Ron Roy on fri 28 mar 97
Corinne asked,
>----------------------------Original message----------------------------
>In response to the pizza-stone thread,Ron Roy wrote:
>>
>>With cone 10 bodies you run into cristobalite problems - especially with
>>high iron bodies.
>>If there is significant cristobalite present thermal shock cracking is the
>>problem at oven temperatures. My dilatometer experiments indicate this
>>reversible cristobalite inversion happens between 150C (300F) to 250C
>>(480F). It means - if the ware is not placed in the centre then one side
>>goes through the expansion before the other side and may crack. There is no
>>cristobalite in low fire bodies - not through firing anyway. At come 6
>>there is not enough time for the cristobalite to build up much. Either of
>>those temperatures would be better.
>>
>Geesh, this is scarry! What you are saying is that cone 10 really isn't
>safe for oven use above 300F? That cone 6 ware WOULD be safe? Aside from
>worrying about the trauma of taking out of the oven a broken casserole and
>ruining dinner, I guess I'm also worried about customer liability.
>
>Also, what about the two different ware handling thermal shock, ie; being
>put into a preheated 450F oven, or coming out of a hot oven?
>
>What is the best claybody and glaze temp for this stuff?
Hi Corinne,
It is possible to make oven ware which will function well. Jim Robinson
wrote two articles for The Studio Potter many years ago. One was called
"Fear of Silica" and the other was "body Building for Potters." They
outline the problems and suggest solutions way better than I can do here. I
will give some brief advice to perhaps prime the pump ( it turns out not
very brief at all.)
Cristobalite begins to show up in bodies about 1100C. It builds in volume
from then on until it's melting point at 1710C.
Our strategy, if we want to avoid cristobalite, is to make sure there is
enough KNaO in our clays to melt the small stating crystal of cristobalite
before it forms the base (seed) for further cristobalite to form on.
Cristobalite is a crystal phase of silica, as is quartz.
In Jims articles he says we must have no less then 10% feldspar in a body
to do the work of keeping the cristobalite under control.
When we use bodies with various amounts of iron we fix it so that we over
flux the body and can't use as much feldspar. In reduction iron is reduced
and becomes a very strong flux. There is a way around the problem if you
must have an iron body I suppose. If you use very refractory clays and keep
the feldspar at 10% or more. There is another problem as well. There just
may be some sort of catalytic affect from iron that helps cristobalite to
form. I will do some experiments on that some day.
To get porcelain to melt well at cone 10 we traditionally use 20 to 25%
feldspar. When I measure the expansion/contraction of porcelains I have yet
to see ANY cristobalite on my charts.
So now you see why I recommend white bodies at the higher temps - you can
get enough spar into them to take care of the problem.
There are other factors with oven ware. Grog makes a body more elastic and
should be considered. If the body is vitrified, as in porcelain) there is
not much elasticity and heat shock will have more effect. If the body is
too open then it is a poor conductor of heat - if there is no cristobalite
then that solution is good - hence many oven cookers are made of low fired
clay - no cristobalite formed below 1100C.
The other problem of course is clay/glaze fit. When any glazed ware is
heated the clay expands differently from the glaze. On cooling they both
contract at different rates. Glazes need to be matched to the body or visa
versa. Once you have it matched to one body don't assume it will fit any
other bodies.
So you see it is not a simple problem. The wonder is how anyone gets it
right at all. We work with materials which are variable in our bodies and
our glazes which makes the task more difficult.
I said cone 6 was an easier temperature at which solve the problem - for
two reasons.
Because it takes time for cristobalite to form - less gets formed than at
higher temperatures. In oxidation iron has nowhere near the fluxing power
of reduced iron. You also need more melters to tighten the clay so it is
easier to get enough spar (KNaO) into the clay. By the way I have some
evidence that using the mid range flues (MgO and CaO) at cone 6 to tighten
bodies does not work as well as Sodium and Potassium at reducing
cristobalite. I will be doing more experiments in that area in the future.
Using the same glaze inside and out helps the problem up to a point. Glazes
which wind up bigger than the body (don't craze) can make the pot stronger
but we don't want the forces built up too much. The strategy here might be
- find out where a glazes stop crazing on a body and drop the expansion a
little lower. That will prevent future crazing but not put too much stress
between the clay and the glaze. Make sure there is little or no
cristobalite in the body to go through it's expansion and contraction at
oven ware temperatures and test each time you are using new materials in
you clay or glaze. If you are buying clay assume it will be different each
time - buy it in the biggest batches you can afford and make sure each
batch is tested. If your clay supplier doesn't number their batches -
change suppliers.
Design of ware to be used with heat is important. Flat bottomed shapes with
vertical sides will concentrate any forces at the corners. Rounded shapes
will transfer the strains more evenly.
When testing your oven ware, tea pots, cups, etc. Make some thiner than
usual versions of what you want to make and glazes them thicker than usual.
Apply heat under the kind of conditions you say they will work. One of the
best ways of stressing a pot is to freeze it first then heat it. If a tea
pot can survive ten trips from the freezer to being filled with boiling
water ten times it should survive normal usage.
I know I have just opened the door on this - I wish I had better answers -
I wish you all had dilatometers - I wish I could say use this clay and that
glaze. I can't. Perhaps we will get to the point where our suppliers will
recommend certain combinations - in the meantime the clues are out there.
Many potters do make ware that works - there other strategies and I hope
they will post their answers.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
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