tetragonal form is usually red or orange color, while the orthorhombic is usually yellow or orange, but the color is not a very reliable indicator of the structure. The tetragonal and orthorhombic forms of PbO occur naturally as rare minerals. Reactions Metallic lead is obtained by reducing PbO with carbon monoxide at around : PbO + CO → Pb + The red and yellow forms of this material are related by a small change in enthalpy: PbO(red) → PbO(yellow) ΔH = 1.6 kJ/mol PbO is amphoteric, which means that it reacts with both acids and with bases. With acids, it forms salts of via the intermediacy of oxo clusters such as . With strong bases, PbO dissolves to form plumbite (also called plumbate(II)) salts: PbO + + → Applications The kind of lead in lead glass is normally PbO, and PbO is used extensively in making glass. Depending on the glass, the benefit of using PbO in glass can be one or more of increasing the refractive index of the glass, decreasing the viscosity of the glass, increasing the electrical resistivity of the glass, and increasing the ability of the glass to absorb X-rays. Adding PbO to industrial ceramics (as well as glass) makes the materials more magnetically and electrically inert (by raising their Curie temperature) and it is often used for this purpose. Historically PbO was also used extensively in ceramic glazes for household ceramics, and it is still used, but not extensively any more. Other less dominant applications include the vulcanization of rubber and the production of certain pigments and paints. PbO is used in cathode ray tube glass to block X-ray emission, but mainly in the neck and funnel because it can cause discoloration when used in the faceplate. Strontium oxide and Barium oxide are preferred for the faceplate. The consumption of lead, and hence the processing of PbO, correlates with the number of automobiles, because it remains the key component of automotive lead–acid batteries. Niche or declining uses A mixture of PbO with glycerine sets to a hard, waterproof cement that has been used to join the flat glass sides and bottoms of aquariums, and was also once used to seal glass panels in window frames. It is a component of lead paints. PbO was one of the raw materials for century eggs, a type of Chinese preserved egg. but it has been gradually replaced due