C.C. Bookout on sat 14 apr 07
The problem that I see with that solution is that if the clay body is
vitrified a glaze on it will not be absorbed into the body of the
clay itself and the glaze will just be sitting on the very surface
and there will be not clay/glaze interface. I guess it is worth a
try, but I would be sure to protect the shelves as the glaze might
fall off.
Dave Finkelnburg on sat 14 apr 07
Dear C.C. Bookout,
Glaze may, can, does "fall off" a pot during
firing...sometimes...but in my experience there is no
reason it should just because the pot is vitrified
before the glaze is applied to it.
My experience applying low fire (cone 08 to 04)
glazes to pots which have been previously fired to
cone 5 has been very acceptable. I have not had any
problem with the glaze either crawling or falling off
the pot. Of course, I have had probably all glaze
problems, at times, with high fire glazes on low-fired
bisque/biscuit.
Firing greenware several cones higher than the
subsequent glaze firing is a common industrial
technique for functional pots. It is very proven and
very workable. It is the result of a conscious choice
to follow a particular path to making pots. They key
is to formulate, mix and apply so that shrinkage
cracks do not develop as the glaze dries. If the
glaze cracks and curls away from the pot upon drying,
that guarantees glaze will fall off onto shelves
and/or other ware. :-(
Good glazing!
Dave Finkelnburg
Date: Sat, 14 Apr 2007 10:05:49 -0400
From: "C.C. Bookout"
The problem that I see with that solution is that if
the clay body is
vitrified a glaze on it will not be absorbed into the
body of the
clay itself and the glaze will just be sitting on the
very surface
and there will be not clay/glaze interface. I guess
it is worth a
try, but I would be sure to protect the shelves as the
glaze might
fall off.
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Donna J.S. Causland on sat 14 apr 07
Hi Dave , Stephanie and others who replied:
Thanks for the input. I appreciate the opportunity to
talk out a problem I am trying to solve.
I am trying to change to cone 6 porcelain from 03
earthenware - stronger more beautiful body.
My forms are 20" bowls that slump at cone 6 but
work if I fire them upsidedown and supported, therefore,
unable to be glazed at cone 6.
After working so long in cone 07-03 glaze range I have
so many combos (hot little sellers) that I hate to give up.
So, as always, I am trying for the best of both worlds.
So far, no problems with applying glaze, brushes on,
doesn't flake off. Reds are gorgeous on the porcelain.
BUT, it seems like the glaze is shrinking, like it is pulling
itself into little globs and between the globs it is thinner,
causing a depression and consequent darker coloring due to
the glaze being thinner, evenly spaced dark dots in the glaze.
I was wondering if I sprayed a thin coating of a flux, like
the ferro frits 3110,3124, or 3134, on the porcelain, let it
dry, then glaze, if it would help flux the glaze. I think it is
coefficient of linear expansion problems I am having.
I am having difficulty understanding how it works from
what I have read. It seems the higher the COE, the more
it expands during firing, the more it contracts during cooling
and therefore causing more crazing as well. It seems my glazes
are not visibly crazing, but thinning. Also, I don't know the COE
of the commercial glazes.
I did pick up those frits and will test this idea. I was wondering
if anyone has any suggestions or experiences they wouldn't mind
sharing.
Dave: I appreciate your comment, "It is the result of a conscious choice
to follow a particular path to making pots. " This path to finding a
stronger
clay body began when you explained to me at the Pomona Tile conference how
crazing in the glaze was weakening my clay body.
Thank you in advance for your time and input,
Donna Causland
djscausland@comcast.net
John Rodgers on sun 15 apr 07
Calculate the dry weight of glaze in a volume of water and glaze, then
calculate what 1% or 2% of that dry weight is. Add that much CMC to the
glaze. Then brush, dip or spray the vitrified piece. I do this quite a
bit. The glaze with the CMC will dry hard and be easy to handle, and
when it fires it will burn out, leaving a glaze well adhered to the
surface of the post.
Works for me.
Regards,
John Rodgers
Chelsea, AL
C.C. Bookout wrote:
> The problem that I see with that solution is that if the clay body is
> vitrified a glaze on it will not be absorbed into the body of the
> clay itself and the glaze will just be sitting on the very surface
> and there will be not clay/glaze interface. I guess it is worth a
> try, but I would be sure to protect the shelves as the glaze might
> fall off.
>
> ______________________________________________________________________________
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Lee Burningham on sun 15 apr 07
Howdy,
=20
Agreeing with Alex Solla about adhesion of the glaze to the pot.
=20
A coat of Heavy Spray Starch will also give your pots a base for the =
glaze to hang on to when glazing a pot that has been fired at a high =
bisque temperature. It even works with pots that have been glaze fired =
as high as cone 10.
=20
Lee Burningham
=20
Alex Solla on sun 15 apr 07
Something else to consider when applying c06 glazes to high fired bisque:
ADHESION.
Without much absorption you are left with glaze trying to slide
off the pot. Need to find someway to either heat the pot prior to glazing
or add epsom salts/calcium chloride to the glaze to thicken it (a LOT)...
or add other fun stuff like syrup, sugar, etc ...
some way for the glaze to adhere and bind. I used to spray glazes repeatedly
through many firings, over and over eachother... best results I had
were from layers of glazes on warm pots, plus a coat of Karo corn syrup
which made a nice hard crunchy layer so when I picked the pots up
the glaze wouldnt powder off into my hands.
Give it a whirl.
cheers,
Alex Solla
Dave F said:
Firing greenware several cones higher than the
subsequent glaze firing is a common industrial
technique for functional pots. It is very proven and
very workable. It is the result of a conscious choice
to follow a particular path to making pots. They key
is to formulate, mix and apply so that shrinkage
cracks do not develop as the glaze dries. If the
glaze cracks and curls away from the pot upon drying,
that guarantees glaze will fall off onto shelves
and/or other ware. :-(
Good glazing!
Dave Finkelnburg
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