At first, like perhaps many players, I thought the Pottery skill was just a joke. But then I began to realize that the hints were too sophisticated for it to be a joke!
Look at this!
1) Top half: Peter, an old retired glassblower, not only sells the material to make glass, he also has a BASEMENT added to his house in addition to two other profession machines. His house is the only one I have ever seen with a basement - there has to be something down there.
2) Bottom left: Isabelle's stall has broken pottery around it. This means her stall was the pottery stall. Additionally, the Giant Tooth quest - the last quest she gives you (as far as I know) - tells you that she carved figurines out of the teeth: That's sculpting! This means she made and sold figurines after her pottery stock crashed - meaning that sculpting is also from Pottery!
3) Bottom right: Rendal's Pottery. The most basic hint, it comes complete with a pottery oven and TWO hints at an actual Pottery skill. Additionally, the dialogue choices show that the player "has no use" for Pottery, but that doesn't mean it doesn't exist - just that it wasn't deemed a necessary profession.
I am certain that there is some secret way to gain the skill...there just has to be! 
Then again, maybe I'm just too big a fan of Moonlight Sculptor 
Edited by Rez, 15 November 2014 - 05:44.
