The Mirror Blank
Making a 4.25 Inch Dobsonian Reflector Telescope
There are a few different options of material to make the mirror. The least expensive is plate glass. The assumed disadvantage of a plate glass mirror is that it has the highest index of expansion. When your first take your telescope out into the code, while the mirror adjusts and its temperature is in flux, the surface will be more out of whack then with a material with a lower index of expansion. Alternatives in increasing price are Pyrex (the trademarked name for borosilicate glass), quartz, and more exotic blends such as Zero-Dur. There is much debate about this issue. Some would suggest that with an appropriately thin mirror, the temperature adjustment will go much quicker than a traditional thickness mirror. Some would suggest that expansion doesn't matter all that much as the biggest problem is surface air currents. Sometimes fans are used to help the mirror adjust more quickly.
I don't have a strong opinion either way. Fancy glass is fine if you have the money. It might adjust more quickly when you are figuring your mirror, but you shouldn't be in a rush anyway. It only leads to problems. For this project the mirror blank happened to be Pyrex. The 12.5 inch mirror I broke was (or at least was supposed to be) plate glass. It turned out to be tempered which is why it broke.
Grinding Down the Edge
If your mirror blank doesn't come with a beveled edge, you will want to grind it down before starting. First Hand Discover sells sharpening stones for this purpose. If during the rough grinding this edge wears off, you will want to regrind it. If you fail to do this, there's a good chance that you will chip the edge of the mirror.