Bisque underfired

[irenepots]

I just opened up my  ^ 04   slow bisque load and it seems to have underfired.  The temperature reached 1940 degrees, but you can see one cone has barely bent.   I did a 30 minute preheat and a 4 minute hold at the end, and the whole thing took 15:12.     I've never had an underfire like this before.  

My previous bisque had an hour preheat and a 5 minute hold at the end and cones from both shelves touched down after 14:38.   

Bisque before that went to 1940 after a 15 minute preheat and an 8 minute hold, both cones touching down

 I'm  concerned because today's  load included a bunch of test blanks and I'm wondering how accurate glaze results will be on them.   Why do you think the cones didn't bend?  I know it's a factor of time and temperature together,  but I don't understand what happened.  The preheats shouldn't matter should they?   Did the kiln just not fire as hot as usual for some reason?  It has relatively new coils.  Should I remake the testers?

Did I just piss off the kiln gods somehow?

 Thanks for your help!   Sorry I don't know why the picture spun or how to un-spin it

 

 

[Babs]

Cones positioned in same spot in kiln?

Load same density?

15hrs a long firing.

What is your ramping?

Are you logging the temp every hour or at each ramp switch?

What are your pots looking like, sounding like, feeling like?

Your test tiles will be more porous. You could dampen them a bit to counter that....or dip for shorter time.

[irenepots]

Hi Babs.  Thanks for your response.  My firings are always long -  it's the slow bisque on my computerized LL kiln.  The load was about the same as usual but the top cone was in a slightly different position because there were two large flattish pieces on the top shelf, along with smaller pieces.    For some reason my top shelf usually fires cooler than the bottom, and I know that's unusual, but this time it fired hotter.  Bottom cone was in its usual place, so the cones were in different  spots, while usually they correspond to each other.   I don't log the ramp temps.  There might have been more ware on the bottom shelf by weight.  I  usually leave the vent running till the load has cooled but this time I unplugged it when the firing was complete.  The pots seem cooked enough to me and feel as usual.  Maybe someone with more experience would know otherwise tho.   The test tiles will be brushed not dipped.    

Can you refire a bisque load?

On 1/24/2022 at 10:58 AM, Callie Beller Diesel said:

10 hours ago, Babs said:

Cones positioned in same spot in kiln?

Load same density?

15hrs a long firing.

What is your ramping?

Are you logging the temp every hour or at each ramp switch?

What are your pots looking like, sounding like, feeling like?

Your test tiles will be more porous. You could dampen them a bit to counter that....or dip for shorter time.

 

 

[dhPotter]

Having self supporting cones that bend to touch the shelf is overfire. For Self supporting cones to be fired to correct temperature the tip should bend no further than  where the cone meets the base, roughly a 1/2 inch above the shelf.

[Babs]

It is common on a top loader for top shelf to fIre cooler and a soak helps to even up the temp wIthIn the kiln.

It Is also common to place a series of cones, one below target temp, target temp, and one over target cone. 

Cones measure heatwork done.

Depending on how touchy your ckay is, I wouldn't refire this load , could be as little as 10 to20 deg C difference..soakfor 10mins  each time, not sure why you are changing. Density of pack?

[irenepots]

20 hours ago, Babs said:

It is common on a top loader for top shelf to fIre cooler and a soak helps to even up the temp wIthIn the kiln.

It Is also common to place a series of cones, one below target temp, target temp, and one over target cone. 

Cones measure heatwork done.

Depending on how touchy your ckay is, I wouldn't refire this load , could be as little as 10 to20 deg C difference..soakfor 10mins  each time, not sure why you are changing. Density of pack?

just trying to perfect the firing.  When I fire a glaze load I use a 5 and 6 cone on each shelf.  So if my cones are usually bending beyond the cone with a 5 minute hold, then I need to change the offset for a 10 minute soak right?  Does a fuller load need more or less time?   I fire 2 instead of 3 shelves because

[irenepots]

oops sorry, because with 3 shelves the temps vary widely, and I also don't produce that much ware.  

 

[Babs]

The 10min soak will even out the temp throughout the firing. If in a glaze firing, cones are bending on  a 5 min hold and your glaze is same on each shelf then no .

If densely packed, tbe soak helps with all pots reaching temp.

Firing time should not alter with density unless you are wantig to do this..

During gla ze firing, to get uniform results the packing of each shelf is important.

@GEP

went i nto this in detail in a previous post.

[oldlady]

irene, what L&L kiln do you have?  do you have any photos of what you are firing?   i have an L&L also and your times seem high to me.  i load as many as 9 shelves in my single firings from greenware to cone 6 glaze and they take about 14 -15 hours when densely packed and the elements newish.

i think you said you only use about 3 shelves,  what is on them?   you mentioned relatively new elements, when did you replace them?   i am just wondering as you are  what takes so long?

