Bisque firing issues

[Gonepotty]

Hi everyone,

Really hoping someone can shed some light on this issue I have had. Slow bisque fired to cone 04 and only some cones have bent but not the target cones....witness cones are in order of (left to right) 05, 04, 03.- photo attached.

2 inch clearance around thermocouple. 1½ inch clearance between lid and pots (could this be an issue?- too little?). Top bung put in at 600 degrees celius. Not overly packed, even distribution of pots. Kiln controller says it got to cone 04 (1062 celcius) and shows on firing graph but I know it didn't due to witness cones.

New kiln (skutt 1027)this is my second bisque fire.  First bisque firing I did had the same issue but it was my fault due to kiln shelf too close to thermocouple so lesson learned but this time I am stumped. Perhaps the cones need to be offset? I am literally clueless as what else to do.

Any input greatly received :))

[Morgan]

For me personally, if I use the pre-programmed bisque I usually get a very small target cone bend @ 04 (sometimes none) but I simply glaze accordingly and do not worry to much about a perfect bisque. 

Also, and I may be mistaken, but most pre-programs simply fire to the actual 04 target temp (1945) which does not mean you are actually getting that heat work.

You could always add a soak at the top or adjust the firing schedule to get what you are aiming for.

Edited May 8, 2022 by Morgan

[Mark C.]

For bisque you are fine as its close enough at least for me. Other's may add more soak time. 

[Hulk]

Hi Potty,
You might have to adjust your offset, however, likely more important that your glaze fire heat work is spot on than your bisque fire.
As long as your bisque isn't too fragile and/or porous at between maybe cone 06 to cone 05+ (as your cone packs indicate), glaze the ware and go.

I believe that adjusting the offset and/or schedule to dial in the heat work is a usual thing, that gets easier as experience is gained.
Cone packs on each shelf, keep doing that for a while. Note the setup on the cones, the eight-degree angle does matter. Angled more, the cone will show more heat work than occurred; angled less, or stood exactly straight up, less (to a point - it will slump when it's hot enough).

[Bill Kielb]

2 hours ago, Morgan said:

Also, and I may be mistaken, but most pre-programs simply fire to the actual 04 target temp (1945) which does not mean you are actually getting that heat work.

FYI - most pre programmed controllers follow the rate in the last 200f of the Orton firing chart to achieve the proper amount of heat work or very close to that published in the Orton chart (1945 = 108 f per hour in last 200f). Calibrating or offsetting a new kiln to tweak it is not uncommon as long as all the elements are working appropriately. Adding a 15-20 minute hold at the end generally adds enough heatwork to drop the next cone. 

[Min]

@Gonepotty, just double checking you aren’t firing an earthenware clay? (Earthenware is sometimes bisqued hotter than the glaze fire in which case it would matter that some of your shelves pots were under fired.)

6 hours ago, Gonepotty said:

1½ inch clearance between lid and pots (could this be an issue?- too little?)

How much space between top shelf and kiln lid? Kilns with one thermocouple can fire more evenly if the greatest mass in in the middle section of the kiln. Tall pots at the bottom and top with shorter pots (therefore more shelves) in the middle can even out the firing.

3 hours ago, Morgan said:

Also, and I may be mistaken, but most pre-programs simply fire to the actual 04 target temp (1945) which does not mean you are actually getting that heat work.

With all due respect the controller takes into account heatwork when using a cone fire program. If a temperature is input rather than a cone it will fire to the temp.

[Gonepotty]

8 hours ago, Morgan said:

For me personally, if I use the pre-programmed bisque I usually get a very small target cone bend @ 04 (sometimes none) but I simply glaze accordingly and do not worry to much about a perfect bisque. 

Also, and I may be mistaken, but most pre-programs simply fire to the actual 04 target temp (1945) which does not mean you are actually getting that heat work.

You could always add a soak at the top or adjust the firing schedule to get what you are aiming for.

Thanks Morgan, it's a new skutt with the touch screen controller so I selected the cone 04 firing which said in brackets 1062 degrees celcius. Will it not get to this target temp?

[Gonepotty]

8 hours ago, Mark C. said:

For bisque you are fine as its close enough at least for me. Other's may add more soak time. 

