I can't seem to place rugs or tapestries into a cask to mordant or dye them like other fabric items. I'm unsure if this is a bug or if there's a different process for dyeing these items but it doesn't hurt to put it up anyway.

1. Have a plain freshly-crafted rug or tapestry

2. Place into a cask of mordant or dye

3. Hit 'seal'. It gives the error "You don't think this will result in anything useful"

In addition, I've found that picking up a tapestry from the ground will appear to duplicate it, but it fixes itself upon relogging