Bisque firing frustrations

[Gonepotty]

Hi all, 

Having very frustrating results with my bisque firing . oxidation. Never seems to get to target temp...elements fine, glaze firing reaches target with no issues. 

Current 08 schedule ( Celcius) 

Step1) 80'C/hr to 200.

Step2)120'C/hr to 915

Step3)60'C/hr to 995 +20min hold. (aiming for top target of approx 1015'C; kiln has always seemed to fire cooler for bisque by about 65'C hence aiming for this higher target in the hopes it will reach 950'C).

RESULTS: Top shelf reached 08. Mid shelf  reached 09. Bottom shelf … no bend to 09. Bottom shelf was densely packed...would this impact the firing.? Typically, my kiln is hottest on bottom shelf and coolest ontop. Was a ful kiln load with shelves and props. 

 

I have tried this below schedule in the past and worked quite well. It is listed as a "high bisque" schedule but wondering if it is too aggressive?:

1)90'C to 300

2) 110'C to 900

3) 60'C to 1050+ 20min hold. 

whilst I was quite happy with this programme, it fired a bit too hot for my bisque ware and reached 1000'C. I would like to aim for 950'C and so thought shall I:

a) drop the final temp down to 1000'C+20min hold in the hope that it will reach 950'C? OR

b) adjust the programme to the 08 schedule? 

Should bisque firings take longer than 10hrs? First firing schedule took 9.5hrs whilst the "high bisque" schedule takes 13.5hrs...Guess I was keen to also reduce firing time but it appears to not have worked that well. 

Any words of wisdom greatly appreciated!

 

[Pres]

@Gonepotty, could you include a little more information on your kiln. Are you doing a manual ramp with a kiln setter, or are you using a programmable controller. Balanced packing on all shelves will help with evening out the temperatures, so yes having a tighter pack in the bottom will effect the firing.

 

 

best,

Pres

[Gonepotty]

56 minutes ago, Pres said:

@Gonepotty, could you include a little more information on your kiln. Are you doing a manual ramp with a kiln setter, or are you using a programmable controller. Balanced packing on all shelves will help with evening out the temperatures, so yes having a tighter pack in the bottom will effect the firing.

 

 

best,

Pres

Hi Pres, I used a programmable controller. Skutt 1027 with the new touch screen controller. Would you mind explaining how a tighter packed shelf will impact heat as this is something I am struggling to grasp. Thanks so much

[Callie Beller Diesel]

5 hours ago, Gonepotty said:

Bottom shelf was densely packed...would this impact the firing.? Typically, my kiln is hottest on bottom shelf and coolest ontop.

Yes. Yes this will affect things. If your kiln is hottest on the bottom and you’ve packed it tightly, but your ware still isn’t getting the heat, the bottom is probably packed too tightly. You may also be going a bit too fast through the middle parts of that firing. 120 C is a really fast rate of climb. And yes, it’s not unusual for a bisque to go for 10-12 hours. Keep in mind the elements aren’t on at full power the whole time. If you’ve got the touchscreen, check out the diagnostics in the last firing. It’ll tell you what your energy usage is.

Re: the density affecting the heat:  Electric kilns are sometimes programmed to emit more heat on the bottom elements, because heat rises, and they’re trying to avoid cool spots in the bottom of the kiln. They’re assuming people pack it evenly, and they don’t expect consumers to have to compensate their loading habits on a new kiln.

If the bottom part of the kiln is packed particularly densely, especially compared to the other shelves, the heat may not have time to penetrate all the way through to the middle of the bottom layer. If the cones aren’t bending on that bottom shelf with a 20 minute soak at the end, that bottom layer and even the middle is probably playing catchup to even hit cone 09. Try slowing your second segment to 90 or 100 degrees C, aiming for the proper cone 08 temperature, and packing the kiln more evenly. 

 

 

 

[Babs]

Not knowing the clay you use, or the kiln, here's my bisque in °C.

And, Callie the expert, so listen to her.

If bisqueware dry and of good thickness i.e not thick sculpted pieces

50°C / hr to 100°C

100°C/hr to 600°C

150°C/hr to 1000°C 

Soak for 10 minutes.

Target is C06. 

I pack bottom shelf with taller stuff, rest of kiln is tumble packed. Soak at end for me is to allow kiln temp to even out and tumble stacked ware to get to the cone target. 

