There are three methods of inducing opacity: 1) liquid drops, e. g. mixing together of glasses of different refractive index and surface tension. 2) gaseous particles, e.g. introduction of fine bubbles into the system by introduction of a compound which will decompose. 3) solid particles; the most common method. Materials used include: tin oxide (SnOI2I), zirconium silicate (ZrSiOI4I), zirconium oxide (ZrOI2I), antimony oxide (SbI2IOI3I) and titanium oxide (TiOI2I). The degree of opacification achieved will be proportional to the degree of fineness and dispersion of the opacifier. For maximum opacity the cooling should be slow. -- AATA