How do I know if bisque is underfired?

[Lauren F]

I recently fired several large greenware tiles (20 x 20") in a custom pre-programmed slow bisque. They came out fine.

When I loaded another bisque today I noticed that this same pre-programmed slow bisque schedule was missing the last segment (to 1860F). I am not sure when or why this was deleted, but I am worried that the previous tiles may not have reached full temperature. In this abbreviated schedule, the final segment was set to 1200F.

My question is—would I be able to tell if the tiles were fired to 1200F instead of 1860F? 

[Min]

If the colour is the same in both (I'm guessing it is if you haven't noticed a difference) then put a drop of water on a piece that was for sure bisqued to 1860F and time how long it takes to soak into the clay. Repeat with the clay that was possibly only bisqued to 1200F and see if takes longer. You can also stick your tongue on both pieces, the lower fired one will stick more. (maybe don't do this in public)

[Kelly in AK]

20” x 20!” Large tiles indeed. No mean feat in my book, 12 or 18 hundred degrees, you’ve passed some hurdles. 

If I had two sets of tile from one body, one set being fired to 1200°F and the other fired to 1800°F I believe I could tell pretty easily which was which. So, yes, imagine you could too.  As mentioned above, get it wet. Try to dig in or scratch it. Properly fired bisque ought not yield to that with a fingernail or wooden tool. A metal tool (steel) can scratch soft bisque with effort, but I’m getting more specific than I intended. You might ask me about what’s proper, then I’d be in trouble. 

If handed a piece of bisque ware and asked what temperature it was fired at, I’d have to ask a lot more questions before I ventured a guess.

Be careful with those tiles, glad you fixed your settings. It’s a blip. Oh yeah, cones! Use those until you have no reason to wonder what your kiln is doing. And again when you do have reason. 

[Lauren F]

Thanks so much Min and Kelly—this is incredibly helpful! I am really grateful.

Unfortunately I have already glazed all of the bisqueware I have in this particular clay body (white stoneware mixed with molochite) so I don't have an ideal point of comparison. 

I did the tongue test on some correctly-fired bisqueware of another body and I did not notice a difference. Did the same with water and noticed a slight difference.  Also, the bisqueware in question doesn't yield to a wooden tool, so that is a good sign! Yet it does seem pinker than some of the other bisqueware I have lying around, but because it's a different clay body, I can't be sure that's abnormal.

In addition to making a couple backup pieces, my plan is to glaze the work as planned but modify the schedule to move slowly on the ascent around 900-1200F.  Any thoughts on this strategy would be appreciated. 

 

[Babs]

Ifit was bisqued to a lower temp. it will take on more glaze if glazed as usual. Maybe test fire one glazed piece through a normal firing, place it on a clay biscuit to protect your shelf.

[Min]

@Lauren F, what cone are you firing the tiles to, cone 6? How are you firing the tiles? How big is the kiln, room to put a shelf above and below the one(s) that will have the tiles? Can you slow cool the kiln?

 

[Bill Kielb]

On 11/17/2023 at 1:01 PM, Lauren F said:

final segment was set to 1200F.

My question is—would I be able to tell if the tiles were fired to 1200F instead of 1860F? 

Just a couple thoughts
1200 f is about cone 020 ( likely not very sintered), folks who intentionally carve soft bisque end up around cone 012. 1860f is cone 05 / 06 so you likely would notice a big difference between the products including: look, touch, size, firmness, absorption, tongue test …… from 1220 f - 1860 f lots of things burnout including sulphur, calcium carbonate, etc…  but they don’t burnout necessarily because of peak temperature. It’s rather time at elevated temperatures so bisque firings are generally slower than glaze.

• You could bisque something to 1200 and compare
• You could re-bisque the pieces in question to let’s say cone 06 to be sure
• You could slow your glaze firing schedule through 1200 - 1860 as sort of a modified once fire schedule. Slow bisque schedules generally go about 180 f from 1000f to 1700 f btw.

I do not believe your actual glaze temperature has been referenced yet. What cone do you glaze fire to? Some of the above more or less assumes you glaze to cone 5/6.

Cone 020 is really a China paint type schedule generally designed to just begin softening glaze, I think the bisque would be incredibly soft at this temperature and not very sintered, but easy to test to be sure.

Edited November 19, 2023 by Bill Kielb

[Lauren F]

4 hours ago, Min said:

@Lauren F, what cone are you firing the tiles to, cone 6? How are you firing the tiles? How big is the kiln, room to put a shelf above and below the one(s) that will have the tiles? Can you slow cool the kiln?

