During the melting process of quartz sand above temperatures of 1700 °C, fused fused silica develops which contains SiO2 a amorphous modification once it is solidified.

 

The low extenion value is an advantage (< 0.1 percent, see Sand expansion). Thanks to this favorable behavior, fused-silica sand can be used as basic molding material for reducing sand expansion defects. It has to be take into consideration that already at temperatures of 1300 °C a slight sintering (see Sintering) sets in. This is the reason why it is used nearly exclusively in the area of light-metal foundry.

Fused silica spec:
 
3.   Particle Size Distribution *Pan designates the percentage of fused silica passing the last reported screen for each size.