Cheryl Shoemaker on thu 11 apr 96
Marcia,
I am no technical wizard but I've had similar strangeness with a midfire cone 6
porcelain body. I'm sure the folks you've been talking to are far better
qualified to lead you to solutions than I am but just maybe some of my
observations will jog a brain cell.
I always get LOTS of blebs with 2 of my glazes if I take them to dead on the
highest recommended cone. I'm also sure to get a few if I take it to 0.5 cone
*under* recommended max. and if I want truly gnarly pots .. well I just have to
overfire by 0.5 cone. Thus far I am putting those 2 glazes in areas of the kiln
that will get to 1 cone below max. recommended and all is fine but I'm not
satisfied with this solution but will continue to handle things this way until I
come up with better. My other glazes cause no such problems, I can take the
body 1.5 full cones over recommended cone and there are no problems. I have
decided that it is a problem resulting from a water soluble glaze component
migrating into the body and therefore lowering the maturation point on the outer
surface of the porcelain. (I throw some pretty thick pieces and on them the
blebs are purely on the surface which makes me certain it is to do with glaze
material migrating into the body especially since other glazes fired much higher
are not at risk.) I have also decided the culprit at least in these particular
glazes is Gerstley borate, which I seriously doubt you use at that temp but
maybe there is another water soluble ? Unfortunately substituting a fritted
form of boron causes other changes in both glazes which I am still slowly
dekinking. I have also thought that a slightly higher bisque may solve this but
I haven t' done it because for me that is a last resort. I have also thought
maybe calcining part of any water soluble material may be a fix but have not
done this yet. I have also tried misting the pots prior to glazing thinking
that maybe the outer surface of the porcelain wont' absorb quite so much of the
soluble material (I've no idea if it worked or not because I keep forgetting to
put a mug in the hot regions of the kiln). Will your glazes mature 0.5 or 1
cone lower than you normally fire? If so, that just might be a fix.
Like I said, I am no technical wizard but if you're not ready to give up that
clay body it might be worth some additional thought. I am certain that my
problem stems from a soluble from the glaze entering the clay body lowering the
maturation point of the body. I use those two glazes very thin so there is a
lot of water to carry such things. Hopefully my observations will set off a
lightbulb somewhere. Good luck.
Cheryl ... it is spring in SC and my eyes are bombarded with various shades of
pink ... ick ...
| |
|
