vince pitelka on wed 10 jul 02
> I don't know if you have a pyrometer, but if you do, i think, generally,
> best to go no faster than 250 degrees F an hour until you pass
> quartz...around 1063 degrees F. After that, you can probably fire the kiln
> fast as your burners will heat it up and be fine.
This perhaps needs a bit of clarification. Quartz inversion is of little
concern in heating a bisque-firing, because the wares are very porous and
have very high thermal shock resistance. As you suggest, one should always
be cautious in heating a kiln through the early stages, but the critical
point where the wares can be heated faster is when dull red heat is visibly
emitted by the wares themselves everywhere in the kiln. That is what is
called "black body emission" and it simply refers to the light emitted by
matter itself when heated adequately. It is important to look for that,
rather than just the light/color reflected from the burners or elements.
Once the whole kiln has reached 1000 degrees dull red heat will be visible
if the room lights are turned off. By then sintering is well underway, and
the thermal shock resistance is very high, so heating can proceed quickly
after that. HOWEVER, it is unwise to fire too quickly even after red heat,
because the "water smoking" period lasts all the way up to 1400 or even 1600
degrees F, and volatiles are outgassing from the wares. If you fire too
quickly through this period, you run the risk of trapping carbon and sulfur
(carbon/sulfur coring) and creating possible problems with bloating in the
claybody and pinholing in the glaze surface later on. Those problems are
often traced to excessively quick bisque firing.
I believe strongly in candling, especially with a gas kiln. It may not be
so critical with a toploader with fairly thin walls, but with any gas kiln
it will give the firing a gradual start. The primary downfall over time
with all intermittent kilns (those that are heated and cooled with every
firing cycle) constructed of brick is breakdown of the refractory. The more
gradual the heating and cooling, the longer the refractory lasts. Obviously
this could be carried to excess, resulting in excessive fuel bills, but
candling is cheap, and it heats the kiln and the wares very gently in the
early stages of the firing, driving off moisture. It is unwise to simply
say that candling wastes time and money, as someone said yesterday.
Best wishes -
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
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Ron Roy on sun 14 jul 02
I believe Vince has a valid point here.
Most kilns - I should say all kilns - when the burners are turned on - heat
very quickly - especially those with two big burners rather say 4 or more
smaller ones. Electric kilns are the same in a way because the bricks
nearest the elements are heated before the others.
Probably the most damage to refractories comes at around 200C/400F when all
the refractories go through the cristobalite inversion. All the bricks and
shelf supports have cristobalite - I am not sure which shelves have the
potential to have cristobalite - perhaps someone else can enlighten us. The
more often refractories are fired above 1100C/2000F the more cristobalite
builds up. The higher you fire the greater the build up, the slower you
fire and cool (above 1100C) the more cristobalite builds up.
Another good reason not to use crushed up used fire bricks for grog if you
don't want more cristobalite in your clay body.
The volume of any refractory containing cristobalite increases a bit as it
goes through the inversion around 200C and reverses again as it cools
through that temperature.
Picture this - a brick in the wall of your kiln - the inner face reaches
the cristobalite inversion temperature before the outer face (remember -
soft brick is designed to transfer heat slowly) it gets bigger while the
outer face says - wait - I'm not ready yet. The result - over time is
called spalling - the inner layer of the brick falls away.
This same thing can happen at the quartz inversion as well but because it
happens when the heat rise in kilns is slowed considerably it is not such a
danger - but it still happens - it is a wonder that spalling is not more
common that it is.
Cristobalite is the one to worry most about - simply because - while the
free quartz says relatively the same - and diminishes slightly with each
firing - cristobalite increases with each firing over 1100C.
An interesting turn on all this - cristobalite is sometimes added to low
fire clay - in controlled amounts - to help keep glazes from crazing. Even
some stoneware potters want a little to help prevent crazing.
Picture this - a pot at 200C suddenly gets a bit smaller - helping to put
the glaze under more compression - and prevent that glaze from crazing over
I don't recommend generating cristobalite in bodies - especially on
ovenware (any ware that has to go through the cristobalite conversion temp
in use. The better answer is to make sure your glazes are formulated with
the proper expansion for the body you are putting it on.
> The primary downfall over time
>with all intermittent kilns (those that are heated and cooled with every
>firing cycle) constructed of brick is breakdown of the refractory. The more
>gradual the heating and cooling, the longer the refractory lasts. Obviously
>this could be carried to excess, resulting in excessive fuel bills, but
>candling is cheap, and it heats the kiln and the wares very gently in the
>early stages of the firing, driving off moisture. It is unwise to simply
>say that candling wastes time and money, as someone said yesterday.
>Best wishes -
15084 Little Lake Road