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odd discoloration on refire

updated sun 24 jun 12

 

Marci on sat 23 jun 12


hey guys,
One of our chinapainters posted this to our forum and frankly ,
its got me stumped. Im wondering if any of you have ever seen
anything like this:
http://ppioforum.com/viewtopic.php?f=3D7&t=3D2910
The piece itself is commercially made clear glazed porcelain .
Its been chinapainted with blue and then fired to overglazse temps
( about cone 015-ish ) ...
The odd brownish discoloration popped up .
I thought it might have been something that dropped and melted into
the piece... but it also seems to have created pits in the surface as well =
..
Mild acid ( which will remove fired chinapaint )( is not doing
anything to it... but physically abrading the surface with a Dremel
rubber bullet is... but the area is now also pitted ..
Refiring to overglaze temps is not removing the stain .
Its not mildew which resembles fine dark spots.. like pepper
sprinkled on the piece.
It there anything in the clay or glaze that could have done this
after being refired to overglaze temps? Im baffled.
I realize its a commercial piece and no way to know what the glaze
or clay composition is.. but I just wondered if any of you have ever
seen anything like it ?
Thanks
marci the chinapainter