On 05/09/2023 23:59, Gary Shaddick wrote:
I recently came across a 6" f/8 reflector where the owner says that the mirror has a quartz substrate. He actually made the mirror several
decades ago. Is there any way I can tell for sure if what he says is true? There are no papers or certifications, just his word that it is quartz. thank you
The difference in density between fused silica (2.22) and Pyrex (2.23)
is too small to measure without fancy kit and the owner may not allow
you to dunk the whole thing in water on a balance anyway.
https://psec.uchicago.edu/glass/Mechanical%20Properties%20of%20Glasses.pdf
If he claims it is from pure quartz natural single crystal then that has
a specific gravity of 2.65 which you could determine by weight alone.
https://en.wikipedia.org/wiki/Quartz
You might be able to do it by a star test after deliberately keeping the
mirror a bit warmer than its environment. Quartz has about 5x lower
expansion coefficient than Pyrex so should have a better figure much
sooner than you are used to for a similar sized instrument. eg
https://adsabs.harvard.edu/full/1966IAUS...27...51M
It could be done non-destructively by portable XRF fluorescence with the
right kit and a helium blanket over the test specimen. eg
https://www.portaspecs.com/detecting-light-elements-with-portable-xrf/
Sodium and/or potassium will be present at a few % level in a glass but virtually absent from pure quartz or fused silica. Your local university geology department might have a bog standard XRF - ones that can get low
mass elements like sodium are still a bit thin on the ground.
If you are allowed very slightly destructive testing then a pinprick
level of damage to the reverse side would allow ICPOES or ICPMS to
detect all the impurities in a microgram sample using laser ablation.
(again local uni geology or chemistry dept)
They might even be able to tell you where the silica it was made from originated on a good day. They can within reason do it for gold, some
precious stones and fine wines (when fakery is suspected).
--
Martin Brown
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