[Roberta12]

@irenepots I have an L&L e23t kiln.  I also slow bisque.  My slow bisque without preheats or holds is around 13 hours.  That is with 5-9 shelves in the load (sometimes lots of flat things)   Thanks @dhPotter for the tip about self supporting cones.  I did not know that.  I was having issues with my glaze firings in November so bought self supporting cones for my bisque load, just as a check. I have always made my own cone packs for glaze loads.  

Anyway, I don't think a hold is necessary for a bisque load.   In my experience, your bisque load will be fine.  Next bisque load check again.  Perhaps you can get more pieces in the next load to see if that makes a difference?  Do you normally only have 2 or 3 shelves in a bisque load?  And were the cones sitting on a shelf by themselves or next to pots?  I know if I have a small load with either glaze or bisque, it definitely changes the length of firing time and can affect the glazes.   My top shelf is cooler than bottom or middle.  I thought that was a normal thing?

Roberta

[Denice]

I really pack my kiln and if it takes longer than 12 hours to fire I start planning to replace my wiring.    Denice

[irenepots]

20 hours ago, oldlady said:

irene, what L&L kiln do you have?  do you have any photos of what you are firing?   i have an L&L also and your times seem high to me.  i load as many as 9 shelves in my single firings from greenware to cone 6 glaze and they take about 14 -15 hours when densely packed and the elements newish.

i think you said you only use about 3 shelves,  what is on them?   you mentioned relatively new elements, when did you replace them?   i am just wondering as you are  what takes so long?

Hi ol, it's an e28s-3, with 4 elements.  I use 2 rows of split shelves, so does that translate to using 4 shelves?   It's a shallow kiln because I'm short and can't reach the bottom of a taller kiln.  I use a lot of dark brown clay and I don't stack it because I want to be sure there's  no blockage to any impurities gassing out.  I hope I'm phrasing that right.  I had more pinholing when i used to stack. I do stack white.   Elements were changed around June 2019,  but I need to check to see how many firings they've had.  To me the time is pretty consistent with my other bisques. What changed was how little the cones bent.   I can't find any pics of my firings but I'll keep looking.  Typically  I fire mugs and bowls and some slab plates.  I had some small pinch pumpkins and flat testers, which were stacked 2-4 high.  I try to put flat stuff at the edges

 

20 hours ago, oldlady said:

irene, what L&L kiln do you have?  do you have any photos of what you are firing?   i have an L&L also and your times seem high to me.  i load as many as 9 shelves in my single firings from greenware to cone 6 glaze and they take about 14 -15 hours when densely packed and the elements newish.

i think you said you only use about 3 shelves,  what is on them?   you mentioned relatively new elements, when did you replace them?   i am just wondering as you are  what takes so long?

 

[irenepots]

8 hours ago, Roberta12 said:

@irenepots I have an L&L e23t kiln.  I also slow bisque.  My slow bisque without preheats or holds is around 13 hours.  That is with 5-9 shelves in the load (sometimes lots of flat things)   Thanks @dhPotter for the tip about self supporting cones.  I did not know that.  I was having issues with my glaze firings in November so bought self supporting cones for my bisque load, just as a check. I have always made my own cone packs for glaze loads.  

Anyway, I don't think a hold is necessary for a bisque load.   In my experience, your bisque load will be fine.  Next bisque load check again.  Perhaps you can get more pieces in the next load to see if that makes a difference?  Do you normally only have 2 or 3 shelves in a bisque load?  And were the cones sitting on a shelf by themselves or next to pots?  I know if I have a small load with either glaze or bisque, it definitely changes the length of firing time and can affect the glazes.   My top shelf is cooler than bottom or middle.  I thought that was a normal thing?

Roberta

Hi Roberta, the cones are always next to pots.  I thought it was odd that my top shelf is cooler too, but until this last firing my shelf temps  had been quite close according to the cones.  When you fire a smaller load does it take longer?  I put in some shelf posts if I have a smaller load than usual, but this load was about usual.  I used to take pics of every load, I guess I'll start doing that again.  

[irenepots]

On 2/5/2022 at 10:31 AM, dhPotter said:

Having self supporting cones that bend to touch the shelf is overfire. For Self supporting cones to be fired to correct temperature the tip should bend no further than  where the cone meets the base, roughly a 1/2 inch above the shelf.

Thanks for that info.  So the one cone is fired to about 04 then?

