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22-inch diameter Blank

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We have a 55.88 cm diameter mirror blank for sale, 8.26 cm thick of unknown origin.

It weighs 130 pounds or 59 kg, so its density is 2.90 g/cm^3, which rules out Pyrex (d=2.23) or Borofloat (2.2) or Zerodur (2.53)*. It has an orange/yellowish cast that doesn't show up very well in the photographs. The closest matches that I find in the Schott catalog are:

N-SF64 with density 2.99, index of refraction 1.706 and Abbe number 30.23 or
N-KZFS4 with density 3.00, IOR 1.613 and AN 44.49 or
P-SK57 with density 3.01, IOR 1.587 and AN 59.60.

By comparison, some of the commonest and cheapest optical glasses are BAK-4 with density 3.05 and BK-7 with density 2.5. 

Not sure how our club member acquired it, and he's now deceased. 

There is one chip near one edge.

This is quite a heavy piece of glass, so shipping is going to be expensive. We are located near Washington, DC, in case you'd like to drive here and pick it up.

*however, if we assume that our measuring tape was reading 2 mm too low and our scale was reading 2 pounds too high, then the density could be as much as 3.05. If the measurements were off by equal amounts in the opposite direction, then the density could be as low as 2.78. So that leaves a lot of different types of glass!!!