Opinion - Bisque Firing Cone 06 or 04

Not sure what glaze problems you were experiencing, but for most of my stuff 06 has worked fine. I used to bisque at 04 until one time I think I over-fired a little and the clay lost so much of its porosity that it was difficult getting glaze to adhere to it; that's why I only go to 06 now and it is fine for my high fire and raku.

Two exceptions come to mind. Some teachers I've known (like my high school teacher) bisqure students' pots at 04 so that it is less vulnerable to breakage while glazing. Another exception is low-fire ware. If you look at the label of many commercial low-fire glazes they say to bisque at 04, even though the subsequent glaze-fire is only to

05 or 06.

We were getting bubbling in a Gerstley based glazed (Erin's base) that had been acting fine. I suspect that the Gerstley is changing with each order. One potter suggested that we try firing at a higher bisque. The studio is meant for students at the University and they have little or no experience so I was not sure if the decreased porousity would be harder or easier on them. It is interesting to hear that the higher bisque can benefit them.

We have actually been trying to replace our glazes that have Gerstley which is what I think is probably the issue.

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Erin's White - BaseHM = 35 Bentonite 100 Gerstley B 3200 Flint 3000 Talc 2000 Soda Spar F4 1400 EPK (Kaolin) 500

10100 Green Copper Carb 200

[We were having pinholing problems in low-fire ware (not just Gerstly Borate glazes). Firing the bisque to a maturity as high or higher than the glaze helped eliminate the holes. I think these clays contain something volatile that outgasses at peak temperatures, and makes its way out through the molten glaze. On the other hand, the body was marginally less absorbant of glaze, if that's an issue.]

Andrew Werby

04 fires it to maturity (or should if your kiln reaches the proper temp.) for best results the bisque firing should be done 2 cones higher than the glaze firing. the extra time and cost is minimal but the result is a whole lot better, eliminating some of the blips that can happen otherwise.

studio is

experience

easier on

Ah ha! Gertley Borate (aka "colemanite" I think?) I've been hearing for a few years to start watching out for inconsistent gerstley coming from the mine, which I think is somewhere in California. I continued using it with no bad results so I started thinking it might be a "Paul is dead" sorta thing. I guess they were right, whoever "they" are. I'll bet you are right in thinking its the gerstley. But I am no glaze chemist. BTW -- the 06 bisqueware that took the gerstley fine often had a glaze on it that was almost all gerstley: the 80/20 clear crackle raku glaze that you see everywhere.

Good luck!

Eric SpunMud

P.S.: by the way, when i mentioned that some people i know have bisqued students' pottery a little higher, to cone 04, in order to reduce breakage during "glazing," i just meant the process where the bisqueware is touched, decorated, dipped and generally handled during the glazing process -- not the glaze firing itself. the higher bisque fire results in a more stout piece of ware that can better withstand "rough" handling. sorry for being a bit vague there.

I thought that was pretty clear. We have on occasion had the kiln underfired due to error and had a lot of breakage because the person unloading did not know the difference or didn't care. A sore point with me since I feel protective of the students.

Some people never learn. Go figure.

-- Lady Chatterly

"Her replies are like those little messages you get at the end of Taipai" -- invntrr

If it wasn't obvious to all, note that the above rule of thumb assumes that you are working with low-fire clay and glazes. The body maturity (such as it is) comes entirely from the bisque firing. With higher-fire systems (cone 6 and up, say) the bisque firing is simply to give the piece enough strength to allow handling during glazing, since the maturity of both body and glaze come in the glaze firing. Some folks bisque lower (06 or even 08) to save on energy, while others bisque to 04 to help get over as much out-gassing from the body as possible to avoid bubbles in the subsequent glaze. But of course with some glaze materials, the glaze itself can cause bubbles. I think Gerstley is one of those.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

I bisque to con 06, well I guess its 06 I hardly ever do a cone reading on a bisque (I know I should) but I always soak for 30 minutes at temp. So I set the kiln for 1000C, (1830F) and soak for 30 minutes. When I did not soak so long I was having problems with pin holing. I also soak my glaze firing for 30 minutes. I do use cones for my glaze firing, so have to take into account the soak. I think though that the soaking ensures that the glaze has time to mature and any volatile substances have time to escape.

Oh I disagree, it would mean that it would not be porous anymore and applying glaze then becomes a difficulty. I prefer dip glazing where possible. I have no problems with pin holing since doing a soak with both bisque and glaze. I have few glaze problems actually other than if I have applied it too thick, too thin, or it is a new glaze I am testing and just didn't work :o)

How long do you do your soak on the bisque? We glaze fire to cone 6 and I am trying to bully the administrator into letting us do ramp firings on those but we are at a University and in today's new world everything is done with an eye to 'making a profit' - don't get me started... It had never occurred to me to soak the bisque to get rid of the LOI. I also dip glaze but I have never really adjusted to the difference in humidity between the west and east cost so I really do like the pots to be porous.

Always soak for 30 minutes. Thats my rule now :o)

Thanks, I will do this at home. I can't imagine how I'm going to get it past the administrator on campus given that they bulked at a 10 min soak for the glaze fire (Have I complained about bureaucrats here? - I try to limit my rants..... ). I'm a bit worried about over firing and I do a ramp firing on the glaze with a 15min soak... how much heat work do you think you get with that long of soak... a half cone?

Dear Jedi,

There is another way around this; slow the firing ramp down in the final sector, firing temperature curves always slow naturally - just accentuate it a bit. I don't soak at all; I just fire slower and it works really well as a technique at both Earthenware and Stoneware temps.

Steve Bath UK

In article , jedi writes

thankyou for your reply, but i answered your query with the info you gave.firing to c4 and c6 ( which you stated in your answer to me is what you fire to !!!)is on a completely different scale to your query which was about 04 and 06. hope you put all the other good people who were kind enough to answer you in the picture.

We seem to have some confusion here. The question was in regards to a BISQUE firing which we do at cone06. The GLAZE firing is cone6. We are getting bubbles in one of our glazes (well three glazes but all with the same base which uses Gerstly). This glaze has only recently been misbehaving and I suspect it is the Gerstly which varies with each dig. One of our potters suggested firing to a higher BISQUE to deal with the problem in the GLAZE firing (the large case is for my benefit so that I am sure I am making the right references... sorry but I'm verbally defective and since whatever I posted seems to have lead to confusion I am doing the best I can to clarify).

So far I think we are at the conclusion that raising the bisque fire does deal with eliminating gasses in the body that may be an issue in the glaze firing but so does soaking or a ramp firing. That yes, the issue is probably the gerstley which we should be getting away from now that frits are just as affordable... Soaking, ramping good, 04 bisque has mixed results (loss of ability to suck up the glaze but sturdier bisqueware that is safer to handle and may aid in the bubbling of glaze).... What I found most interesting was the concept of ramping or soaking a bisque. I have always gone with the candle forever then fire to cone. It never occurred to me that there would be a reason to do otherwise. Now I know.