I made one of these and deployed a portable kiln.
I did it next to Marius Lenn to see if I could advance my trophy via the free clay he gives you when hailing him (part of mercenary recruitment quest). I was able to use it to create the table wear.
After about eight minutes, my attempts to combine it (via the Combine button after having learned it), I got the message saying, "Your request failed because the item was not found." I then attempted the combine in experiment mode and that similarly failed.
The fixed kiln was still visible, but clicking on it did nothing.
I was able to get about ninety combines in before it stopped working. Despite having my Master Potter Trophy equipped, it did not advance at all.
While the characters who were in the zone at the time could still see the kiln, subsequent characters that zoned in later could not.