How to tell fused silica from pyrex

[mura_gadi (Steve)]

Hello,

I am after a simple test for checking my numerous unlabeled glass blanks I recently became the proud owner of. Soda glass is easy, I can just check to see if the edges are green on the blank. The Pyrex over fused silica is proving to be a lot harder to find an answer for though.

While I have numerous labeled Pyrex 6" blanks in cardboard packaging, I also have a few that are packed in wooden boxes and that makes me think they might be fused silica blanks. Wishing thinking maybe, but both the wooden box and the internal packaging seems a step up from the rest of the items on offer.

Weight doesn't seem to get me a good answer, pyrex comes in at 2.2g/cc and fused silica at 2.2-2.23g/cc. I'm thinking my kitchen scale will struggle with that at 6" blanks as they're pretty cheap.

I'm after suggestions that can be done at home, or with minimum expense, laser pens/pointers and scatter patterns maybe?


Thanks
Steve

[Rerouter (Ryan)]

Well this is far harder than I first thought.

Melting points - almost the same
Refractive index - almost the same
Density - almost the same
Mohs hardness, 6.5 vs 7.5 maybe able to try a scratch test
Light frequency cutoff - so far into UV that no one likely has a source. And practically the same in IR.
Dielectric constant - near identical
Thermal expansion constant - 4 vs 0.6 parts per million per degree c. So every 2 degrees would cause 1 micron delta between the diameter of the 2. E.g. heat them 30 degrees and measure the change with a vernier.

I would probably start with a scratch test or the thermal expansion test. The silca has the lower expansion constant.

[AstralTraveller (David)]

One thing that would determine the difference in a minute is a portable XRF spectrometer. And if this inquiry had come up last year I might have been able to help but I am now on the scrapheap - a victim of the government's abandonment of the Tertiary education sector.

Portable XRF is a common tool in geology/earth science departments and in the mineral exploration industry. I don't know who else uses them. I'm not sure how well it will detect boron (it doesn't like light elements) but the borosilicate glass also contains Al which it will detect.

[mura_gadi (Steve)]

Hello,

Sorry to hear about your scrapping, both at a personal level and at the country level.

However I know most of the admin staff at Canberra Astronomy Society do work at the ANU and I think I might have a friend at GEOScience Australia HQ still.

So, that might provide two avenues, won't even need to be portable as I can take the glass to them, with a nice bottle of scotch to be left forgotten as I leave.


Thanks heaps
Steve
Ps. Otherwise its a compass needle on a pivot board I can load up with increasing weights.
Pps. The bottle bribery offer applies to my GEOScience friend only, not meaning to impede ANU staff in anyway.

[astro744]

The three Pyrex blanks I’ve seen and two I’ve worded on all had a yellow tinge especially noticeable when looking through the edge. Two were Astro-Optical Supplies glass and one was Coulter Optical. I read that soda glass has a green tinge.

I have a 16” blank (unknown glass type, 29mm thick polished to 5% from edge). It is distinctly blue/green. It’s a project I may finish one day (or not) as the case may be.

[AstralTraveller (David)]

Originally Posted by mura_gadi

Hello,

Sorry to hear about your scrapping, both at a personal level and at the country level.

However I know most of the admin staff at Canberra Astronomy Society do work at the ANU and I think I might have a friend at GEOScience Australia HQ still.

So, that might provide two avenues, won't even need to be portable as I can take the glass to them, with a nice bottle of scotch to be left forgotten as I leave.


Thanks heaps
Steve
Ps. Otherwise its a compass needle on a pivot board I can load up with increasing weights.
Pps. The bottle bribery offer applies to my GEOScience friend only, not meaning to impede ANU staff in anyway.

The reason for suggesting a portable is that you simply put the probe up to the sample, measure and get data much better than you need. Lab XRF provide even higher quality data but the sample has to be prepared and placed in a sample holder - you would have to powder some mg of sample. So a portable is easier.

There is a part of me that thinks that there must be an even easier test but it eludes me at the moment.

Yes, the treatment of the tertiary sector has been appalling. Apart from the strategic need for an educated population this sector was recently (if my fuzzy memory serves) the country's third biggest export earner.