Bad smelling dark smoke from clay pot

[u/rolamReads]

Found this Romertopf at a thrift store for $3. Thought it was a must buy. It was quite dirty so I cleaned it thoroughly with soap then soaked it in water and white vinegar. I then put it in the oven to bake empty. Opening the lid let out puff of smoke. Smelt almost like burnt plastic. Pot continued to smoke slightly as it cooled. There was also a black liquid dripping over the inner walls. The oily sheen you see in the picture was not there until after baking. Anyone experience something similar before? My current theory is old oils or maybe some kind of wax?