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how many times can a midrange stoneware clay sagger be refired in

updated wed 12 jan 05


Ivor and Olive Lewis on tue 11 jan 05

cone 6 firings?

Hello again Alisa,
Hope you are surviving the stormy weather in your part of the Globe.
What happens to clay during repeated firings. An interesting question
since, if you think about it, we rely on it to do "nothing". It is the
fundamental ingredient for a Potter's Kiln.
First of all, raw clays such as Kaolin or Ball clay, are rather
unstable and during their first firing undergo several transformations
which result in that hard permanent substance we call Pottery.
The ability for a fired clay to survive repeated firings depends upon
the original mixture because that determines the nature of the
eventual fired fabric. But in the main clay becomes an intimate
mixture of Mullite and Silica. Original clay bodies that were high in
Kaolin and low in Silica have more Mullite, which is the strong stuff.
Mullite is a good refractory and will not readily melt. Service
temperature goes above 1400=BA C. Silica is a refractory stuff as well
but, as you know it suffers phase changes. So try to avoid it in
refractory mixtures
Sounds as though your saggars have a lot of Mullite.
I think you have only one thing to fear, that they will become brittle
and may break if you drop them or knock them with a hard object. They
may also fracture if heated suddenly due to thermal shock. But if they
have lasted for ten firings they should not bloat or melt down.
Anyone contemplating making saggars should choose high alumina clays
that have good working plasticity that can be easily formed, together
with well fired refractory grog or Sillimanite and no fluxing material
such as felspar.
Michael Cardew gives very readable guidelines for making Saggar Clays
and Refractory kiln furniture
Best regards,
Ivor Lewis.
S. Australia.

Ron Roy on tue 11 jan 05

cone 6 firings?

Hi Alisa,

There is no doubt that any clay will change somewhat being fired to cone 6
- much depends on how much KNaO is resent - but there will be some melting
each time - less free quartz and perhaps eventually cristobalite

I would say - you will be well aware that melting is happening well before
any "melt down" - it will only happen gradually with perhaps the first sign
being warping.

As the clay becomes tighter - you may well see cracking before you see any
sign of overfiring.


>Recently it dawned on me that maybe I am waiting for a meltdown, or maybe not.
>I use an unglazed pot to fire small unfired pieces in, that I use later on
>other work.
>The sagger pot has been fired at least 10 times, up to cone 6 or 7.
>I am wondering if clay has a firing life? Can it be unlimitedly fired to
>maturing temperature without
>it eventually deteriorating? This may be a question for Tom B. or Ivor.
>Do clay molecules
>remain stable firing after firing or does there occur a sort of breakdown
>including loss of mass?
>(which I imagine opening my kiln one day and finding my sagger pot bloated
>or sagging or worse).
>regards from Alisa in Denmark

Ron Roy
15084 Little Lake Road
Brighton, Ontario
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513