Will it have reached about a cone 06 at least? Will the bisque not be too porous?

[Gonepotty]

8 hours ago, Hulk said:

Hi Potty,
You might have to adjust your offset, however, likely more important that your glaze fire heat work is spot on than your bisque fire.
As long as your bisque isn't too fragile and/or porous at between maybe cone 06 to cone 05+ (as your cone packs indicate), glaze the ware and go.

I believe that adjusting the offset and/or schedule to dial in the heat work is a usual thing, that gets easier as experience is gained.
Cone packs on each shelf, keep doing that for a while. Note the setup on the cones, the eight-degree angle does matter. Angled more, the cone will show more heat work than occurred; angled less, or stood exactly straight up, less (to a point - it will slump when it's hot enough).

Thanks so much - very helpful

[Gonepotty]

8 hours ago, Bill Kielb said:

FYI - most pre programmed controllers follow the rate in the last 200f of the Orton firing chart to achieve the proper amount of heat work or very close to that published in the Orton chart (1945 = 108 f per hour in last 200f). Calibrating or offsetting a new kiln to tweak it is not uncommon as long as all the elements are working appropriately. Adding a 15-20 minute hold at the end generally adds enough heatwork to drop the next cone. 

Thanks for your reply.  If I offset the cones or add a 15-20mim hold will the bottom shelf get too hot as it seems like that's the hottest part of my kiln?

[Gonepotty]

6 hours ago, Min said:

@Gonepotty, just double checking you aren’t firing an earthenware clay? (Earthenware is sometimes bisqued hotter than the glaze fire in which case it would matter that some of your shelves pots were under fired.)

How much space between top shelf and kiln lid? Kilns with one thermocouple can fire more evenly if the greatest mass in in the middle section of the kiln. Tall pots at the bottom and top with shorter pots (therefore more shelves) in the middle can even out the firing.

With all due respect the controller takes into account heatwork when using a cone fire program. If a temperature is input rather than a cone it will fire to the temp.

Thanks so much Min, there was a 1½ clearance between the pots and lid...is this too little?

With regards to the packing/arranging of pots throughout the kiln- perhaps I need to invest in other props and half shelves as currently I have 4 standard shelves and only 15cm props.

I will try figure out if there is a way to fire to temp rather than cones and this may be quite helpful

[Bill Kielb]

3 hours ago, Gonepotty said:

Thanks for your reply.  If I offset the cones or add a 15-20mim hold will the bottom shelf get too hot as it seems like that's the hottest part of my kiln?

That was really a response to @Morgan as ther was uncertainty in how automatic controllers fire to a preprogrammed cone. In your scenario your bottom fired to 05 and the rest of your kiln looks like 06. One of the first questions would be at what speed? So the faster one goes, the more uneven firings CAN be. Assuming you went for a SLOW bisque then you would gradually offset the cone until the hottest part made it to the programmed cone. From there you would slowly offset the thermocouple zones until your kiln was balanced.

You could do zone offsets first to get them even and then slowly offset the cone to get you there as well. Just slow and watchful to dial it in is the key.

Now as others have said, before offsetting anything: use good practice - stagger your shelving so wares can see the glowing elements. This is primarily how your kiln heats after red heat. There ain’t much air in your kiln and it’s too thin to hold any reasonable amount of heat. Heating by convection over 1000 degrees is almost nil. Try and make sure your wares can see the the elements as much as practical - stagger shelves. Slow down if going fast. The top will usually fire cooler because of the lid losses, load the top lighter. Did you load the bottom lightly for this firing, if so, new knowledge, this needs to be loaded more heavily to fire more evenly.

Before balancing anything, check your cone six operation. Often the controllers will perform better with higher temperatures (mainly because of speed) so before you offset for bisque, check your glaze firing, especially if using tcouple offsets. Then check your bisque. Bisque firings are not nearly as critical so most find that a 05-04 bisque is often just fine.

While adding a soak is a technique, my experience - most of the time folks misuse the technique. Try to fully understand and have a real verified reason to hold and soak. Ordinary wares transfer heat far faster than the normal pre programmed heating rates so really no reason the center should be heating significantly slower than the outside. Adding holds generally drive things to the next cone with the same uneven firing result.

