Mary/Adams on sat 6 aug 05
Thanks all for great info. No, my 40 books didn't help. And, of course, I
tried to return the unordered clay because I wanted what I ordered. Laguna
was good enough to very quickly respond to their error without shipping
charge and send what I had ordered--- and told me to keep the EM212 Cone 06
I will take all your advice plus some I got from Laguna that I could use
this clay for tiles. I'm interested in tiles and will be researching that.
Of course, I will use cone 06 glaze and he suggested to bisque fire to 03
and glaze fire to 06. More questions to come on tiles.........
I also appreciate all the good info on the exchange of cone clay with firing
cone. I wasn't very clear. Sorry. I did mean that the pieces had been
bisque fired to 04. I thought somehow that the Cone 6 clay might be safe
then to put into a cone 10 glaze firing; but, I guess what I'm
understanding from all the comments is that .
1. You CAN bisque fire Cone 10 clay and then turn around and put a Cone 6
glaze on it and fire it at cone 6 but it will not be vitrified enough and so
will absorb liquid and not be food -safe???
2. You CANNOT bisque fire Cone 6 clay and then turn around and put a Cone
10 glaze on it and fire it to cone 10 because it could STILL melt???
Snail, eliz, all, thanks!!!!! Again !!!!! and maybe you could please
Someday, maybe even I will be able to add some info to this great group.
Snail Scott on sat 6 aug 05
At 07:48 AM 8/6/2005 -0700, you wrote:
>...I guess what I'm
>understanding from all the comments is that .
>1. You CAN bisque fire Cone 10 clay and then turn around and put a Cone 6
>glaze on it and fire it at cone 6 but it will not be vitrified enough and so
>will absorb liquid and not be food -safe???
If the glaze crazes, it will absorb a bit of
moisture and probably be a bit less sanitary
if bacteria grow in the crazing, but I wouldn't
hesitate to eat off it for that reason. It's a
pretty trivial factor, in my mind. (I don't wipe
my kitchen counters with bleach, either, or throw
out old milk if it still smells OK.) The whole
'bacteria-in-the-crazing' concern doesn't worry
me in the least - I'm sure that I'm exposed to
worse things every day than what's stuck in the
cracks of a glaze. But that's me.
Absorbency can be a hazard in microwave applications,
though. If it soaks into the clay during washing,
(through the unglazed foot even if it's uncrazed,)
it won't dry very fast with glaze covering it, and
the moisture can turn to steam in the microwave
and break the pot. Rare, but possible.
Mainly, the clay just won't be as strong, because
it's not fully vitrified. Many suppliers sell clay
that they call '^6-10' clay, because they know that
they can get away with it, but when it says ^6-10,
it's NOT TRUE. They know, as I've just mentioned,
that a ^10 clay can be underfired to ^6 and work
pretty much OK, so they take a ^10 clay and call it
^6-10, but it's not. It's still a ^10 clay. A lot
of people fall for this, though, and ceramists all
over the country are making stuff out of this
so-called ^6-10 clay and underfiring it to ^6,
because it mostly does work OK. It just doesn't work
as well as it would if it were a true ^6 body.
>2. You CANNOT bisque fire Cone 6 clay and then turn around and put a Cone
>10 glaze on it and fire it to cone 10 because it could STILL melt???
Bisque temperature does not affect the final
temperature of vitrification AT ALL. Whether
you bisque fire it to ^04, or to ^012, or all
the way to ^6, or don't bisque fire it at all,
if it was a ^6 clay, it will always be a ^6 clay.
That will never change. Bisque temperature does
not affect the temperature of vitrification.
CLAY THEORY 101:
Clays are said to fire to certain temperatures
because that temperature is the point at which
that particular combination of minerals reaches
a degree of melt which is considered optimum.
This basically means that it's as melted as it
will get (i.e. 'vitrified') without actually
starting to warp or sag. This temperature is
mainly determined by the amount of 'flux'
materials in the clay, such as potassium,
sodium, and others. (The rest of the clay is
mostly silica and alumina, which won't melt on
their own without fluxes to help.) More flux =
lower melting temperature. If a clay has a
certain amount (and type) of fluxes, it will
vitrify at a certain temperature, always.
(This is a little oversimplified, but it'll do.)
In a nutshell - always try to fire to the cone
that's recommended for that particular clay.
Underfiring has fairly minor consequences (usually),
but overfiring can ruin both the work and the kiln.