Jorge Nabel on thu 13 mar 03
Ron,Im having a terrible time with a batch of white earthenware and asked a
lot of people here but cant find an answer.The problem is that some glazes
that ussually work fine are felling apart from rims, handles or the sides of
flat plates.I bisque to 1000C in about 8 hours, being the last four hours to
a rate of 50/80C an hour.So, and reading what you said, could it be possible
that this batch has more talc than ussually and this ramp gives enough time
to form a lot of cristobalite???or this can only cause cracks?
Ive been working on one glaze adding a very high expansion frit, and thought
I fixed it,but no, and I began to be a little depressed now as I have a lot
of orders to fill and few time to make things work-
Jorge en Buenos Aires, again in the net
""Talc is not a low fire flux - it acts in a catalytic way to produce
cristobalite in low fire bodies to help avoid crazing - and also to reduce
rehydration in low fired bodies.
.........
Warning - glazes with high amounts can have a low enough CTE to make them
pot busters and can promote shivering.
RR ""
Lily Krakowski on fri 14 mar 03
Jorge: I certainly am not Ron, but I was told, a while back, by people who
know, that it is ok to chime in, even if and when a messge such as yours is
addressed directly to someone.
Questions: Is this a new batch of earthenware? Did you mix it, or did the
dealer/manufacturer? THERE ARE MANY STORIES OF ERRORS MADE BY THE
MANUFACTURER. THIS COULD BE IT.
When I wrote a piece called "Why That Glaze Does Not Work" some 15 years
ago(!) I did some research, and several people who make up clay bodies for
sale told me that they sell by characteristic not material contents. In
other words the people making up SnowWhite c.04 Throwing Body will change
spar, and clay and whatever in it when the original is too hard or
impossible to get. At the time Mr Isaacs, a wonderful man ( Hammill &
Gillespie) told me that many feldspars are actually blends, and sold under
names because they have the stated characteristic. In other words the clay
body may have changed.
The same applies to your glaze ingredients. I hate to be a smart-arse but
that is why one should ALWAYS keep a jar of one's old glaze on hand, and
always keep a lump of one's old clay on hand, to have a control when bad
things happen. You say "old glazes" but are they new batch of the old
recipes?
My one suggestion right now is that you mix up a small batch of a similar
body and test that---there are recipes around.
Good luck
Jorge Nabel writes:
> Ron,Im having a terrible time with a batch of white earthenware and asked a
> lot of people here but cant find an answer.The problem is that some glazes
> that ussually work fine are felling apart from rims, handles or the sides of
> flat plates.I bisque to 1000C in about 8 hours, being the last four hours to
> a rate of 50/80C an hour.So, and reading what you said, could it be possible
> that this batch has more talc than ussually and this ramp gives enough time
> to form a lot of cristobalite???or this can only cause cracks?
> Ive been working on one glaze adding a very high expansion frit, and thought
> I fixed it,but no, and I began to be a little depressed now as I have a lot
> of orders to fill and few time to make things work-
> Jorge en Buenos Aires, again in the net
>
>
>
> ""Talc is not a low fire flux - it acts in a catalytic way to produce
> cristobalite in low fire bodies to help avoid crazing - and also to reduce
> rehydration in low fired bodies.
> ..........
> Warning - glazes with high amounts can have a low enough CTE to make them
> pot busters and can promote shivering.
>
> RR ""
>
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Lili Krakowski
P.O. Box #1
Constableville, N.Y.
(315) 942-5916/ 397-2389
Be of good courage....
Snail Scott on fri 14 mar 03
At 11:45 PM 3/13/03 -0300, you wrote:
>...glazes
>that ussually work fine are felling apart from rims, handles or the sides of
>flat plates...
This sounds like shivering, not crazing. Is the
glaze a new batch? A change in your raw materials
may be to blame.
-Snail
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