Mark Potter on thu 10 jan 08
CLAYARTERS
I need help with a good bisque firing schedule for HEAVY PIECES
1.5-2"" . . . . these are sculptures made with a groggy red clay . . .
I'll be bisquing in a 24 cu. ft gas-fired Alpine . . .
I have little experience bisque firing heavy work . . . and my past
successes have been intermittent at best . . . . .
I need to know how long to take to get to each temperature . . . and
how long to hold . . . . I'm happy to err on the side of
inefficiency. . . .
I realize in the past that I've posted this same question . . . .but
feel I need to get some more input . . . as I'm still bursting
pieces . . . clearly water related.
My work is BONE DRY . . and I plan on soaking with full pilots for at
least 24 hours. . . .
Any other suggestions?? Close to proven schedules??
best,
Mark
>>
>>
>> The critical phase is driving off the water through the lower
>> temperatures. Getting to - and through - the 200 F mark is
>> where slowness and caution are rewarded. Once you are
>> past the first 400 degrees or so, you can go pretty quickly
>> and smoothly up. If you go too fast, you'll have to do a long
>> soak to make up for it, since you want to be sure to have a
>> nice clean burnout. How long it 'long enough' depends
>> entirely on your clay, though: some clay bodies are 'dirtier'
>> than others, and need more burnout time. Quartz inversion
>> is of very little concern when firing greenware. It's mainly an
>> issue when firing (and cooling) high-fired clay.
>>
>> I like a nice long candling period, and (on my manual kiln)
>> I set the top ring to low with the lid propped open about
>> the thickness of a brick. I start this in late evening and then
>> begin firing in earnest the next morning. Really, this is
>> longer than necessary, even for my thick handbuilt work,
>> but I'll use the extra power (not a lot on one-ring-low) and
>> by candling overnight, I can be sure the kiln will reach
>> temperature conveniently during working time the next day.
>>
>> In the morning, I shut the lid and leave it for about a half-hour
>> to let the temperature equalize a bit, then turn the remaining
>> rings to low and wait an hour. I give it usually no more than
>> a half-hour on medium, then straight to high. It doesn't heat
>> so quickly that I worry much about going too fast - in most
>> electric kilns it's just not possible. I don't bother leaving a
>> peephole open, since I figure the gaps around my lid give
>> a similar amount of ventilation in a more distributed way. ;-)
>>
>> My own candling schedule is based on my own convenience,
>> and thinly-thrown work of coarse clay may need no more
>> than an hour of candling. Tighter clay bodies, thicker work,
>> and damp clay will require proportionately more time.
>>
>> You are wasting energy leaving the lid propped open after
>> the first hour or so - a peep (or a gap) is plenty of venting
>> for any remaining moisture. Greenware breakage, in my
>> experience, is always caused by moving too quickly past
>> the 200-degree mark, never due to firing too fast thereafter.
>> Incomplete burnout will show up in glaze defects later on,
>> but most electric kilns (if loaded even moderately full) just
>> don't fire fast enough for that to be much of a problem.
>>
>> In my opinion, 'medium' only exists to cushion the shock to
>> the elements of suddenly being turned to 'high'; it's not really
>> a necessary step for firing. I mainly use the 'medium' setting
>> to even out an uneven glaze firing, turning any section that's
>> hotter than the rest down to 'medium' until it all even out,
>> and I can turn it all to high to finish up. I also use 'medium'
>> to control the rate of cooling, by turning everything to
>> 'medium' after my cones read the proper heat-work, letting
>> my matte glazes develop as I wish. I seldom spend much
>> time on 'medium' otherwise. I don't know it anyone out there
>> shares my opinion on this, but it works for me.
>>
>> Spend the extra time on low power, up to about 400 degrees,
>> but then kick it into gear.
>>
>> -Snail
>>
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>
> ______________________________________________________________________________
> Clayart members may send postings to: clayart@lsv.ceramics.org
>
> You may look at the archives for the list, post messages, or change
> your
> subscription settings here: http://www.acers.org/cic/clayart/
>
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Ron Roy on sat 12 jan 08
Hi Mark,
If you can get the atmospheric water out just below boiling temperature you
will not blow anything up. Just hold something cold and shiny to an upper
spy (like a mirror or shiny metal) to see if there is any condensation. It
may take days but when that water is gone you can proceed at a normal pace
- like 100C per hour.
I fire portrait heads for someone and have not lost one using this technique.
RR
>I need help with a good bisque firing schedule for HEAVY PIECES
>1.5-2"" . . . . these are sculptures made with a groggy red clay . . .
>
>I'll be bisquing in a 24 cu. ft gas-fired Alpine . . .
>
>I have little experience bisque firing heavy work . . . and my past
>successes have been intermittent at best . . . . .
>
>I need to know how long to take to get to each temperature . . . and
>how long to hold . . . . I'm happy to err on the side of
>inefficiency. . . .
>
>I realize in the past that I've posted this same question . . . .but
>feel I need to get some more input . . . as I'm still bursting
>pieces . . . clearly water related.
>
>My work is BONE DRY . . and I plan on soaking with full pilots for at
>least 24 hours. . . .
>
>Any other suggestions?? Close to proven schedules??
>
>best,
>
>
>Mark
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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