Types of glass
Glass Type Thermal Coefficient of Expansion (PPM/Deg C Thermal Conductivity (W/M/Deg C) Specific Heat (J/Kg/Deg C) Relative Material Cost (Plate Glass=1)   Comments
Plate Glass > 8.6 0.5 730 1   Produced by all manufacturers. Suitable for elliptical flats. Quite suitable
  for mirrors up to say 12" as these smaller thinner mirrors stabilise
  quickly, but most customers are now going for a lower expansion glass
  even for these smaller sizes.
Suprax 8488 4.3 1.2 ? 1.1   Produced by Schott. Our main low expansion glass provider up to mid
  2004 after which the kiln was shut down. A supply became available
  again in 2006. It is popular because its price is close to that of plate glass
Pyrex 7740 3.25 1.13 726 1.3  Produced by Corning. Thermal coefficient marginally better than Suprax,
 but a bit more expensive. Very expensive thicker than 25mm
BK7 7.1 1.11 858 1.1  Produced by all manufacturers. Our main use is Cassegrain secondaries
 where we need the good transmission characteristics for testing.
BVC 2.4 - 2.8 ? ? 1.2  Produced by ASM products of Canada. Definitely a serious option for
 very large mirrors, but not as popular at the moment for smaller mirrors
 due to its looks (it's black!) Started getting more use from mid 2004.
 Readily available up to at least 30".
Fused Silica 0.55 1.38 703 #1  
Supermax 33 3.25 1.2 830 2.5  Produced by Schott. Same materials as Borofloat but produced by a
 rolling process.Potential for mirrors up to 1m diameter up to 65mm
Zerodur > 0.02        Produced by Schott. This has extremely low thermal expansion, but you
 have to be able to afford it!
ULE 7971 > 0.05 1.1 776 #1  Produced by Corning as an alternate to Zerodur.
Borofloat 3.25 1.11 830 #1  Produced by Schott as an alternative to Pyrex.
E6 2.8 1.1 730 #1  Produced by Ohara as an alternative to Pyrex.
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