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MotownPaul

First porcelain fire: questions

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I am a hobbyist with some training in high school and college and have, so far, only worked with low-fire clays.  I’ve decided to try a box (one kiln-load) of porcelain.  So far I’ve thrown / built enough to fill my kiln.  I’ve received conflicting instructions from my pottery supply house, glaze manufacturer and kiln manufacturer so I am coming here now to verify what I think is the correct approach to bisque fire and glaze fire.  First, I plan to bisque my cone 9/10 clay to cone 04 on my Skutt electric kiln at slow speed with top peep hole open.  Then I will glaze with cone 10 glaze and slow-fire to cone 10 with top peep hole open. I will not use stilts but rather keep glaze from bottom of pieces and be sure to have fresh kiln wash on the shelves.    Is this the correct approach?  Thanks in advance for your help.

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Works for me. Porcelain is still clay that some find a little different to throw but is generally smooth, no grog, fairly white so glazes can be a bit bleached compared to other clays. It can slump more than other clays when fired depending upon the shape and for that reason folks avoid stilting and can stick to the shelf given the right clay and circumstances.

As far as bisque, 04 your regular schedule should be fine, ventilation in the bisque as normal. Glaze fire as you normally would as well. Not sure why the top peep is open in the glaze fire, if you have a counterflow exhaust run as you normally would for bisque and glaze. If you have an automatic control then slow glaze in cone fire mode is about 10-12 hrs in bisque and 8 hours in glaze.

our porcelains and glazes work just fine in fast glaze actually. What have you heard that concerns you?

Edited by Bill Kielb

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Thanks  for the quick response, Bill.  My biggest concern now has to do with the pieces sticking to the shelves, especially during the glaze firing, as I understand I can’t use stilts with wire prongs at cone 10.  I guess I will need to be very careful not to get the glaze too close to the bottom of the pieces.   If you feel a medium or fast mode on glaze firing with all peep holes closed is OK I will go that route.

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13 minutes ago, MotownPaul said:

Thanks  for the quick response, Bill.  My biggest concern now has to do with the pieces sticking to the shelves, especially during the glaze firing, as I understand I can’t use stilts with wire prongs at cone 10.  I guess I will need to be very careful not to get the glaze too close to the bottom of the pieces.   If you feel a medium or fast mode on glaze firing with all peep holes closed is OK I will go that route.

I would stick with your normal practice for your glaze and some porcelains can stick to the shelves, not all but some. Kiln wash  shelves for sure and  grog under or you could paint your foot with alumina wax for insurance. 

Edited by Bill Kielb

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When I fire porcelain, in addition to the precautions re: glaze you've already mentioned, I put a bit of sand under each piece, so they can move ever so slightly and I do not leave the peep holes open, regardless of whether a fast or slow fire or during a slow cool. 

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Do you need to fire to ^10?

As stated in other threads, ^10 is usually for gas/reduction.  You might get just as good results at ^6 if firing electric.

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I believe I need to fire to cone 10, because the clay is labeled cone 9/10 and the pre-mixed glaze I have from Spectrum is labeled 9/10.    Do you know if I should be concerned about the clay sticking to the shelves during  the cone 04 bisque firing with porcelain?   I will be sure to have a fresh coat of kiln wash on the shelves.  Do I need to take further measures?

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For functional work, it’s best to fire your clay to maturity. 

Porcelain shouldn’t stick at all in the bisque. As long as you leave about 1/4” at the bottom of the piece unglazed (wax resist is your friend), you shouldn’t have too much glaze trouble, unless your glaze is unusually runny. Make sure the bottoms are wiped clean after glazing and you should be fine. 

Its only some porcelain, usually the really translucent ones, that will “pluck” the kiln shelf, not all of them. As long as you’re kiln washed and have clean bottoms it’s not a big deal. 

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On 10/7/2019 at 7:50 AM, MotownPaul said:

I believe I need to fire to cone 10, because the clay is labeled cone 9/10 and the pre-mixed glaze I have from Spectrum is labeled 9/10.    Do you know if I should be concerned about the clay sticking to the shelves during  the cone 04 bisque firing with porcelain?   I will be sure to have a fresh coat of kiln wash on the shelves.  Do I need to take further measures?

I add alumina hydrate to my wax resist for bottoms of porcelain AND flanges on lids. I put about a cup in a jar and stir in about a tablespoon of alumina hydrate. Porcelain can flux enough to "pluck" or stick to shelves and where bare clay touches bare clay as in flanges.  Alumina in the wax prevents that. 

Marcia

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