Gillian Parke on thu 19 apr 01
I recently made a test batch of Malcolm Davis' shino
glaze. The first test piece came out beautifully with
lots of carbon trapping. So, I made up a larger batch
and put on another piece. This time the piece had
huge bubbles on the surface that never healed over and
again, lots of carbon trapping. I thought I my have
made the glaze incorrectly so I fired two more pieces,
one with the original test batch and one with the new
batch. The test pieces came out looking the same, no
bubbles. I did not fire these pieces myself but I am
planning to fire my kiln this weekend. Being a
chemist, I am curious as to what may be the
difference. Any thoughts? Also, has anyone ever
tried adding activated carbon to a shino to promote
carbon trapping?
Thanks
Gillian
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Cindy Strnad on thu 19 apr 01
Gillian,
I wonder if the glaze thickness on your test pieces might have been less
than on the full-sized pieces. As you probably are aware, thinner-walled
pieces will attract less glaze, while the greater moisture capacity of
thicker (larger) pieces will draw a great deal more glaze. And of course
bubbles are less likely to heal in a thicker glaze.
As both of the glazes worked well on your test pieces, I suspect this is =
the
cause of the problem. If so, you would simply need to thin the glaze a bi=
t
before applying it to full-sized pieces. I do highly recommend you try it
out on one or two less important pieces before you commit your masterpiec=
es
to it, though.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
cyberscape on sun 22 apr 01
Gillian,
With carbon trapping shino glazes the firing decisions are critical, so
start firing the pieces in your own kiln, keep really accurate records
of what you are doing and how you do it and experiment a lot. Carbon
trap shino glazes respond differently to streaming body reductions at
different temperatures. When we do a streaming reduction for 25 minutes
at ^012 we get heavy black carbon trapping, especially when the damper
is not too much open. By contrast, when the reduction is done at ^08,
all else being equal, the shinos are rich in color, with smoky
patterns. The way we load the kiln makes a difference, too, as the
streaming flame and smoke only touch some areas of the pieces if the
kiln is tightly stacked. You can get some great flashing effects by
blocking access of flame to areas of pieces in the kiln. Most carbon
trap shinos do not run. You must get them hot enough. Shinos like
long, slow firings. We go about 18 -20 hrs. including a long soak at
the end, and fire Malcolm's shino and other such glazes to ^10 bending
or down. I know that this is the short answer, but the real answer is a
workshop's worth of information. Good luck.
Harvey Sadow
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