Recycled glass comes from everyday window glass, table tops and bottles. Since the manufacturers of such glass are simply creating a fast setting/stiff glass as a consumer product (i.e, a bottle to hold soda pop, etc), they are understandably not concerned about small changes in the glass manufacturing formula. Therefore, the COE's between even bottles from an identical product or sheets of window glass can vary typically from the 70's to 80's. However, if you use glass all from the same piece (i.e., a broken glass table top), that glass is always compatible with glass from the same exact piece. If you want to mix glass from various sources, you have to do pretesting with several pieces in a small kiln and examine the cooled pieces for stress cracks.
Recycled glass takes on a fascinating stone like appearance after several kiln firings. It has to be fired at higher temperatures than art glass (over 1550) and one only gets about 2-3 firings before the glass stiffens up so much it will not flow any longer in a kiln. If you look closely at a glass table top or a clear bottle, you will often see a greenish tint which becomes very pronounced after the glass is kiln-fired.
The second photo above (Glass Flower Reborn, Copyright 2008) shows a sculpture I created using clear bottle glass for the top/flower (with copper foil inclusions), bottle glass for the stem and window glass for the base. The first photo (Glass Flower Reborn, Copyright 2009) shows a sculpture using various shades of green bottles.