 

I fire these to cone 6, in a pretty fast firing. The kiln is on the larger side (https://coneartkilnsshop.com/index.php/product/2827d/) 

Yes, I could slow cool it.

[Lauren F]

1 hour ago, Bill Kielb said:

Just a couple thoughts
1200 f is about cone 020 ( likely not very sintered), folks who intentionally carve soft bisque end up around cone 012. 1860f is cone 05 / 06 so you likely would notice a big difference between the products including: look, touch, size, firmness, absorption, tongue test …… from 1220 f - 1860 f lots of things burnout including sulphur, calcium carbonate, etc…  but they don’t burnout necessarily because of peak temperature. It’s rather time at elevated temperatures so bisque firings are generally slower than glaze.

• You could bisque something to 1200 and compare
• You could re-bisque the pieces in question to let’s say cone 06 to be sure
• You could slow your glaze firing schedule through 1200 - 1860 as sort of a modified once fire schedule. Slow bisque schedules generally go about 180 f from 1000f to 1700 f btw.

I do not believe your actual glaze temperature has been referenced yet. What cone do you glaze fire to? Some of the above more or less assumes you glaze to cone 5/6.

Cone 020 is really a China paint type schedule generally designed to just begin softening glaze, I think the bisque would be incredibly soft at this temperature and not very sintered, but easy to test to be sure.

Thanks Bill! I am unfamiliar with (but now learning about) soft bisques.  Yes, I glaze to 2175F, and after a 12 min hold it's around cone 6.

I like your idea of the 1200 tests, but because this work is time-sensitive (and I have a lot of other firings going on), I'm more inclined to just redo the pieces.

As for the rebisque, I am reluctant because it risks cracking the piece, but perhaps it's worth a shot. 

Your third suggestion is appealing. My bisque schedule goes 100f/hr from 1200–1860 so maybe I just insert that into the glaze.

 

Thanks!!

[Jeff Longtin]

Good advice so far...I'll just weigh in with this: one big difference between a soft bisque firing and a normal one is that the pots are more fragile. If you use glaze tongs you need to be especially careful not to grip the pot too hard. Super easy to go thru the clay wall or shatter the piece entirely.

Tongue method - in a pinch it works however I prefer to lick my finger then touch the pot. Bisque pots always seem to have a slight dusty/salty surface that seems a little unpleasant.

As mentioned, an underfired pot will absorb more glaze than a properly fired one. Sometimes wetting the pot with water, a gentle sponging, will counter the very dry surface.

[Min]

15 hours ago, Lauren F said:

I fire these to cone 6, in a pretty fast firing. The kiln is on the larger side (https://coneartkilnsshop.com/index.php/product/2827d/) 

Yes, I could slow cool it.

Thanks for answering a couple points, makes it easier to give suggestions when we have more info. 

I would glaze fire the tiles on waster strips of clay. These don't need to be bisque fired if they are bone dry. For waster strips I would roll out a slab of clay (same clay as the body) to about 1/8" thick and about 19" X 16" then cut it into strips about 2" wide. There can be gaps between strips but have the edge ones fairly close the edge of the tiles. Make the strips a bit thicker if you plan on glazing the edges of the tiles right the way down with no wax line. Extruded coils would work too. Reason for the waster strips (or coils) is twofold, one to allow the tiles to shrink / move freely without getting hung up on the shelves and secondly to allow airflow under the tiles. During cooling the shelf is going to retain heat much more so than the air will, this means the bottom of the large tile can be much hotter than the top surface which can lead to cracking.

I would place the tiles in the middle section of the kiln. If you use half shelves make sure they are lined up perfectly. I would also suggest ringing the tiles with either other pots or square or triangular kiln posts laying on the side, this also helps even out the heating and cooling.

I know you said the tiles are time sensitive so you might not have time to dry waster strips / coils, if so some people have success with using silica sand under large slabs, sculptures also.

To be super safe you could also add a slow cool through the quartz inversion zone. (slow from 1100F - 1000F, tc's measure air temp not shelf temp so the 100 degree range is pretty safe) Don't open the kiln until it has cooled down well below 400F. I would also glaze fire them slowly, not a fast firing.

Edited November 19, 2023 by Min

[Lauren F]

Min—thank you! I appreciate all of this thoughtful advice. 

I will let you know what happens.

-Lauren