Alumen (potassium aluminium sulfate, KAl(SO4)2·12H2O; the dehydrated form is known as 枯矾)
Nature and flavour. Sour, astringent, cold. Lung, Spleen, Liver and Large Intestine systems.
Alum — a double salt of aluminium, and one of the widest-travelled substances in the materia medica, since it has also served as a dye mordant, a water clarifier and, historically, a food additive. The classical texts describe it as drying dampness and relieving itch, counteracting toxin and killing parasites, and record it externally for eczema, weeping skin complaints, ringworm and sores, used as a wash or a powder. The dehydrated form, 枯矾, is the more strongly drying and astringent of the two and is the one preferred on a discharging sore. Internally it is recorded for checking bleeding and diarrhoea, and in some classical sources for phlegm and for seizure disorders. Those internal uses belong to the traditional record: they are not supported by controlled human evidence, and the aluminium question below is why they have largely been dropped in contemporary practice.
性味归经。酸、涩，寒。归肺、脾、肝、大肠经。
明矾，一种铝的复盐，也是本草中流传最广的物质之一 —— 它同时充当染料媒染剂、净水剂，历史上还作过食品添加剂。古籍记载燥湿止痒、解毒杀虫，外用治湿疹、渗液性皮肤病、癣与疮疡，多作洗剂或散剂。煅后失水所得的枯矾燥涩之性更强，用于渗液外溢的疮面更宜。内服记载为止血、止泻，部分本草还用于化痰与癫痫。内服诸用途属于传统记载，未被对照人体证据支持；下面的铝暴露问题正是现代临床已基本弃用内服的原因。
铝盐，这一点就是安全问题的全部。铝不是营养素，机体处理铝的能力很差：国际食品安全机构为膳食铝设定过暂定每周耐受摄入量并逐年下调，中国与其他国家也已陆续限制含铝食品添加剂。药用的内服份量不是自用范畴，以完全避免为宜 —— 铝负荷主要落在肾功能减退者（排铝能力差）与儿童身上。外用时收涩，久用则刺激皮肤：不宜大面积、用于破损皮肤或长期反复使用；口腔、鼻腔、眼与外生殖器黏膜皆不宜。白矾呈酸性，误服大量曾有口腔与消化道腐蚀性损伤的报告。孕期与哺乳期避免使用。持续渗液的皮疹、久不愈合的疮面、以及任何部位的出血（包括肠道出血 —— 那正是古籍内服用途之一）都须先明确诊断，而不是靠一味收涩药。