light scattering.

The extent of light emerging, dependent on its wavelength, further influences the film's ability to obscure variations beneath it when colored—such as blue or red tones.

Ultimately, the interplay of the pigment's absorptive qualities and the light scattering performance collectively contributes to the film's overall hiding power.

Refractive Index

The effectiveness of hiding power in white or near-white paints relies significantly on the refractive index difference between the pigment and the vehicle. Hence, the same pigment yields better hiding ability in water than in linseed oil, with even superior results in air ("high dry hiding") owing to the expanding refractive index disparity.

To maximize hiding power with a specific pigment type, consideration of the pigment's particle size and its dispersion degree within the medium is essential.

Table 1 lists refractive indices for many white opaque pigments and extender pigments used in paints.

Table 1'Refractive Indices of White Pigments and Extender Pigments

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 represents the color perceived when a pigment-vehicle mixture is viewed under reflected light, such that the background is completely obscured. Mass color includes aspects of lightness, hue, and saturation. While this term predominantly applies to colored pigments, it can be applied to white pigments as well.

Tinting Strength

Tinting strength reflects a pigment's capability to impart color to a standard paint or pigment. When considering white pigments, tinting strength indicates resistance to discoloration by other colorants. It's frequently used to gauge relative hiding power. Hence, the same fundamental properties affecting hiding power—such as refractive index, particle size, and degree of dispersion in the medium—remain relevant for both mass color and tinting strength. For colored pigments, the inherent light absorption properties also play a major role.

Undertone

The undertone of a particular hue or paint color is the color observed when applying it very thinly or mixing it with white. This can be achieved by physically spreading the color in a thin layer, scraping with a palette knife, or diluting it with a medium.

White Pigments

Aluminum Silicate

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

Known commonly as clay or kaolinite, aluminum silicates are utilized as fillers in paints and as raw materials in the manufacture of ultramarine blue and violet pigments.

Antimony(III) Oxide

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

This white pigment is rarely used in artists' materials due to its toxicity and abrasiveness, although it occasionally serves as a flame retardant in other paints.

Barium Sulfate

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

Barium sulfate naturally occurs as barite, a mineral with a density range of 4.3 to 4.6 g/cm3. Its low Mohs hardness renders it less abrasive than other inorganic pigments. Highly resistant to acids, alkalis, and organic solvents, barium sulfate excels in lightfastness and weather resistance. Generally stable without a tendency to flocculate, it disperses easily. A refractive index of 1.64 means it's not typically classified as a white pigment; rather, it finds its role as a filler and mineral base for lake pigment precipitation. Its transparency allows for light transmission through its film while still scattering it, thus creating diffuse light effects.

Commercially, two types of barium sulfate are available: one is a natural finely ground mineral barite, while the other is artificially precipitated synthetic barium sulfate (termed blanc fixe). Due to increased costs, the natural version tends to be preferred. This pigment also enhances the flow properties of organic pigments and contributes to their dispersion during paint creation.

Health and Safety

Barium sulfate is generally regarded as non-toxic and thus permissible in several regions, including the United States and much of Europe.

Additionally, barium sulfate is a component of lithopone, Pigment White 5.

Calcium Carbonate

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

Calcium carbonate ranks as the most pervasive mineral globally, alongside silicates. Popular forms in paints include:

Calcite: Clear to opaque dense crystals with perfect rhombohedral cleavage; fluorescent varieties exist under ultraviolet light.

Chalk: A very pure limestone formed from fine calcite crystals originating primarily from the fossil remains of microscopic plankton.

Limestone: Compacted chalk developed from marine organisms’ calcareous skeletons.

Marble: The metamorphosed version of limestone, marble is primarily calcite but may comprise varying dolomite and other minerals—a coarse, compact mineral.

This versatile ingredient exhibits semi-hard qualities and is relatively less abrasive, achieving a Mohs hardness between 2 to 3 and a density close to 2.6 to 2.8 g/cm3. Calcium carbonate dissolves in weak acids but remains insoluble in alkalis. Treatment often occurs to enhance its dispersibility across diverse paint binders.

Calcium carbonate does not qualify as a white pigment due to its refractive index of 1.58, primarily serving as a filler to economically reduce paint costs by replacing pricier pigments. Similar to barium sulfate, it optimizes the flow of organic pigments and enhances dispersion during paint manufacturing.

Lead White (Basic Lead Carbonate)

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

For insights about Natural Pigments' stack process flake white, including its historical context and differences from modern lead white, consult articles on the subject.

If clarification is needed regarding the distinction between flake white and Cremnitz white, examine specific literature that addresses these terminologies.

For an exhaustive overview of the white pigments used in artists' colors, consider referring to dedicated articles about white pigments.

Lead White Pigment

Shop for Stack Process Flake White

Lithopone

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