《天工开物·中篇·冶铸》
铜钱质量的高低，是以锌的含量多少来辨别区分，至于从外在质量看铸钱成品，轻重与厚薄，那是显而易见的。
凡钱高低以铅多寡分，其厚重与薄削，则昭然易见。
The quality of copper coins is distinguished by the zinc content. As for the external quality of cast coins, weight and thickness are obvious.