Seems brutal but works

[Pres]

Density of the load has an effect on the way heat moves in the kiln. Kilns are heated by 3 different forms of heat transfer: Radiant, Conduction, and Convection.  Understanding these will help you to understand the need to pack the kiln consistently. Radiant heat is occurs first in the kiln as the elements radiate heat to heat up the air and the pottery. The pottery on the outer edges will heat up to radiate heat towards the center of the kiln. Please note that while Radiant heat is going on Convection is also occurring in the kiln as air heats up and begins to move to the top of the kiln forcing cooler air downward. Overly tight loads will inhibit the movement of air in the kiln causing areas that do not get air to not reach expected cones. However, as this is going on Conduction, where heat is moving through a solid, is taking place, Conduction is a slower process but it allows the pots to heat up from the outside to the inside, and the kiln from the inside to the outside. This good reason to have proper distance from the kiln to outside walls or other materials that could ignite.

Hope this helps to answer some of you concerns.

 

best,

Pres

[Min]

As much of the density or mass in a firing comes from the shelves themselves it's often helpful to put the taller pots on the bottom shelf, shorter pots and more shelves in the middle of the kiln and tall again on the top shelf. (assuming it has with just one thermocouple or is a manual kiln)

[Gonepotty]

14 hours ago, Callie Beller Diesel said:

Yes. Yes this will affect things. If your kiln is hottest on the bottom and you’ve packed it tightly, but your ware still isn’t getting the heat, the bottom is probably packed too tightly. You may also be going a bit too fast through the middle parts of that firing. 120 C is a really fast rate of climb. And yes, it’s not unusual for a bisque to go for 10-12 hours. Keep in mind the elements aren’t on at full power the whole time. If you’ve got the touchscreen, check out the diagnostics in the last firing. It’ll tell you what your energy usage is.

Re: the density affecting the heat:  Electric kilns are sometimes programmed to emit more heat on the bottom elements, because heat rises, and they’re trying to avoid cool spots in the bottom of the kiln. They’re assuming people pack it evenly, and they don’t expect consumers to have to compensate their loading habits on a new kiln.

If the bottom part of the kiln is packed particularly densely, especially compared to the other shelves, the heat may not have time to penetrate all the way through to the middle of the bottom layer. If the cones aren’t bending on that bottom shelf with a 20 minute soak at the end, that bottom layer and even the middle is probably playing catchup to even hit cone 09. Try slowing your second segment to 90 or 100 degrees C, aiming for the proper cone 08 temperature, and packing the kiln more evenly. 

 

 

 

Thanks so much for your repl

 

14 hours ago, Callie Beller Diesel said:

Yes. Yes this will affect things. If your kiln is hottest on the bottom and you’ve packed it tightly, but your ware still isn’t getting the heat, the bottom is probably packed too tightly. You may also be going a bit too fast through the middle parts of that firing. 120 C is a really fast rate of climb. And yes, it’s not unusual for a bisque to go for 10-12 hours. Keep in mind the elements aren’t on at full power the whole time. If you’ve got the touchscreen, check out the diagnostics in the last firing. It’ll tell you what your energy usage is.

Re: the density affecting the heat:  Electric kilns are sometimes programmed to emit more heat on the bottom elements, because heat rises, and they’re trying to avoid cool spots in the bottom of the kiln. They’re assuming people pack it evenly, and they don’t expect consumers to have to compensate their loading habits on a new kiln.

If the bottom part of the kiln is packed particularly densely, especially compared to the other shelves, the heat may not have time to penetrate all the way through to the middle of the bottom layer. If the cones aren’t bending on that bottom shelf with a 20 minute soak at the end, that bottom layer and even the middle is probably playing catchup to even hit cone 09. Try slowing your second segment to 90 or 100 degrees C, aiming for the proper cone 08 temperature, and packing the kiln more evenly. 

 

 

 

Thanks so much for your help. I think I'll adjust the firing schedule and make sure I pack it evenly! Really appreciate you taking your time to reply

[Gonepotty]

10 hours ago, Babs said:

Not knowing the clay you use, or the kiln, here's my bisque in °C.

And, Callie the expert, so listen to her.

If bisqueware dry and of good thickness i.e not thick sculpted pieces

50°C / hr to 100°C

100°C/hr to 600°C

150°C/hr to 1000°C 

Soak for 10 minutes.

Target is C06. 

I pack bottom shelf with taller stuff, rest of kiln is tumble packed. Soak at end for me is to allow kiln temp to even out and tumble stacked ware to get to the cone target. 

Seems brutal but works

Thanks Babs, its really helpful to have many reference points/info from different folks as it helps build a bigger picture for me. 

