WarmGlass.com

I've been casting plates to a thickness of 1 inch with an inclusion of thin copper cut outs that have been etched with
ferric chloride. The plates have broken and refused in the first firing or in subsequent drop ring firings have either ripped
and stayed curiously intact or fractured. There are issues with off gassing through the casting. That's not the issue it
may seem as the castings are laid out so that air can escape during the cast firing. Have any of you had experience
with doing inclusions with copper etched with ferric chloride?

There's a huge difference in CTE between copper and glass. You can get away with copper wire or foil inclusions as copper is pretty malleable, but depending on the thickness, a copper plate may have sufficient stiffness to break the glass during cool down.

Tony

The copper I'm using is 'shim' copper. It can be cut with jewelery shears

We really need much more information about the firing conditions and schedules to help.
But, don't expect the copper to stretch and form in the same way the glass does. If I understand you correctly you attempted a drop ring with copper imbedded in the glass. I would expect this to break - every time.

Copper shim is relatively thick--I've stretched glass with copper inclusions but they've been copper foil, not actual copper sheet metal. If you need jewelry shears to cut it, it's too thick.

What you want is copper about the thickness of aluminum foil, i.e., something you can easily cut with scissors. And it will break apart in a drop slump as the glass stretches.