The blackening of the areas where carbon black is produced is due to the nature of the substance. Because it is soot from the incomplete combustion of petroleum products, it is of extremely fine consistency, to the point that although it is a solid, it behaves like a liquid. The Texas panhandle is notoriously windy and, the least little spillage, especially on top of a railroad car, will travel like a cloud of smoke. What is seen in Borger is mainly due to wind action. This fine texture is also the reason carbon black cars have much smaller loading hatches than regular covered hoppers. Another unique feature are the faucet-like sampling spigots on the car sides.
In addition to the boxcar-like cars of Thrall and others, ACF built 449 4589 cu. ft. Center Flows for carbon black service with 20 inch diameter loading hatches and, three sampling spigots per side, like their square counterparts.
For those wanting more info on these cars, the Society of Freight Car Historians published a 87 page monograph by Eric A. Neubauer in 1990, entitled “Carbon Black Cars”, ISSN 0884-027x. This book not only explains the manufacture and uses of carbon black but, also lists the carbon black manufacturers and their locations as well as car rosters by cubic foot capacity with line drawings and, by operator, from 1933 to 1989. The January 1997 issue of Railroad Model Craftsman had a photo and drawing of a 1964 vintage Thrall 4727 cu. ft. car.