Randy and Cheryl Weisz on tue 17 jul 01
Thanks to all who made suggestions on getting better suspension of one of
our favorite glaze bases.
The original glaze base (cone 6) recipe was:
nepheline syenite 20%
dolomite 20%
frit 3124 20%
OM-4 Ball Clay 20%
Silica 20%
Here are the suggestions and results from testing.
1) Add 0.5 to 0.75% Epson Salts. This resulted in a much better suspension,
very little settling even over a period of several days, and good coverage
of bisque ware. However, after firing, the surface went from gloss to
semi-matte.
2) Increase the specific gravity from 1.45 (where we currently mix our glaze
to) up to 1.6 or higher. Surprising to us... this had a dramatic effect.
We mixed to specific gravity of 1.67 and had good results. The glaze still
settles but does not result in hard concrete like formations in the bottom
of the bucket.
3) Re-formulate the recipe to reduce the frit content, and remove the
nepheline syenite. We did this with the following recipe which has nearly
the same unity formula as the original:
custer feldspar 28.6%
dolomite 20.2%
frit 3134 11.8%
OM-4 Ball Clay 26.2%
Silica 13.2%
On firing this resulted in a very similar surface, but the color was
noticeably darker.
4) Add 2% Bentonite. We choose instead to re-formulate to include 2%
bentonite. The resultant recipe is:
nepheline syenite 19.6%
dolomite 20.0%
frit 3124 20.0%
OM-4 Ball Clay 18.4%
Silica 20.0%
Bentonite 2.0%
This resulted in a very workable glaze that was the easiest to apply to
bisque ware, did not settle out, and on firing was virtually identical to
the original. If anything, it produced a more pleasing and uniform surface
and color.
Of everything we tried reformulating with bentonite was by far the best
solution.
Thanks again to all who made suggestions.
Randy Weisz
(Cheryl Weisz Pottery)
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