Around 1450, Johann Gutenberg was the first to bring together the complex systems and subsystems necessary to print a typographic book, including a thick, tacky ink that could be smoothly applied and did not run off the metal type; a strong, sturdy press; and a metal alloy that was soft enough to cast yet hard enough to hold up for thousands of impressions. But the key to his invention was the __________ used for casting the individual letters.