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bisque fire temperature question


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I glaze fire to Cone 5.  I have been bisque firing to cone 06, but I have seen references to those who bisque fire to cone 04.  Does it depend on the clay?  What does the higher bisque firing change?  thank you!

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Hi Grackle!

A higher bisque can burn out more organics, which could be helpful, particularly for red/brown/black clays*.

Higher bisque should close the clay up, perhaps significantly, hence, if you're having to dip quickly to avoid getting too thick a glaze layer, it could really help.
On the other hand, you may have to slow down, perhaps even dip twice to get a thick enough layer, it depends!

Yes, my experience says it very much depends on the clay, and as for glaze application, how you mix your glazes and your application processes.

I really like having a longer window of time to pour in/out and dip, where I don't have to hurry and can move deliberately while glazing.

Surely there will be more responses to your question!
Please check back in a day or so.

*I not only bisque to a solid 04, but I've also added a half hour (or so) of hold at 1500F for all the clays I use; if there's red or brown clay in there, then I'll hold on the way up and again on the way down, with the kiln vent running the whole time.
The higher temperature, oxygen, and holds seem to really help - especially where there's anything much over 5mm thick - with the off gassing and resultant pinholes, also bloating.
Even the well behaving clays will sometimes have a bit of something that causes a pinhole. With the hotter and longer bisque, I get much less of that.

see also:
Bisque (digitalfire.com)

Edited by Hulk
oxygen - kiln vent, added link
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Hulk:  Thank, you so much!!  I wondered about the clay, as I use red, white, porcelain and a very dark clay which is quite rough, and I am still learning how it works.  I wish I could just settle on one clay, BUT I find I like each of them for different work.  

Thinking now, I will just switch to cone 04 for bisque, since my loads at this point are mixed with the different clays.  I am not a production potter, retired now, and really just having fun with it all.  LOL

Sandra

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It's all about fun for me too.
That said, I'm interested in production, how the repetition and focus sharpens skills...

Be sure to check your glaze thicknesses.
You might find some clays "close up" quite a bit, while others take the glaze almost the same.
If the layer is thin, going back for a second pour or dip can do the trick.

Check out what Tony Hansen has to say about thixotropy (the link above).

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Grackle:

the technical aspects of your question: clay continues to expand up to 2050F; at that point, the porous body begins to contract until it hits maximum density. (Vitreous state). At cone 04; (1975F) the pores are smaller; therefore the absorption is less. As Hulk pointed out: cone 04 bisque absorbs less glaze. I have used cone 04 for decades because I can control glaze application better. 

Inorganics in clay; primarily in red, and darker brown bodies burn out between 1250-1750F. The reason a slow bisque fire is required for these types of clay: it takes up to 30 additional minutes of the ambient kiln temperature to penetrate an 1/2” clay wall.. The slow bisque cycle allows that to occur. As with all things clay: dive in and take cone 04 for a test drive and see if it works for you.

Tom

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Thank you!  I just did a bisque fire to cone 05, so will see how the various clays and glazes perform.  I tend to keep my best notes about which glazes do and do not work together, and also which glazes fit the various  clays the best.  I also note where in the kiln is best for certain glazes (I have my favorites).  Had not thought too much about how the bisque temp affected it all, so that will be a fun new element to add to the mix.

Bisque 04 will be coming soon.

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I have often considered the +/- side of moving the bisque to 04, but then realized that in the studio I would have to change a few things that I have come to expect. My glazes are all set to pretty much match my dipping time on the 06 bisque ware. My spraying situation also uses a set timing as does my washing of pots. To change the bisque temp would change much of my internal clock when glazing so i have no made the move.  Call it old school as I have been doing it for more than 50 years.

 

best,

Pres

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I have found with red or other dark clays, a slow speed is just as good, if not better than raising the bisque temperature. It will depend a bit on your clay body. I don’t notice a huge difference between 06 and 04 bisque in the clay I use, but a slower bisque makes a noticeable difference. 

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I'll share another aspect to the bisque temp decision: thinness of the clay piece.  Using the process of slip casting often results in a clay wall that is thinner than thrown pots. (Especially porcelain.) Firing to cone 06 can result in a very absorbent pot. Sometimes this overly absorbent clay wall can absorb too much glaze. (Crawling can result.)

Firing to a hotter temp, 05 or 04, reduces the absorption and makes it easier to get a thin glaze application.

