These are exposed air blisters and are completely normal. The blisters are a result of small air pockets getting trapped just at the edge of the surface in our castings. Depending on how much air is trapped inside these, they sometimes erupt and take a piece of the ceramic off. These will not affect the performance of the fire at all.
There is over 35mm of thickness to the cylinder, and this is then wrapped in a thick insulating blanket and a complete steel band to ensure that nothing can escape from the cylinder walls.
Example:
