Differentiating Pyrex from glass

[Speedi]

Hi all. I recently decided to undertake the endeavour of fabricating my own Newtonian telescope. I've drawn out all the plans, ordered most of the materials, and am getting ready to begin.
However, I do have a question. I recieved my blank today and was a bit surprised when I opened the box. For starters, the blank is 10.75" in diameter and 1.75" thick. I had ordered a 10" pyrex blank. Here in lies the question. I was under the impression that Pyrex came with kind of a frosty appearance on the edges. This blank is completely clear. Also, there seem to be ripples across both faces of the glass. They are not pronounced enough to be felt, but can be seen by looking at the glass from an angle.
Having calculated the density of the glass, it lies much closer to 2.23 g/cm^3 than 2.45gm/cm^3. I was curious if this must indeed be Pyrex, and if the ripples are remniscent of the annealing process.
Two final questions: Since the glass is 10.75" in diameter, should I be left with say, 10.5" of mirror surface (because of the bevel)?
Should I grind the backside of the mirror with a beveling stone? Or the sides, which have a small step (as if the glass were cut halfway from both sides)?
Any information would be greatly appreciated.
Thanks in advance,
Abel

[Satchmo]

Abel

Can you post a digital camera picture of your blank from a few angles? Sounds like Pyrex if it isn't green , but the odd diameter and homogoneity swirls make me guess it might have been one of the blanks made in Australia during the second world war when they were making Pyrex here. There are still a few changing hands. Whom did you buy the glass from?

Mark

[Garyh]

Hi abel,
Its a great project to start with and I found it lots of fun!!..
Mark is on the mark there as nearly all plate blanks I have seen are the usual greenish look. If its density is lighter than plate I say its pyrex just its a odd size disk? and have no idea where it would have come from. I ground a 6" disk which also was pyrex and was 6"1/4 full size..(American pyrex). I kept the bevel small which was a pain and had to rebevel at least 5 times through the grits. Better of to put a nice large bevel at the start, say 5-6mm.
Pick the worse side for surface defects for the back and grind it flat with coarse 120 then medium grit 180/220 on a steel plate or glass plate, give it a bevel too before you start.
Better of with a flat base to help stop asigmatism and sit well in your mirror cell. I also cleaned up the edges as my blank was very rough and not quiet round, but I have a lapidary grinding wheel which makes this easy. I would not worry about the edge if the mirror blank is nice and round..
I am only a beginner in this so thats about all the advice I can give you..
Hope you make that 1/10 + mirror and have fun on the way...
Cheers Gary

[cristian abarca]

Hi Abel. it's good to see someone else trying to make their own mirrors, there aren't too many of us around. The pyrex blanks that I have purchased are clear on both sides there is no frosting (they are from germany), however a friend of mine purchased one from the US and it did have the frosting as you suggested. plate glass has a green tinge to it if your's isn't greenish then it isn't plate. more than likely if you purchased it from a reputable source then it is more than likely pyrex. It's important to beville the edge. Always beville. I have taken chunks of the side of two mirrors due to inadequate bevelling. 2-3 mm should be enough but like gary said you would have to do it regularly. I do it after every grit change. While hogging out the depth I do it every 1 hour. Also be gentle when using a sharpening stone to bevel, too harsh and you'll chip it. Go to this thread and scroll down to see what I mean, I also have a picture of a pyrex 4 inch blank and a plate glass tool http://www.iceinspace.com.au/forum/s...ad.php?t=11384

regards Cristian