Marshall Colman on thu 17 dec 09
I am using Contem underglaze colours on a tin glaze, painting overglaze w=
=3D
ith
a soft brush. The red and yellow give trouble, appearing refractory and
forming an unpleasant crust. I have increased the firing temperature
steadily from cone 04 to cone 01, with no success. In fact, when I re-fi=
=3D
red
from cone 03 to cone 01 the problem got worse on some pieces. The troubl=
=3D
e
is usually bad when the glaze is thin and the colour is thick, but it is
unpredictable.
Oxides give no trouble at all and the problem only occurs with these
commercial stains, which I believe are frits.
This has been bothering me for years and if it weren't for the fact that =
=3D
I
want to use read and yellow I would stick to the good old cobalt, copper,=
=3D
iron and magnesium.
The glaze is:
Lead bisilicate=3D0955%
Cornish stone 14%
Borax frit=3D09 8%
Tin oxide=3D09 7%
Zirconium silicate=3D09 7%
China clay=3D09 5%
Bentonite 1%
Zinc oxide 1%
Any suggestions?
Marshall Colman
St Albans, UK
Paul Herman on thu 17 dec 09
Marshall,
It sounds like you are using underglazes, but putting them over the
glaze. If this is the case, it seems like you should try using the
colors UNDER the glaze. An underglaze may be formulated more as a slip
than a stain, and contain clay or other refractory ingredients. I
think what you need for overglaze decoration are stains, not
underglazes.
Best wishes,
Paul Herman
Great Basin Pottery
Doyle, California US
www.greatbasinpottery.com/
On Dec 17, 2009, at 11:28 AM, Marshall Colman wrote:
> am using Contem underglaze colours on a tin glaze, painting
> overglaze with
> a soft brush. The red and yellow give trouble, appearing refractory
> and
> forming an unpleasant crust. I have increased the firing temperature
> steadily from cone 04 to cone 01, with no success.
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