Physical Characteristics
The common type of borosilicate glass used for laboratory glassware has a very low thermal expansion coefficient (3.3 x 10−6/K), about one-third that of ordinary soda-lime glass. This reduces material stresses caused by temperature gradients which makes borosilicate a more suitable type of glass for certain applications (see below).
The softening point (temperature at which viscosity is approximately poise) of type 7740 Pyrex is 820 °C (1,510 °F).
Borosilicate glass is less dense than typical soda-lime glass due to the low atomic weight of boron.
While more resistant to thermal shock than other types of glass, borosilicate glass can still crack or shatter when subject to rapid or uneven temperature variations. When broken, borosilicate glass tends to crack into large pieces rather than shattering (it will snap rather than splinter).
Optically, borosilicate glasses are crown glasses with low dispersion (Abbe numbers around 65) and relatively low refractive indices (1.51–1.54 across the visible range).
Read more about this topic: Borosilicate Glass
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