[Hulk]

Yep, watch that glaze fire target. If possible and safe (kiln glasses against harmful rays and any flying bits), actually check your cone pack through the peep.

A new day!
Ah, found this, cone setup; here are two arguments for the self-supporting cone*:

“Height Setting” impacts cone deformation dramatically. Figure 6 and 7 show the effect of setting height against the angular deformation of various height cones. The cones at the highest heights deformed the most and the cones mounted at the lowest heights deformed least.

“Angle Setting” also significantly effects cone deformation as shown in Figure 8 and Figure 9. To illustrate this, a similar procedure was used in “Height Setting”, where several cones were mounted at various angles with all other factors remaining constant. Cones set less than the designed 8° angle deformed less than those set at 8° angle. The Large cones in Figure 10 are correctly mounted at an 8° setting angle and at approx. 5.08 cm (2 in.) setting height.

from 69281-Pyrometric Cones Brochure.qxp (quantotec.com), where there are similar charts for cone 10 cones and other stuff about cones.

If there's about fifteen degrees angle difference between straight up and the 8 degree, that's about twenty to one or two degrees F, depending on where in the range the heat work achieved, the most discrepancy being at the low end of the range, eh?

*I'm using the small cones, which are easier for me to see, are cheaper, also fit in the sitter, and bend about 9 F hotter than the big ones.

[Bill Kielb]

7 hours ago, Gonepotty said:

Thanks so much Min, there was a 1½ clearance between the pots and lid...is this too little?

Just noticed this, could any of these pots see an element? Also as Hulk mentioned above setting the angle on these cones is very critical. Maybe use a  2” setting height reference to set them all the same.

[Gonepotty]

5 hours ago, Bill Kielb said:

That was really a response to @Morgan as ther was uncertainty in how automatic controllers fire to a preprogrammed cone. In your scenario your bottom fired to 05 and the rest of your kiln looks like 06. One of the first questions would be at what speed? So the faster one goes, the more uneven firings CAN be. Assuming you went for a SLOW bisque then you would gradually offset the cone until the hottest part made it to the programmed cone. From there you would slowly offset the thermocouple zones until your kiln was balanced.

You could do zone offsets first to get them even and then slowly offset the cone to get you there as well. Just slow and watchful to dial it in is the key.

Now as others have said, before offsetting anything: use good practice - stagger your shelving so wares can see the glowing elements. This is primarily how your kiln heats after red heat. There ain’t much air in your kiln and it’s too thin to hold any reasonable amount of heat. Heating by convection over 1000 degrees is almost nil. Try and make sure your wares can see the the elements as much as practical - stagger shelves. Slow down if going fast. The top will usually fire cooler because of the lid losses, load the top lighter. Did you load the bottom lightly for this firing, if so, new knowledge, this needs to be loaded more heavily to fire more evenly.

Before balancing anything, check your cone six operation. Often the controllers will perform better with higher temperatures (mainly because of speed) so before you offset for bisque, check your glaze firing, especially if using tcouple offsets. Then check your bisque. Bisque firings are not nearly as critical so most find that a 05-04 bisque is often just fine.

While adding a soak is a technique, my experience - most of the time folks misuse the technique. Try to fully understand and have a real verified reason to hold and soak. Ordinary wares transfer heat far faster than the normal pre programmed heating rates so really no reason the center should be heating significantly slower than the outside. Adding holds generally drive things to the next cone with the same uneven firing result.

It was a slow bisque and the bottom was loosely packed so definitely something learnt here:) Yes all pots exposed to elements .

I don't think my kiln has different zones as it only has 1 thermocouple in the middle. Glaze firing reached target cone 8 perfectly and cones all looked good. Would it best to just off set the relevant cones in this instance? 

When you say 'adding holds result in the same uneven firing', I'm not too sure what you mean by this?

[Gonepotty]

2 hours ago, Hulk said:

Yep, watch that glaze fire target. If possible and safe (kiln glasses against harmful rays and any flying bits), actually check your cone pack through the peep.

