Color and Undertone
Many industries measure color using the CIELab scales based on a three dimensional representation, sometimes known as a color sphere. This is shown schematically in Figure 7.1.
Thus brightness is measured as the L* value (black being zero and pure white being 100), the red-green tone is measured as the a* value (for TiO2, positive is red toned, negative is green toned), blue-yellow tone is measured as the b* value (positive is yellow toned, negative is blue-toned). The standard of 100% reflectance is generally taken as freshly smoked magnesium oxide, although more stable, ceramic sub-standards are used in practice.
Although the color of TiO2 pigment is broadly described as 'white', anatase has a somewhat bluer mass tone due to greater reflection at the blue end of the spectrum. The brightness and color of both forms may also be degraded by impurities in either the core particle or the surface treatment. In practice, the a* value rarely changes but L* and b* can both be affected by coloring ions such as iron, niobium, vanadium and chromium present in the ore, in surface treatment chemicals or picked up from processing equipment during manufacture.
For TiO2, color is generally described in terms of brightness (L*) and mass tone (a* and b*) but a further factor, its 'undertone' which is related to its particle size distribution is also important especially in colored binders and tinting systems.
The undertone or tint tone is a fundamental property of a TiO2 pigment and is a function of its particle size distribution. Thus, the relative light scattering power of rutile TiO2 pigments at different wavelengths varies with its particle diameter. Figure 7.2 shows the effect.
In practical terms, reflectance at the blue end of the spectrum may be enhanced by making the mean particle size smaller, with maximum blue reflectance occurring at circa. 150 nm (although the mean psd for real pigments is not normally made this small because overall opacity would be significantly reduced). Changing a pigment in this way has a small effect in white systems where the overall incident energy is scattered back from the film but it may be sufficient to produce a 'blue white' instead of a 'neutral' white.
However, changing the undertone of a pigment has a significant effect in tinted systems, especially gray, where the effect is clearly visible. Thus changing to a blue tint tone pigment would change the tone of a gray from a red through a neutral to blue tone. Other colors are also affected to a greater or lesser extent.
Conversely, a pigment with a large mean size scatters red light more efficiently, producing a reddish tone in gray films.
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