I scrounged up:
I coated a stainless steel cup and bowl with kiln wash to stop the glass sticking.
I sanded off the silvering from some of the mirrored float glass, crushed it up and put it in the cup.
I crushed up a blown glass bottle and put it in the bowl. I washed the crushed glass in water and poured off the fine glass in suspension.
I experimented with chemically removing the silvering from the back of the mirrors. I daubed a bit of chemical on the back of the mirror and put some scratches through the mirroring with a screwdriver. The silvering appears to have three layers: Grey, red and silver.
Things that failed: