Two types of lead oxide are used in ceramics: litharge, or lead monoxide, and red lead.
Litharge: Lead monoxide (PbO) has a specific gravity of 9.3 to 9.7; a melting point of 888°C. It is insoluble in water but soluble in alkalies, certain acids, and some chloride solutions.
Red lead: Lead oxide (Pb3O4) has a specific gravity of 9.0 to 9.2; it decomposes between 500 and 530°C. It is insoluble in water and is decomposed in some acids, leaving insoluble lead peroxide, PbO2.
Lead oxide is used quite extensively in optical glass, electrical glass, and tableware. It increases the density and refractive index of glass. In addition, it can be cut more easily than other glasses and has superior brilliance, both of which make it good for cut glass.
Lead glasses may be formulated with a wide variety of electrical and acid-resisting characteristics; desirable properties, such as weather resistance, electrical resistivity, etc., will depend upon the total composition of the glass.
Lead has many advantages as a glaze ingredient. The superiority of lead glazes lies in their brilliance, luster, and smoothness, which are due to their lower fusion point and viscosity. Lead glazes are, in general, highly resistant to water solubility and chipping, and have few faults in texture and bond. They have high mobility, refractivity, and elasticity and are softer than leadless glazes.
All investigators agree that the use of fritted glazes, in which all of the lead is fritted, has important health advantages. In this way the raw lead oxide is converted into relatively harmless lead silicates, which are much less soluble in dilute acids or gastric juices. Lead silicates are more slowly absorbed after entering the respiratory system, and can be eliminated with less lead absorption.
Lead oxide also is used in enamels. With an increase in lead and a corresponding decrease in potash, with flint constant, enamels become more fusible, have less tendency to craze, and become more refractive, but are less durable in acid fumes.
