《天工开物·下篇·五金》
陕西省中部制造的皮金，是用硝制过的羊皮拉至极薄，然后把金箔贴在皮上，供剪裁服饰使用。这些器物皮件因此都显出辉煌夺目的美丽颜色。
秦中造皮金者，硝扩羊皮使最薄，贴金其上，以便剪裁服饰用，皆煌煌至色存焉。
The sheepskin is made in central Shaanxi province. Nitric acid is used to make the sheepskin extremely thin, and then gold foil is attached to the skin, which is used to cut clothes. These articles made of leather therefore display a brilliant and eye-catching colour.