Also shinchū. The brass in the Shōsō-in is a copper alloy with twenty to thirty percent lead, resulting in a yellow tinge. The relative elasticity makes it suitable for containers and plaques, and it serves as a good base for decorating with engraving lines. As it is hard to dent, it was also used frequently for making tools, particularly after the early seventeenth century, when brass became a common commodity. One theory has it that the naturally formed copper alloy known as chūjaku in the Nara period refers to brass.