[ATM] How to test a mirror blank?

[Aaron L Taylor <lists@subgeniuskitty.com>]

Three of my friends and I are planning to make telescopes this winter. 
For my scope, I have just purchased a mirror blank with an unknown 
history. It is fused quartz that is 13.25" in diameter, 0.8" thick and 
comes with a ten day inspection period. While asking around for ways to 
test it, this list was highly recommended as the place to ask for advice.

The first order of business would be to check for defects. Chips and 
cracks are bad. I can shine a light in the side to look for any bubbles 
or large impurities. Strain can be checked with a pair of polarizers. 
Are there any other tests you guys would recommend that can be performed 
with relatively common tools?

I would also like to non-destructively verify that the blank is fused 
quartz. So far I know that Pyrex could also appear clear/white and have 
been told that waterglass could do the same. One method would be to 
check density. For rough figures, until the glass is in hand, if I 
assume a volume of approximately 1800 cm3 and the following densities:

Fused Quartz: 2.203 g/cm3
Pyrex: 2.23 g/cm3
Waterglass: 2.4 g/cm3

I calculate the following weights:

Fused Quartz: 3965.4g
Pyrex: 4014g
Waterglass: 4320g

I'm going to see if the local postal worker that is also a fellow 
motorcycle enthusiast will let me use their scales. However, failing 
that, I'm not too confident that I will be able to measure with 
sufficient precision to definitively identify the composition of the 
glass. Do you guys have any more reliable methods to verify what this is 
made out of? Are there any other glasses that could have a similar color 
that I should know of?

Thanks,
 -Aaron Taylor
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