S.E.W. on mon 22 mar 04
I slip cast, and only slip cast. I also low fire so I assumed he was =
using low fire slip because he mentioned bisque fire to cone 06. That is =
what the problem is. What you said is completely WRONG. If you are using =
porcelain slip then you MUST fire the bisque to cone 6, not 06. It seems =
that you have it backwards. You can use low fire glaze on porcelain, but =
you can not fire the glazes to cone 6. They must fire to cone 06. If you =
are bisque firing porcelain slip to cone 06, it is severely underfired. =
If you then apply glaze (low fire glaze) and fire to 6, you are glazing =
an immature piece of bisque and then over firing the glaze also. This =
will result in the cracked pieces because the piece is still releasing =
its gases. That combined with "frying" low fire glaze is going to result =
in major problems. As far as soaking the piece, it should only be soaked =
on the bisque fire. If you use low fire glaze (cone 06) there is no need =
to soak them on the glaze fire. I can link you to multiple web sites =
that will further inform you on this process. If you would like them let =
me know. It really doesn't matter what kiln he uses. The kiln wouldn't =
cause pieces to crack like that. Try doing it that way and I'm sure you =
will have great results.
Snail Scott on tue 23 mar 04
At 08:22 PM 3/22/04 -0800, you wrote:
>...I assumed he was using low fire slip because he mentioned bisque fire
to cone 06. That is what the problem is. What you said is completely WRONG.
If you are using porcelain slip then you MUST fire the bisque to cone 6,
not 06. If you are bisque firing porcelain slip to cone 06, it is severely
I've had excellent results bisquing ^6 porcelain
slip to low temperatures, (anywhere from ^012-^04,)
then glaze firing to ^6. If the glaze is ^6 and the
clay is, too, why not?
(Doll-makers often bisque high and glaze-fire low,
but I don't know of anyone else who does. Pottery
tends to either be bisqued low and glaze-fired high,
or one-shot fired.)
Did the original poster say they were using low-fire
glaze? I don't recall that. If they are, it might
account for crazing. In fact, differences in CoE
tend to cause most 'standard' glazes to craze on
porcelain even when they are designed for the same
temperature as the clay. (Yes, casting slip is clay,
and I prefer to call it that rather than 'slip' when
discussing surface treatments, to avoid confusion.)
That doesn't account for the 'cracking' described,
though unless they meant crazing only.
Porcelain needs glazes meant for porcelain in
order to achieve best results. Most commercial
glazes are not very compatible. Opaque and matte
glazes show crazing less, but tend to defeat the
reason most people want porcelain anyway, and they
still do craze. (Were they commercial glazes? Just
an unfounded assumption on my part.) Try using ^6
recipes with a low CoE for porcelain. If you don't
feel up to calculating, etc. just start with a few
base recipes written for porcelain - Chappell's
book has several that I've 'tweaked' with success,
and used on slip-cast ^6 porcelain.
p.s. Terms like 'wrong' and 'must', tend to put my
hackles up, especially written in all caps. 'Wrong'
is so often true only for certain specific intents.
So many techniques which work poorly for some
purposes work very well for others, making 'wrong'
purely a matter of 'not a good means of achieving
the desired result in this case'. Likewise 'must'
generally comes down to one person's preferred method
of achieving a particular goal. Other means may be
possible, and if the desired outcome differs, so
might the method. ;)