During dicussions on Rome, China, and Industrialization, one of the oft-cited reasons why Roman and Hellenistic countries couldn't develop powerful steam engines or guns is because they didn't have the proper "metal casting techniques," or something along those lines.
The question is, what are those techniques and breakthroughs in metallurgy that are required for large cast-metal object to withstand high pressures? Is it something involving inpurities in the metal and high-temperature furances, or what?