Janet H Walker on sun 17 may 98
...I am looking for a simple recipe for a clear glaze for a
single firing for cone 6 porcelain. Something you have used, and
that you know works.
Carrie -- Almost any glaze can be single fired. The bigger problem
you face is that of the firing cycle for the piece itself. If you
are in a community pottery where the kiln is shared by everyone, you
may have a problem. First, make sure that whoever loads and fires
the kiln, if it isn't you, agrees that they are going to put all of
your pieces into that kiln load! Get them to agree that they will
single fire your work -- many people with many years of experience
do not understand single firing and think that pieces will
automatically "explode". Ain't so.
The glaze. The clear glaze that you would ordinarily use is
probably fine if it contains say 15% or more of EPK. The problem
that you will run into is the thickness of the glaze. If the glaze
in the studio is normally used for bisque, it will be too watery to
put onto greenware. Take out as much glaze as you need in order to
apply it the way you want (e.g. enough to dip), let it settle awhile
and pour off the water. Greenware can be dipped, poured from, or
brushed onto. But if there is too much water in the glaze mix, it
will go into the clay body and soften it up too much.
Putting on the glaze. I spritz the piece with water first so
the clay won't just suck up water from the glaze real fast. then
put on glaze on one side. If you want two coats, as you would for
brushing, put on the second coat as soon as the shine goes off the
first one. Wait until the pot is dry enough to handle and glaze
the other side. Some people will tell you that you have to wait
until the next day to glaze the other side. Not so. Not unless
your glaze has too much water in it. Just do it.
Once the glaze is dry, smooth over any pinholes or fine cracks just
the way you normally would with any bisqued piece. And let it dry.
Or dry it right next to something hot, like in the kiln room. I put
my pieces directly into the kiln and let the kiln finish drying them
with a long (4 hr) sit just about 250F. If the firing cycle at your
place includes "overnight on low" you should be fine. Since you are
using porcelain and a clear glaze, I am not too concerned about the
actual firing schedule -- it would be a worry with a stoneware clay
that has more impurities that need burning out. Being on a short
schedule like this is a risk because so few things work out just
right the very first time you do them, even if you do everything
right (I believe the gentleman's name is Murphy) but in principle I
don't see that what you are trying to do shouldn't work. That is to
say, go for it.
Oh yes. About handling. Greenware is quite strong. Handle it
gently and it will survive fine. Some normal methods for putting
glaze onto bisque won't work. For example, don't bother trying
glaze tongs. The pressure that those exert on the wall of the piece
will break it. Don't pick pieces up by the very edge and shake.
Ditto. Hold pieces in such a way as to distribute the stresses.
Use two hands. Pick up mugs around the base, not by the rim or the
handle. Just be gentle and they'll be fine.
I'm sure there is more but can't think of it at the moment. Ask any
questions you can think of. If there is anyone in your studio who
has done single firing, ask if you can watch them do it or if they
will talk you through doing one or two until you feel comfortable
with the moves.
Those of us who work this way swear by it.
Regards,
Jan WAlker
Cambridge MA USA
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