Massicot is lead(II) oxide in an orthorhombic lattice structure.Lead(II) oxide is also called lead monoxide and also called PbO. PbO molecules can occur in one of two lattice formats, orthorhombic and tetragonal. The tetragonal form of PbO is called litharge. PbO can be changed from massicot to litharage (or vice versa) by controlled heating and cooling. At room temperature massicot forms soft (Mohs hardness of 2) yellow to reddish-yellow, earthy, scaley masses which are very dense, with a specific gravity of 9.64. Massicot can be found as a natural mineral in the Earth, though it is not common. In bygone centuries it was mined. Nowadays massicot arises during industrial processing of lead and lead oxides, especially in the glass industry, which is the biggest user of PbO.The definition of massicot as orthorhombic PbO dates from the 1840s, but the substance massicot and the name massicot has been in use since the late medieval era. There is some evidence that the ancient Romans used the substance.