They are usually a pretty soft terra cotta that would probably fail at high temperature. They sometimes come up for free when they get a little ugly. I could see using one or several encased in cob as either a bell for a very small system or as an elbow, when clay tile is used for a cob encased flue.
And tomorrow is the circus! We can go to the circus! I love the circus! We can take this tiny ad:
Hope in a World of Crisis - Water Cycle Restoration