Titanium dioxide is obtained from ilmenite ores using either a sulfate or chloride production process. The sulfate process yields both anatase and rutile crystal forms of TiO2, while the chloride process yields only the rutile crystal structure. Rutile TiO2 is preferred for use in most coatings due to its higher refractive index (R.I.) and hence improved ability to scatter light in paint films. Figure 1 illustrates that light traveling through a film with a higher refractive index pigment will have a shorter overall path length (top illustration) when compared to a film with a lower refractive index pigment (bottom illustration). As long as the film thickness is greater than that indicated by the dotted line in the illustration, both films will appear white and opaque. However, if film thickness is reduced to the dotted line, light traveling through the film with the lower R.I. pigment will be partially absorbed by the substrate causing reduced hiding when compared to the film with the high R.I. pigment, which will remain white and opaque. Thus, choosing a high R.I. pigment such as rutile TiO2 (R.I. =2.73) to maximize the refractive index difference between the pigment and its medium allows for the greatest opacity development in the film. Additionally, TiO2 is unique among pigments in paint formulation because it provides both wet and dry hiding of the substrate by the paint film.