Jeff Tsai on mon 11 apr 05
Hi,
It seems like a long time since I last even looked on Clayart...been too busy. I have a question however, and have turned to you as my usual source of info is out of town, and alos I wanted to hear your opinions.
Our Val Cushing Satin Black Glaze was probably improperly mixed using an old formula that contains Albany Slip. Several years ago we found this formula had a problem with bubbling and causing large bloats and cracked bubbles whereever the glaze was able to pool; thus making all of the insides of our pots look like mini volcanos about to erupt. Well, we reformulated and got good results, but some new techs may have accidently used the old recipe as several pieces came out with the old problem.
Here's my question. Is there a chance that simply refiring these pieces, without reglazing them; may cause them to simply melt over the imperfections. These are not food-bearing pieces; so the glaze doesn't need to be perfect. I'll probably refire them anyway, but just thought I'd see what people thought, if the problem might correct itself in a second firing.
-jeff Tsai
karen gringhuis on tue 12 apr 05
Jeff -
You can try refiring your pots - it might work. If
you do, be sure to SOAK for at least 2 hrs. betw. C/8
and top temp. shut down. Do this even in an electric
kiln.
Working backwards, VC Black Satin Matt or VC Satin
Doll, whichever recipe you're using, is high in
metallic oxides trying to get out of the glaze during
firing. Add to this Albany Slip as an ingredient
which has so many impurities which also must get out
during the firing and problems such as you describe
can occur. BSM was formulated as an Albany-free
replacement for SDoll.
Checking with a friend who is extremely familiar with
this glaze, I understand that it has always behaved to
some degree as you describe . For this reason, it's
not a great idea to use it INside a pot. Obviously
another reason for using a "liner glaze" - which has
always struck me as a bother; but here we have a
Reason. I use VCBSM on open tumblers and bowls with
no problem however; I wouldn't be without it. Nor
would I mess with its colorants. (If this recipe
isn't in ClayArt archives or SDSU Glaze Data base &
you want it, write me back off list.)
But any glaze high in metallic oxides, (manganese is
a constant offender) can have this bubbling problem.
Tomato Red with iron ox. is another possibility. The
more confined and enclosed the space, the greater the
problem. And VCBSM is a matt glaze i.e. viscous &
stiff, which compounds the problem.
When firing such glazes (IMHO when firing period),
it's a good idea to soak at least 2 hrs. between C/8
and top temp. to let bubbling heal. The firing cycle
in a large redtn kiln tends to do this anyway. As a
True Believer in SLOW firing, I fire this way even in
my electric to mature all my glazes.
Hope this helps. Pls. let me know your refire
results.
Karen Gringhuis
KG Pottery
Box 607 Alfred NY 14802
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