The previous post didn't work out to well.

I used to use a piece of 1/4" plate glass with not frame. In using Azo I found the printing time quite long. Just happened to switch to a vacuum frame which uses a piece of clear 1/8" glass, the printing times were significantly shorter with the same negatives. I asked a glass person why and he said that plate glass is inherently stronger than ordinary glass and actually has a light reducing effect, he simply asked if the sides of the glass were at all green, that is an indication of "plate" glass which reduces light transmission.

Cheers!