Proper bisque temperature in relation to final firing temp
Posted 02 October 2012 - 09:58 AM
Recently in reading posts by posters who clearly have extensive knowledge, I have found references to firing to a higher temperature for the bisque firing, and lower for subsequent firings. In researching the subject, I did discover a reference in a book to using that order when working with low fire clays, and reversing the order when using mid or high fire clays, but this is contrary to what I was originally instructed, so now I'm a bit confused. Let's say I have a clay that fires in the 1-4 range, and I want to bisque it, then underglaze fire it -- and possibly finish it off with additions of a cone 4 glaze. How should I proceed, in terms of firing temps? And why? Would that procession be different if the clay were a mid range clay, firing from 2 to 6? And why?
Posted 02 October 2012 - 10:36 AM
Bisque firing is very misunderstood because many of the institutions where pottery students learn and work want to keep everything as simple as possible so they make a sound decision to bisque at 04-06 then fire at Cone 6 or 10. This gives good results across a wide spectrum of situations. Unfortunately this is often presented as a RULE rather than a studio option, so students leave thinking this is the only correct way to fire.
The fun part comes when you get your own kiln and realize that these numbers are not carved in stone. Bisque is simply a way to get rid of impurities and get the clay past quartz inversion so it becomes a new material. It can be done across a wide range of temperatures with no one solution being "right" for everyone.
Clays that are bisque fired at the cool end ... say 08 - 010 ... stay open and will absorb more glaze ... this is good for pit firing and raku where every little nook and cranny is just waiting to grab color. The downside being that the piece is much more fragile than one bisqued at 04 - 06. A cone 10 clay bisque fired this low will be very open whereas a Cone 2 clay will be more closed.
Porcelains are often bisque fired at Cone 10 then glaze fired at a lower temp. This is because they need to be supported so cannot have glaze on both sides.
Sometimes potters bisque hotter than the final fire because they like the color at that higher Cone but their surface treatments can only be fired a lot lower ... say 018 for lusters and such. People who make intricate surface designs fire high first to get the piece to maturity, then fire dozens of times at low temp as they apply underglaze color.
Bisque firings don't even need to be done if you learn the skills of once firing.
The trick is to match your bisque to your clay and what you want ... what you are looking for. The only way to find out is to try different variables with your clay. See if you get better results at 07 than you do at 04. See if you like the way your colors look at Cone 5 rather than 6. If you are now in your own studio, you can try whatever combinations you want until you find your look, the one that suits you and your work.
Contemporary Fine Colored Porcelain
TRY ... FAIL ... LEARN ... REPEAT
Posted 02 October 2012 - 11:02 AM
Thanks Chris for your insight, I am a newbie with my own kiln and bisque to cone 04; low fire to cone 04 sometimes to cone 05; mid range fire to cone 5 because I really like the colors. Reading your post has really jazzed me up..........can hardly wait until the weekend to start potting.....
Posted 02 October 2012 - 03:47 PM
Posted 02 October 2012 - 07:07 PM
Any oxidation refining will destroy your reduction effects ... Everything is a trade off. If you high fire first to make it mature you lose the openness ... As to the underglaze colors again it all depends ... Do you like them better at 05 or 6 or 10?
BORING hours of testing ahead for you!
I have clay tests of all my underglaze colors at 05, 6 and 10 so I can quickly see which one I want to use.
Make a tile, cut lines into it like ruled notepaper then divide those into four parts. Name of the underglaze, Cone 10, Cone 6, Cone 05. Then apply the underglazes to the highest temp squares and fire. Then add to the next and fire. Then to the lowest and fire ... Bingo you have a test tile. Adjust this as needed for the temps you fire at.
I will be off the forum for the next three weeks but will check in to see what's up when I can get Internet access.
Contemporary Fine Colored Porcelain
TRY ... FAIL ... LEARN ... REPEAT
Posted 02 October 2012 - 07:42 PM
Kiln Repair Tech
Owner, Neil Estrick Gallery, LLC
Posted 04 October 2012 - 10:19 AM
Unfortunately this is often presented as a RULE rather than a studio option, so students leave thinking this is the only correct way to fire.
You are (as always) making some great points in that posting, Chris.
There is an interesting problem in teaching in any field that crops up time after time after time which often leads to this "rules" business, and many other things. It really ultimately deals with the "disconnects" between various instructor's teaching styles and various student's learning styles. And a bit of how the typical "educational system" works.
Many times information is presented by an instructor in a certain manner. But then, if there is a failure on that teacher's part to utilize the real-time "check for understanding" step in the dynamic back-and-forth teaching process, what actually got communicated and what was intended to be communicated are sometimes two very different things. But the instructor does not KNOW this... because he/she never enquired what the student was recieving. So he/she goes on merrily to the next thing.... assuming that everyone is on the same page now.
