United States Bureau of Mines Bulletin 636

Abstract

From Introduction: "In connection with the overall research by the Bureau of Mines on the nature of coal and its products, and extensive investigation was conducted on the composition of the resins from pitch of low-temperature tar. The pitch resin is that material in pitch or tar that is insoluble in saturated hydrocarbon solvents. It varies in appearance from a dark brown semisolid to a black, granular solid.