Low temperature clays are far more likely to have this issue. And if present, it is more likely to be unsightly. The salt-free specimens have 0.35% added barium carbonate.
A common problem with dry and fired ceramic. It is evident by the presence of a light or dark colored scum on the dry or fired surface.
|Hazards||The Use of Barium in Clay Bodies|
The Majolica Earthenware Process
How to make strong, durable functional ware from red terra cotta odies based on the traditional of majolica.