Because everyone does not learn the same way, if a teacher solely utilizes a single modality (most will default to the one that THEY themselves learn best by), students that do not fit that particular pedagogical approach are not getting the message that the teacher thinks they are.
Teacher is saying/showing "It depends"..................... student is hearing/seeing "Do it this way".
So sometimes it is not that the concepts are presented intentionally as hard-and-fast "rules", but it is the impact of what might be called "less than highly skilled teaching" that results in the student's understanding of the idea as being some sort of rule. That is not the case all the time.... but unfortunately it happens that way a surprising number of times.
One of the downsides of the current format for collegiate-level teaching of ceramics is that the qualifications for the teaching positions typically solely require an MFA degree. The typical course of study in that overall BFA/MFA path has its emphasis on lots of important aspects of developing a successful, productive, professional creative artist. But it does not typically provide a lot of courses forcused on the art and science of education. So an MFA graduate might be a GREAT artist. But that does not make them a great TEACHER.
Some of these folks have some inherent skills in teaching, and that is maybe what tends to lead them toward a teaching career in the first place. They start off with some "tools in the kit" and then because they know a bit about how education seems to work, eventually expand their capabilities over time and experiential learning on the job. The students unfortunately end up being the "guinea pigs" as the professors develop their teaching skills so that their effectiveness rate with a diverse pool of students improves.
Many people learn what they know about the art of teaching ceramics by the experiences that THEY have had in the institutions. They use their teachers as role models for "how it is done". Sometimes the role model being used is not all that skilled a teacher, but their teaching style and this particular future grad's learning style meshed nicely.... which is why that student is shortly graduating (or whatever). So that approach to teaching ceramics tends to gets passed along as "the" way to teach clay. As opposed to A way to teach clay.
There certainly are many exceptions to this idea, and there are some really GREAT ceramics professors out there who are supurb teachers as well as artists. (I've had a few, met many, and currently work with a great team of ceramics educators at NHIA.) And some have HAD educational development courses as well in their course of study.
Unfortunately, there are also some folks out there that DO teach stuff as hard-and-fast "rules" too when they shouldn't.
Personally, I have always been very glad that I was an education minor in college, and that also in another field I was given years of a high level training in intructional methodology. I still screw up frequently to this day, but I feel that training has helped me greatly in teaching at the college level (and in workshops and other venues too).
So sometimes the root of the misunderstandings you hear "on the street" are not what we all think they are.
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art
Posted 04 October 2012 - 02:02 PM
Posted 04 October 2012 - 06:14 PM
Trying to remember if any colleges/universities require their professors/assistant professors/adjunct professors/teaching assistants to have teaching degrees (or couresework) -- like primary and secondary schools require. I don't recall any of my college instructors having teaching degrees or being licensed to teach in a particular state.
Posted 04 October 2012 - 07:39 PM
- Firing to a higher bisque(looking at the vitrification temperature of the clay body) than glaze temperature, came about with the onset of the manufactories that produced ceramics in bulk and were interested in a high success rate. By bisquing to vitrification the warpage factor was dealt with before any time or effort was invested in glazing and decorating the ware which could be lost through warpage. It must be remembered that profit was the motivating factor for this.
- However this comes at a price as ware fired at a lower glaze temperature does not fully develop the glaze interaction layer (that is the zone where the glaze and the body interact). This is the zone where glaze fit comes into play as well the the development of certain glaze attributes. This can be seen by comparing the quality of glazes fired on high bisque at a lower temperature to glazes fired at a higher temperature at a lower bisque. These glazes do not have the depth or "soul" for want of a better word. They are more akin to a coat of paint than a full integration with the clay body.
- Another downside is that the glazing process becomes more problematic. The glaze has to be "doped" to be able to adhere to the non porous bisque. Industry has the technology to deal with this whereas for the studio potter it creates more difficulty in getting enough glaze on the ware to get a good depth of coverage.
- I work extensively with multiple firings at 800.C (cone016-015 depending on firing speed). However I still glaze at vitrification point (Cone 10) after a Cone 04 bisque. This allows me to add extra colour and effects to the surface with overglaze (otherwise known as onglaze or china paints). However these colours just sit on the surface and don't integrate with the glaze. However if I were to fire this higher say at Cone 04 upwards these colours (though NOT lustres) would begin to sink into the glaze and become not so intense. This then then becomes known as Inglaze colours.
- Overglaze work can be done at any temperature. Cone 6 glazes can be fired over a Cone 10 glaze as can any of the low fired glazes. The problem then is the same as for any overglaze work: getting it to adhere to a higher fired surface.
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