Glass and Mirrors - How flat is plate glass?

I need a very flat glass as a base for sharpening blades and was hoping plate glass would fit the bill. I know that float glass is very flat so how does plate glass compare to it? Between a 3/16" thick float glass and 1/4" plate glass which tends to cost more? Thanks.

I believe all modern produced glass is made using the float glass method.

what kind of blades are you sharpening? How large of a piece of glass do you need?

I have several mirrors I have reclaimed from construction projects that serve as a flat base for any such need I have. They are quite thick. Much more so than the cheap mirrors at the home building supply stores. You also need to understand that the glass must be supported full as glass will flex. Thicker glass is more resistant to the flexing.

I'm using the glass to sharpen hand plane irons and also to lap the sole so I'll probably need one at least 36" long (I have a
22" jointer plane) and however many inches wide. I was a little worried about the glass flexing since I don't have a completely flat surface to lay it on - 36" seems to be pushing it a little. I was hoping that having a thicker piece would make the glass rigid but the place I called only carries 3/16 float glass and 1/4 plate glass, maybe not thick enough huh? Man, I wish I had a local scrap yard like everyone else does so I can get the goodies like old mirrors, oh well. You think adding width might increase the rigidity?

As nap says, all glass made today is float. Float glass is made from 3/32" to 1" and is perfectly flat. If flexing is your concern use at least 3/8" thick glass. Call a few glass shops. They have many cut offs and may even have a used piece. Furniture stores use 3/8" glass for table tops and may have an odd one that you may get at a good price.

Thanks, three-eighths it is