Ushered into the ring as “The Heaviest Typeface in the World,” Stefán Kjartansson’s Black Slabbath has earned its title. Kjartansson, an Iceland-born designer who now runs an interactive agency in Atlanta, admits that he bestowed the superlative on the font with a note of hyperbolic humor: Some typefaces may use more ink, but none do so as functionally. Released in February, the design began as an experiment to push the maximum amount of black into white space, yet still carve each character judiciously from its initial block. The letterforms fill spaces with line weights that challenge expected boundaries, push geometric potential, and get as thick as possible before the counters close. When put together, letters become building blocks, modular toys. But Kjartansson maintains that the font’s real success is the rhythm it achieves within such geometric simplicity, its muscle balanced with versatile footwork. The winking play on Ozzy’s metal band wasn’t suggested until the end of the design process, and is merely a happy coincidence: Black Slabbath reverberates with similar intensity and volume. It’s heavy and will not equivocate.