银饰为何会变黑?是纯度不够吗?

　　daiguoyinshidehenduorenhuifaxian，youdeyinshidaijiulehuichuxianbianheidexianxiang，neime，yinshiweihehuibianhei?shiyinweitadechundubugouma?

　　yinshiweihehuibianhei?shouxian，yindehuaxuexingzhibijiaohuoyue，yincihenrongyiyuliudengyuansufashenghuaxuefanying，congerbianchengleliuhuawu。dengshijianzhanglezhihou，zheizhonghuahewujiuhuizaiyinshidebiaomianxingchengyicengheisedeliuhuayinmo。dangrangangkaishideshihou，tahuibiaoxianweiyixiexiweidebanbo，ranhoumanmandifensanchengpian，zuihouchengweiyingxiangwaibiaosecaideguangzedeheidian。

　　lingwai，rentidehanyezhonghanyouliudenghuahewu，youxiekongqizhongyefuhanliuhuaqingdengwuzhi，congeryinfayinshiwaibiaodouhuibeiyanghua，yinqileyinshibianheidexianxiang。

　　yinshibeiyanghuadeguochengshibukefangzhiwukebimiande，youxieyinshideguangmianzaipeidaideshihouhuiyujifuchumo，zaisuoyizaizheizhongzhuangkuangxia，buguangbuhuibianhei，youdeshihouhuiyuelaiyueliang。dangranhuawenbijiaofuzadeyinshi，fengxibuweihuiwufaqingli，suoyishuojiandankuanshibeiyanghuadezhuangkuanghuihaoyixie。

　　yinshibianheiheyindechunduyouguanxima?xiaofeizhezaixuangouyinshishi，keyixuanzekangyanghuanengligengqiangde“925yin”(92.5%deyinhanliang，7.5%deniedenghejinhanliang)，erbuyaoxuangouqianzuyin。