[Gonepotty]

5 hours ago, Pres said:

Density of the load has an effect on the way heat moves in the kiln. Kilns are heated by 3 different forms of heat transfer: Radiant, Conduction, and Convection.  Understanding these will help you to understand the need to pack the kiln consistently. Radiant heat is occurs first in the kiln as the elements radiate heat to heat up the air and the pottery. The pottery on the outer edges will heat up to radiate heat towards the center of the kiln. Please note that while Radiant heat is going on Convection is also occurring in the kiln as air heats up and begins to move to the top of the kiln forcing cooler air downward. Overly tight loads will inhibit the movement of air in the kiln causing areas that do not get air to not reach expected cones. However, as this is going on Conduction, where heat is moving through a solid, is taking place, Conduction is a slower process but it allows the pots to heat up from the outside to the inside, and the kiln from the inside to the outside. This good reason to have proper distance from the kiln to outside walls or other materials that could ignite.

Hope this helps to answer some of you concerns.

 

best,

Pres

Thanks so much Pres, super helpful information and great explanation. Feel more confident now with bisque firing

[Gonepotty]

5 hours ago, Min said:

As much of the density or mass in a firing comes from the shelves themselves it's often helpful to put the taller pots on the bottom shelf, shorter pots and more shelves in the middle of the kiln and tall again on the top shelf. (assuming it has with just one thermocouple or is a manual kiln)

Thanks Min, yes it only has 1 thermocouple. I'll make note of this info for my next bisque firing

[neilestrick]

Pack the middle the tightest.

Have you tried just using the cone 08 preprogrammed firing schedule?

[oldlady]

your kiln manufacturer has a long history of getting the best out of it's kilns and providing simple controls for users.    why do you think your program is necessary and better than theirs?  and what is special about cone 09?   yours is the first post to mention it in all the years i have read almost every post.   are you glazing some special way that requires your bisque  so soft?  

once you fix the loading as suggested, just try what Neil, who is a kiln specialist and dealer, suggests. 

[High Bridge Pottery]

Is it frustrating because it is causing problems or just because you would like the cones to read the same numbers? If it's the second I wouldn't worry about it.

 

Neither of those schedules you have posted are aggressive. For me 100c per hour is slow, 200c fast and 250c is aggressive.

I bisque to cone 016/017 (that's a guess, I have never put a cone in a bisque kiln) in 4 hours doing 20 min to 100c, hold for 20 min, then ramp at 250c an hour up to 800c and hold for 20 min. You can see the kiln drop to under 250c an hour while it struggles from 500-600c as chemical water is lost but it catches back up to 250c and hour after that.

 

 

 

 

[Babs]

That is a pretty low bisque...250°c/hr ìs very aggressive, what type of kiln is it? What clay body? I thought cone017 was for enamel work.. 

[Gonepotty]

On 4/17/2024 at 6:06 PM, neilestrick said:

Pack the middle the tightest.

Have you tried just using the cone 08 preprogrammed firing schedule?

Thanks Neil, this was my next thing to try 

[Gonepotty]

23 hours ago, oldlady said:

your kiln manufacturer has a long history of getting the best out of it's kilns and providing simple controls for users.    why do you think your program is necessary and better than theirs?  and what is special about cone 09?   yours is the first post to mention it in all the years i have read almost every post.   are you glazing some special way that requires your bisque  so soft?  

once you fix the loading as suggested, just try what Neil, who is a kiln specialist and dealer, suggests. 

Thanks for your message. I am going to try the preloaded cone 08 bisque firing next. 

[Gonepotty]

20 hours ago, High Bridge Pottery said:

Is it frustrating because it is causing problems or just because you would like the cones to read the same numbers? If it's the second I wouldn't worry about it.

 

Neither of those schedules you have posted are aggressive. For me 100c per hour is slow, 200c fast and 250c is aggressive.

I bisque to cone 016/017 (that's a guess, I have never put a cone in a bisque kiln) in 4 hours doing 20 min to 100c, hold for 20 min, then ramp at 250c an hour up to 800c and hold for 20 min. You can see the kiln drop to under 250c an hour while it struggles from 500-600c as chemical water is lost but it catches back up to 250c and hour after that.