A new day!
Ah, found this, cone setup; here are two arguments for the self-supporting cone*:

“Height Setting” impacts cone deformation dramatically. Figure 6 and 7 show the effect of setting height against the angular deformation of various height cones. The cones at the highest heights deformed the most and the cones mounted at the lowest heights deformed least.

“Angle Setting” also significantly effects cone deformation as shown in Figure 8 and Figure 9. To illustrate this, a similar procedure was used in “Height Setting”, where several cones were mounted at various angles with all other factors remaining constant. Cones set less than the designed 8° angle deformed less than those set at 8° angle. The Large cones in Figure 10 are correctly mounted at an 8° setting angle and at approx. 5.08 cm (2 in.) setting height.

from 69281-Pyrometric Cones Brochure.qxp (quantotec.com), where there are similar charts for cone 10 cones and other stuff about cones.

If there's about fifteen degrees angle difference between straight up and the 8 degree, that's about twenty to one or two degrees F, depending on where in the range the heat work achieved, the most discrepancy being at the low end of the range, eh?

*I'm using the small cones, which are easier for me to see, are cheaper, also fit in the sitter, and bend about 9 F hotter than the big ones.

Thanks for the link- I'll have a good read. Yes I need to invest in some kiln glasses- didn't know there was such a thing!

[Gonepotty]

1 hour ago, Bill Kielb said:

Just noticed this, could any of these pots see an element? Also as Hulk mentioned above setting the angle on these cones is very critical. Maybe use a  2” setting height reference to set them all the same.

Yes they were exposed to the elements.  Great idea to use a height reference- would never have thought of that 

 

[Min]

It's not the 1 1/2" space from the top of the pots to the kiln lid that might be an issue, it's about the distance from the top shelf to the kiln lid. If you were firing very shallow pots on the top shelf and it was only a few inches from the lid then yes it would slow down the heating of that area. Kiln shelf will have far more mass to heat up than typical pots on it (unless its a massively heavy sculpture, garden pot or that type of scenario).

If you don't use one of those problematic "dirty" claybodies with lots of gunk to burn out then I wouldn't worry about these pots being underfired. If you dip your glazes just do a test dip on one and scratch away the glaze to see how thick it is. If it's too thick then either dip faster or spritz the pot with water 10 minutes or so before you dip them.

[Bill Kielb]

3 hours ago, Gonepotty said:

When you say 'adding holds result in the same uneven firing', I'm not too sure what you mean by this?

A technique to get to the next cone without reaching a higher temperature is to add a hold. So one example - often you might find someone realizes that some of their coloring in glaze or underglaze begins to change and burnout at cone six temperatures. One technique to avoid the cone six peak temperature but still fire to cone six heatwork would be to fire at cone 5 with a 20 minute hold. So the peak temperature stops at cone 5 but because of the extra time (20 minutes) it will fire to cone 6 heatwork, dropping the cone 6 cone. So adding a hold at top temperature doesn’t necessarily even things out, more often it just adds heatwork and drives the entire kiln to the next cone.

[Gonepotty]

11 minutes ago, Bill Kielb said:

A technique to get to the next cone without reaching a higher temperature is to add a hold. So one example - often you might find someone realizes that some of their coloring in glaze or underglaze begins to change and burnout at cone six temperatures. One technique to avoid the cone six peak temperature but still fire to cone six heatwork would be to fire at cone 5 with a 20 minute hold. So the peak temperature stops at cone 5 but because of the extra time (20 minutes) it will fire to cone 6 heatwork, dropping the cone 6 cone. So adding a hold at top temperature doesn’t necessarily even things out, more often it just adds heatwork and drives the entire kiln to the next cone.

Ahh ok , understand now Thank you for explaining 

[Denice]

I have manual kilns,  I put a C5 cone in my kiln sitter when firing to C6.   Most of the glazes I use need a hold at the end,  I can get my C6 firing and also have a long hold without over firing  my glazes from heat work.   I monitor the temperature with a Skutt digital thermocouple  set.    Have fun while you are testing and  learning about your kiln.     Denice

[Hulk]

"Glaze firing reached target cone 8 perfectly..."

That's what, good call.

My kiln set up does about the same, more variation shelf to shelf in the bisque fire than glaze fire.