Two catalytic additives are added to a mixed solution of acetone and furfural to produce a resinous solution for use in impregnating a carbonaceous product. A carbonaceous article is impregnated with the solution at a reduced pressure, and the article is immersed in a concentrated sulfuric acid bath. Then, it is withdrawn from the acid bath, rinsed, and dried. Finally, it is subjected to a heat treatment at elevated temperatures to carbonize or graphitize the carbon product impregnated with the resinous solution.