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