ceramic used in welding

Only one I know of is the ceramic nozzles on TIG torches. Ceramics are a good insulator, though. I have heard of guys making firebrick tables, small furnaces from piled up firebrick, or piles of firebrick to warm large castings.

A better-phrased question might get a more informed answer.

Ceramics are a good

tables, small furnaces

large castings.

answer.

Root pass backup.

Use of ceramics as a backing material is widely accepted. Most commercially available ones I've seen come with a sticky-sided steel tape pre-attached. Simply peel and stick the ceramic against the back side of the work (far side of the root pass from where you are welding) and fill in the gap.

That said, it does take some skill to end up with a U.T. or R.T. passable weldment. In my place of employment, you can only use it if the engineers say you can (backside only accessable enough to remove the ceramic, but not enough to backgouge and weld).

Typically it is only used in flat, horizontal, and vertical welds.