Glass hardness is measured by four standard methods: the Mohs scratch test (qualitative, 5.5 for soda-lime, 6 for borosilicate, 7 for quartz), the Knoop hardness test (HK, used for brittle materials per ASTM C730), the Vickers hardness test (HV, with a diamond pyramid indenter per ASTM C1326), and the Brinell test (rarely used on glass).
To tell glass from quartz, use 5 quick tests: (1) UV transmission — quartz transmits UV-C down to 185 nm, glass blocks below 320 nm; (2) thermal shock — quartz survives 1,100 °C → ice water; glass shatters above 200 °C ΔT; (3) Mohs scratch — quartz scratches glass (7 vs 6); (4) thermal expansion
One of the most important thermal properties of fused quartz is its extremely low coefficient of thermal expansion: 5.5 x 10-7 /°C (20-320 °C). Its coefficient is 1/34 that of copper and only 1/7 of borosilicate glass. This makes the material particularly useful for optical flats, mirrors, furnace windows and critical optical applications which require minimum
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