Optimal sections have been applied to the classification of bituminous coals according to the magnitudes of the parameters Vr and (G)R.I., which reflect metamorphism and caking properties of coal, respectively. The points at which the sections are divided and the number of groups are determined on the basis of the diameter of the group and the minimum error function. Coincidentally, all of these section points correspond to changes in the petrographic characteristics during the genesis of bituminous coal. The section points corresponding to Vr are 20, 28, and 36%. These points divide the coal into four groups, namely low, medium-low, medium-high, and high-volatile bituminous coals. Considering coking properties, the section points on the basis of the value of (G)R.I. are 5(20), 40(60), and 75(90). These points also divide the coal into four groups, namely, non-caking, weakly-caking, medium-caking, and strongly-caking coals. When subdivisions are made, the coals may be put into seven groups, viz. non-caking, slightly-caking, weakly-caking, medium-caking, medium-strongly-caking, strongly-caking, and extra-strongly-caking coals. Thus, a natural classification is formed and a numerical system can be used for classification.
