*2.2.4 Color*

FA from bituminous coal has a darker gray color which comes from lignite or sub-bituminous coal and also can be buff to tan in color. It is thought that the gray color could be explained by the presence of unburned carbon (UBC). If the

**Figure 2.** *Scanning electron microscope (SEM) micrographs of fly ash particles [30].*

percentage of carbon is low or absent in ash, then the color might be brown, due to the presence of iron (+3) compounds. The color changes to bluish gray to gray if the iron compounds are (+2) [7].

Tanosaki et al. [13] have reported the use of colorimetric methods or as is known as the Munsell system to identify colors by following the next three dimensions, hue, value (lightness), and chroma (color purity). In 1905, Professor Albert H. Munsell created the Munsell system. According to Malacara [31], **Figure 3**, describes the color circle system. The system is divided into five principle hues: red, yellow, green, blue, and purple, along with five intermediate hues halfway between adjacent principle hues. Each of these ten categories is used to divide into other ten sub-categories, so that 100 hues are given integer values.
