Increasing Al2O3 stiffens the melt and gives it stability over a wider range of temperatures (although excessive amounts may tend to cause crawling, pinholes, rough surfaces). The addition of Al2O3 prevents devitrification (crystallization) of glazes during cooling because the stiffer melt resists free movement of molecules to form crystalline structures. Thus crystalline glazes tend to have less than .1 molar equivalents of Al2O3. The addition of small amounts of CaO will help reduce the viscosity of a melt and make it flow more freely.