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so many things to consider.  I have always been a "quick" dipper when it comes to glaze.  Not exactly sure why though.  Maybe the times the glaze dripped off the bottom onto the shelves--did not happen that often, but still something.  And then, was it the dip time or was the glaze a bit too thick????  i know that potters should always work towards consistency and predictable results, but often it just seems like instinct (through experience, no doubt), just takes over.

i was taught in school to use 06 for bisque.  Had a 20 year gap in my ceramic life and when i started up again, just went with the 06.  curious to see what differences the 04 will make!

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41 minutes ago, oldlady said:

on this subject, i was told that i should dip glaze for 6 seconds before firing.   there must be more than that since a watery glaze is completely different from a thicker one?  anyone?

Not sure this helps
I can say I always used a three second dip, so everything I make has to accommodate that. I can also add several years ago I was at a John Britt workshop and while dipping a tile we were both counting one one thousand, two one thousand …… we both laughed. Not speaking for him but he seemed used to my three second dip and it made me feel better about my dipping practice. My base reason,  if I wait too long my stuff becomes water logged. Same with pour in and pour out, fill it, pour it out and roll the rim even while pouring. About three seconds. Always bisqued to 04 as well.

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1 hour ago, Pres said:

My glazes are mixed to cuticle visible but covered thickness, no hydrometer. Wash is done with wet sponge or dip, with 20 minute dry. Base glaze is done with 1-2-3 dip, with over dips or sprays. 06 bisque with slow bisque.

I think sort of like a golf swing. If it works for you and you can repeat it ……..

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Does the bisque temp/cone have any affect crazing in the glaze firing? 

Good question!
Given everything else is same - same clay, same glaze, same glaze thickness, same glaze fire schedule - then crazing should also be the same.

Crazing (digitalfire.com)
See second paragraph under Details

Edited by Hulk
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12 hours ago, grackle said:

or maybe the other way around.....

Yes, Grackle...the latter. 06 leaves your body more porous so a quicker dip is best. HOWEVER, it is a balance. Tightening up the clay body, firing hotter, will result in the glaze taking longer to dry on the pot. You might not mind if its a short period...but if its a long period you might find it very annoying. (As I did.) 

The first time I fired hotter, bisque, I was certain I found the magic bullet. I dipped my first pot, in my fairly thin glaze, and it took FOREVER to dry. Most annoying thing EVER! 

So I went back to 06 and refined my application process. If your glaze dries too quickly you might see unwanted streaking. If it dries slowly it might slow down your glazing process.

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@grackle, what's really important to get consistent results is to measure the specific gravity (sg) of the glaze.  X seconds in a watery glaze slurry won't give the pots the same coverage as the same number of seconds in a thick glaze slurry.

Specific gravity measure the glazes density in relation to that of water. Just in case you aren't doing this already a link here on how to quickly measure sg. with a syringe if you need it.

Generally speaking the less porous the clay (from hotter bisque firing) the higher the sg could be if the number of seconds the pot is being dipped remains constant. SG is the third variable in the process. If the sg remains the same but a clay is bisqued hotter then the X number of seconds a pot is dipped could well need to be increased.

 

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i am tuned into measuring specific gravity--fascinating in itself.  I do notice that different glaze recipes recommend different SG's.

Have not dived into the stuff with Darvan yet tho.  When I was first learning about ceramics way back in the late 90's, we used our hand and arm to mix glaze buckets, and the professor just sort of knew when the thickness was good.  I do not put my hand and arm in the glaze anymore, and I wear a mask when I mix glazes.  I do notice when the glaze seems a bit thick on a dipped piece, and does not run off smoothly, and I usually add some water.

I guess in many ways I am more of a seat of the pants potter, in my 70's now and not as precise.  And this is fun, I don't have to make money, but do sell a few pieces.

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My clay says to bisque to 04, but I usually do 05. Once when firing to 04, it overfired to almost 03, and it was nearly impossible to glaze it right. I don't see much difference between even 06 and 04 for glaze application and firing results with my clay, so I slow bisque to 05 and whatever variations happen are acceptable to me. I have also got my specific gravity and dipping times consistent, although I don't actually count, it's just how I got used to doing it.

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Way back when, folk bisqued to c08. 

I would say that If all is good at C04 why would one change to 04. It would be more economical to stay at 06 unless there is a problem there. 

I do what Pres does above BUT if someone comes by wanting an absolutely exact replica, glazewise, of a pot they bought a few years ago, I don't promise  because as a potter, I don't need the exact same of anything.. ... .  I appreciate the folk who do this, wondering now if @ Mark saw any differences in the pots that came home last month.

 

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