Metallography is the study of the patterns in metals, the nature of fractures, and the microscopic crystal-formations. However, neither metallography as a science nor the crystal theory of metals emerged until nearly a century after the invention of the microscope.[30] The ancient swordsmiths had no knowledge of metallurgy, nor did they understand the relationship between carbon and iron. Everything was typically learned by a process of trial-and-error, apprenticeship, and, as sword-making technology was often a closely guarded secret, some espionage.