Litharge or lead oxide is a toxic yellow substance, formula PbO, created by heating lead in air. It can also be formed by heating lead nitrate (Pb(NO3)2. It is used in the manufacture of paints and in glass, as well as in vulcanization.
Do I know that is lead oxide in the picture? No, I do not. But it LOOKS toxic, and it made me think of lead compounds, thus the name.