《明史·志·卷五十七》
洪武末年，因工部上言，又全部开放，让人民得以自己开采冶炼，每三十分抽取二分。
末年，以工部言，复尽开，令民得自采炼，每三十分取其二。
In the last year of Hongwu, because Ministry of Industry reported, it was all open again, and people could mine and smelt by themselves, two out of every thirty minutes.