Hi  I want to give raw firing a shot, cone 6 electric ( of course!) I have a digital programmer> I know the cycle is longer than a biscut firing> Can I have some help with some ideas on what type of sequence would be a good starting point. I know that one must jump into the water to learn to swim, but a little guidance to get me started would go along way. I will be glazing my pots in the bone dry stage.
Thanks

Bill

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I am on my way to my first load of raw glazed / single fired stoneware pots.  I'm worried about the candling time in the firing.  I live and work in "the rainiest county east of the Mississippi" and am worried about pieces blowing up.  I don't plan to load until everything is bone dry, but all bones are not alike. Some of the potters around here candle for up to 12 hours before firing based on no scientific reason that I know of.  I have an electric kiln with a digital controller.  My question is, have any of you changed your firing schedule from that mentioned above and why?

If your pots are no more than 3/8 of an inch thick at their thickest, i would suggest a 100 degree per hour rise to 190 deg f. and hold there for 2 hrs (three hours for less than bone dry, longer for thicker work). This gently invites water to vacate at less than boiling vapor pressures. Rise 100 deg /hour again to 220 deg f and hold for 1 more hour. This is a stronger eviction notice to the unbound water, when there is probably not enough left for there to be a riot of explosive violence. Rise again at 100 deg/hour to 500 deg f. Peep holes open, lid cracked open 1/2 inch or so, and power vent drawing during all of this to evacuate water vapor. 

Some have claimed they don't find the 12.5% weight loss to steam from the kaolin to kaolinite conversion around 1,060 F, +/- 100 F, depending on the kaolin structure, to be a problem for very thick ware - but we definitely have. 

We've fired pieces, not only previously over fired to 400 F, and candled the next day in the kiln for four hours along with the rest of the ware - and these thick pieces disintegrate with great force somewhere above 1,000 F  taking other pieces with them.  Once-fire or bisque-firing, we treat thick ware with caution.

In my experience the conversion of chemical water to steam can be just as problematic as evaporative water.  Obviously this depends on how porous the greenware is.  A high percentage of grog creates lots of pores in the dry greenware allowing the steam to escape at 1,050 F.

We also have a ^06 cadmium-sulphide gloss red glaze which becomes denatured into a bubbly black mass when exposed to carbon monoxide.  So I could imagine this is not the only glaze which is not compatible with the bisque firing portion of a once fired ware.  

George Lewter said:

If your pots are no more than 3/8 of an inch thick at their thickest, i would suggest a 100 degree per hour rise to 190 deg f. and hold there for 2 hrs (three hours for less than bone dry, longer for thicker work). This gently invites water to vacate at less than boiling vapor pressures. Rise 100 deg /hour again to 220 deg f and hold for 1 more hour. This is a stronger eviction notice to the unbound water, when there is probably not enough left for there to be a riot of explosive violence. Rise again at 100 deg/hour to 500 deg f. Peep holes open, lid cracked open 1/2 inch or so, and power vent drawing during all of this to evacuate water vapor. 

Do you slow the rate down going from, say, 960 to 1160 deg. f to 100/hr or less. to compensate for the chemical conversion and water loss?  I've glazed so far with the high calcium, semi-matte glaze, Raw Sienna (MC6), a modified version of Variegated Slate Blue (MC6), and a dry matte glaze, Dry Red Brown, which has a high kaolin content and a yellow iron oxide as a colorant.  Thanks for your comments. When this load comes out I'll share the results, good or bad.  

The pre-programmed bisque firings in the Bartlett kiln controller slow the rate of rise to 100 degrees F between 1,000 F and 1,100 F - compared to a rise of 400 F per hour in our Medium-Slow profile both below and above that temperature range.  

Extremely thick pieces need a custom program with the kaolin to meta-kaolin transition zone slowed to 50 F per hour.

Rodney Allen Roe said:

Do you slow the rate down going from, say, 960 to 1160 deg. f to 100/hr or less. to compensate for the chemical conversion and water loss?  I've glazed so far with the high calcium, semi-matte glaze, Raw Sienna (MC6), a modified version of Variegated Slate Blue (MC6), and a dry matte glaze, Dry Red Brown, which has a high kaolin content and a yellow iron oxide as a colorant.  Thanks for your comments. When this load comes out I'll share the results, good or bad.  

Thanks. I didn't think of using the pre-programmed firing.


Simon,

I'd be very glad to hear more ramblings on getting in touch with your kiln please.  I have finally secured a kiln but it has no controller.  I'm also raw glazing.  Out of necessity I am going "wild"  using only intuition and cones.  I have fire a few times in the past with a controller that seemed to be out of control anyway.  Any tips/secrets/suggestions for firing au natural would be much appreciate.

Simon said:

It's an old thread, but thought I'd add my 5 cents anyway ;)

On the initial stages of a once fire, the 'drying out' phase, I personally try to stay clear of having a precise time in mind. Yes, good to know for planning (will it finish before I'm too tired to stay awake !?) .. but inaccurate in reality.

Different kilns, different loads, different pots, different weather ... all lead to variations.

Think of the 'reason' you are holding the temperature, it is to let all the water out of the clay/glaze before moving onto the high temps where it would otherwise cause explosions.

Hence, try to only move on from the initial drying temperature when the water has indeed finished escaping, for fact.

A cold piece of mirror/steel held over the vent of your kiln for a second or two, will mist up and be wet to the touch when the water is being pushed out.

Doing this at regular intervals during the drying stage, you will also get a good feel and connection with what is happening inside, you'll see it being lite, become heavy and saturated, and yes eventually decline.

It's a nice feeling to be 'in touch' with the physical events rather than simply watching the hand on a clock.

Yet most importantly, you will know 'for sure' when it is good and safe to ramp up to the next temp.

I could go on and on, but try to connect with your kiln in a similar fashion at other stages of the cycle.
Yes you will need a time schedule in the early days just starting out, but with time you will be able to judge for yourself how to tweak for individual circumstances.

Look through the spyhole and note when the atmosphere becomes hazy, when clear, the textures and reflectance of the glazes.

Anyway, I'm rambling now !

.. happy firing :)

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