HSQ 900 and 910 are two varieties of synthetic fused silica with different contents of OH and Cl. As a result of synthetic precursors, both materials feature highest possible purity levels with total contamination in the ppb range. Whereas HSQ900 is the low OH variety with a typical content of 0.2 ppm, HSQ910 is characterized by a relatively low Cl concentration. The viscosity of fused silica is lower than that of other fused quartz types.