BLISTERS - Forming in pottery.
Blistering is caused by gases escaping during firing which cause faults in the process. Many things can cause blistering, over firing, too thick a coating of glaze or a color, inadequately dried clay.
Manganese is a temperamental color and it is impossible to know why when two pieces of work are prepared in the same way and fired at the same time, one will blister while the other is perfect. The color manganese or colors mixed with a high percentage of it often cause this. It forms bubbles and craters; craters occur when the bubbles burst.
Blistering is very difficult to repair. Height-wise there is no problem, it can be filed down, filled up with a color or glaze-base, then painted and fired but if the line of the profile is affected there is no solution, the lines would have to be made wider and this would completely spoil the work.