 

 

 

 

Thanks for your reply. Not having any problems with the pots but rather feel that its not achieving the desired bisque of 950'C - trying to keep everything consistent so I know how the glazes will behave when applying and during the firing

[neilestrick]

26 minutes ago, Gonepotty said:

not achieving the desired bisque of 950'C

It's not about temperature, it's about heatwork, which is what cones measure. Setting it at 950C doesn't necessarily mean cone 08. It depends on the rate of climb of the final 100C degrees. If it's not hitting the cone, then it could be due to the rate of the final ramp, or simply that the system needs to be calibrated. You can do a thermocouple offset which will affect all temperatures, or a cone offset to change the final temp of a specific cone. Using the pre-programmed firing schedule (I'd use Medium speed) will give you a better idea of how accurately the system is firing compared to a custom schedule.

[High Bridge Pottery]

3 hours ago, Babs said:

That is a pretty low bisque...250°c/hr ìs very aggressive, what type of kiln is it? What clay body? I thought cone017 was for enamel work.. 

It's my zero3 fritware clay, kiln is just a 40 litre rust bucket.

 

 

2 hours ago, Gonepotty said:

Thanks for your reply. Not having any problems with the pots but rather feel that its not achieving the desired bisque of 950'C - trying to keep everything consistent so I know how the glazes will behave when applying and during the firing

I would go with the cone program as it may dynamically alter the firing depending on how it keeps up with the rate of climb. Do skutt controllers do that? 

[Babs]

On 4/20/2024 at 1:28 AM, High Bridge Pottery said:

It's my zero3 fritware clay, kiln is just a 40 litre rust bucket.

 

 

I would go with the cone program as it may dynamically alter the firing depending on how it keeps up with the rate of climb. Do skutt controllers do that? 

Ah I remember you posting abiut that process.

[Gonepotty]

On 4/19/2024 at 3:09 PM, neilestrick said:

You can do a thermocouple offset which will affect all temperatures, or a cone offset to change the final temp of a specific cone. Using the pre-programmed firing schedule (I'd use Medium speed) will give you a better idea of how accurately the system is firing compared to a custom schedule.

If I do a cone offset will this only be effective if I fire using a preloaded cone programme? Don't think I want to offset thermocouple as my glaze firings are fine. I have a smaller kiln which has a S type thermocouple and its always spot on. - think i might consider upgrading mine when it comes to end of its life. 

Next bisque firing I'll try the preloaded cone 08 on medium speed as see how things go. Thanks so much for your help

 

[Gonepotty]

On 4/19/2024 at 4:58 PM, High Bridge Pottery said:

It's my zero3 fritware clay, kiln is just a 40 litre rust bucket.

 

 

I would go with the cone program as it may dynamically alter the firing depending on how it keeps up with the rate of climb. Do skutt controllers do that? 

Yes they do have preloaded cone programmes so Im going to give that a go next.

[Kelly in AK]

1 hour ago, Gonepotty said:

…as my glaze firings are fine.

 

This is it. I’m not sure what you’re worried about. I understand the need for consistency and control, but if the glaze firing is working it seems like you’ve got it. On the other hand, I do appreciate the desire to know what’s going on in there and how what’s on the display relates to reality. 

A packed kiln will fire slower and take more energy than one sparsely loaded. A bisque load with pots stacked and full top to bottom, for example, is a significant mass to heat. The controller handles it as programmed, using the thermocouple. I’ve found the programmed cone firing schedules more useful, easier, than anything I’ve concocted trying to program schedules myself. Believe me, I’ve tried second guessing cone fire modes. Use physical cones for when you want to know with confidence what’s happening in the kiln.

I use a Skutt and a Paragon for bisque. The manuals for both are helpful for understanding how the programming works. If the kiln is jam packed “slow” speed works best. My Paragon’s elements are near the end of their lifespan and if I try to fire a bisque with it fully loaded (think hundreds of tiny pinch pots made by little children, stacked and packed) at a regular speed it’ll error out. To put a finer point on it, if it errors fatally and shuts off it’s done, usually a bit under fired. If it errors and continues to the programmed temperature it will plod along soaking everything for hours near the target cone and things get over fired. Simply choosing “slow” avoids this trouble. Not a second spent thinking of segments, rates of climb, or offsets, just a single button press. 

As @High Bridge Pottery mentioned, the cone firing modes are dynamic and adjust themselves as the firing progresses. This is a good thing, because the density of your kiln load and the wear of your elements are dynamic too. 

[neilestrick]

7 hours ago, Gonepotty said:

If I do a cone offset will this only be effective if I fire using a preloaded cone programme?

The thermocouple offset will effect everything. It makes the thermocouple read higher or lower. If your glaze firings are fine, then a cone offset would be the way to go to dial in your bisque firings, but do the pre-programmed firing first.