text: Thallium ( I ) oxide is the inorganic compound of thallium and oxygen with the formula Tl2O in which thallium is in its +1 oxidation state . It is black and produces a basic yellow solution of thallium ( I ) hydroxide ( TlOH ) when dissolved in water .
simplified: It is made by heating thallium ( I ) hydroxide or thallium ( I ) carbonate in air .