Hi Celeste,You can use it up
You can use it up or down — but some irid deteriorates if fired down twice or more. I like the look of irid fired down – the irid textures nicely.
Face, capped with clear – they all work fine. Iridized glass is pretty tough stuff.
As for firing, when I have irid in a piece I fire much more conservatively. I believe the thin, metalic coating can really mess with even heating.
Helios Kiln Glass Studio