Lily Krakowski on fri 14 mar 03
Ok. Two sources, Fraser and Cooper suggest that colemanite is the cause of
spitting. Also several lesser possibilities.
It is possible that the new clay outgases differently from the previous
clay. So you may need to bisque a bit higher. It may be that the new clay,
unbeknownst to you, is actually finer in grain than the old, making it
denser and therefore making it harder for the gasses to exit in bisquing, so
you may want to bisque slower. Do remember that whaterver gives it the
black color may produce more gas than what was in the original clay. It
also may be that the denser clay give a tighter surface and your glaze
simply may not adhere as well...Make yourself a little sample of the new
clay to which you have added fine grog or sand....Test that with the glaze.
Recalculate the glaze and use a frit instead of Ersatz GB.
Sandy Cryer writes:
> Has anyone used B-3 Black clay from Laguna. I had used B-3 Brown without
> prob, so thought I would try this black clay. It threw ok, fired ok, but a
> blue glaze we use at out club spits off of it during the cone 6 elec. firing.
> This happened on several pieces thrown by different potters. It didn't just
> spit off, it flew off, plus where it overlapped other glazes, what was left
> on the pot after spitting off, crawled away from the other glaze in a stripe
> equivilent to the overlap. Very strange. The glaze formula is listed below,
> a nice bright blue, anyone have any ideas?
> The firing schedule is the one in Mastering Cone 6 Glazes
> Cone 6, Electric
> Saphire Blue
> Custer feldspar 25.8
> Gerstley borate 20.4 (this is prob Laguna Borate as they mix it for
>
> us now)
> Whiting 3.8
> Silica 27.1
> EPK 1.6
> Talc 2.8
> Nepheline syenite 13.6
> Zinc oxide 4.8
> **
> Copper carbonate 3.0
> Cobalt carbonate 1.0
>
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Lili Krakowski
P.O. Box #1
Constableville, N.Y.
(315) 942-5916/ 397-2389
Be of good courage....
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