Selenium is used in photocells. The gray metallic form is used, as it changes its electrical conductivity when light shines on it. Selenium is also used as a catalyst. The largest use is to color glass red. It can be used in special brasses instead of lead. It is used in some rectifiers. Most use silicon instead. It is used to tone photographs. It can be used to remove dandruff from hair in the form of selenium sulfide.