[Callie Beller Diesel]

The cone that is more bent looks like a cool cone 04, because the edges are still pretty distinct. The other looks like it’s probably only at 05. It’s started to move, but hasn’t gone over at all.

On 2/5/2022 at 7:38 AM, irenepots said:

.  There might have been more ware on the bottom shelf by weight. 

I would venture that the difference in this firing is because of this. If your load is denser, it can take more time and/or energy expended to heat all the way through. That’s why it’s more useful to us to fire by cone and not just temperature. Cones measure melt. 

It sounds from everything you’re describing, your firings are very close. If your work isn’t pinholing, it’s probably bisqued enough. I was chatting with a few fellow dark clay users on FB about bisque times vs temperature, and we have a tentative hypothesis that time might be more important than end temperature.  A slow bisque to 05 could be just as effective as a faster firing to 04, especially if going hotter begins to affect your clay’s porosity. It doesn’t for mine, but it does on other clays. It’s all a big game of do what works best for you.

[irenepots]

8 minutes ago, Callie Beller Diesel said:

The cone that is more bent looks like a cool cone 04, because the edges are still pretty distinct. The other looks like it’s probably only at 05. It’s started to move, but hasn’t gone over at all.

I would venture that the difference in this firing is because of this. If your load is denser, it can take more time and/or energy expended to heat all the way through. That’s why it’s more useful to us to fire by cone and not just temperature. Cones measure melt. 

It sounds from everything you’re describing, your firings are very close. If your work isn’t pinholing, it’s probably bisqued enough. I was chatting with a few fellow dark clay users on FB about bisque times vs temperature, and we have a tentative hypothesis that time might be more important than end temperature.  A slow bisque to 05 could be just as effective as a faster firing to 04, especially if going hotter begins to affect your clay’s porosity. It doesn’t for mine, but it does on other clays. It’s all a big game of do what works best for you.

It will be interesting to see how  glazing on this load turns out.   Hopefully it won't make that much difference.  But I'll be taking pictures again  before I unload my bisque from now on.   I should do that for glaze too since the same glaze can very so much depending on placement.  Thanks for your thoughts and experience

 

[Callie Beller Diesel]

If you’re concerned about it, you can slow the first part of your glaze cycle down to allow for any further burn off you think might be necessary. Pause it for another 10 minutes around the cone 04 mark, and that seems like it would hedge your bets.

[irenepots]

10 minutes ago, Callie Beller Diesel said:

If you’re concerned about it, you can slow the first part of your glaze cycle down to allow for any further burn off you think might be necessary. Pause it for another 10 minutes around the cone 04 mark, and that seems like it would hedge your bets.

I'm going to see what happens the next time I bisque.  Also it will be interesting to see if something similar happens with my next glaze load.  I have to count up how many firings I've done since the elements were changed, and how many I did before I changed them

 

[Babs]

2 hours ago, irenepots said:

.  But I'll be taking pictures again  before I unload my bisque from now on.   I should do that for glaze too since the same glaze can very so much depending on placement.  

 

So your glaze does come out different depending on placement  Then I would be slowing down the last ramp of firing and adding a 10 minute soak.

Also bottom of kiln packed lightly ,middle more densely and top lightly also

[Roberta12]

22 hours ago, irenepots said:

Hi Roberta, the cones are always next to pots.  I thought it was odd that my top shelf is cooler too, but until this last firing my shelf temps  had been quite close according to the cones.  When you fire a smaller load does it take longer?  I put in some shelf posts if I have a smaller load than usual, but this load was about usual.  I used to take pics of every load, I guess I'll start doing that again.  

I just fired a bisque load.  It was packed.  Lots of shelves, I had a lot of plates.  It was 14hours.  That isn't that unusual for a really packed load for me.  I might have had a shelf too close to a TC also.  If it's a light load, it will be a shorter time.   Do you have a kiln log?  If not I will send you one you can copy.  It is a life saver.  I write down not only every firing but when repairs were made.  It's handy tool. Just DM me. 

Which L&L kiln do you have??  

 

[Morgan]

I do very slow bisque firings for dark clay bodies and it takes close to that long.

frankly I would not worry about it much, one cone is almost perfect by the pic and one at least started bending so we are not looking at a huge difference here, especially for a bisque. Sounds just like normal kiln placement and packing temp differences. 

[irenepots]

1 hour ago, Morgan said:

I do very slow bisque firings for dark clay bodies and it takes close to that long.

frankly I would not worry about it much, one cone is almost perfect by the pic and one at least started bending so we are not looking at a huge difference here, especially for a bisque. Sounds just like normal kiln placement and packing temp differences. 

Hi Morgan, I agree that the bisque time is fine.   As for the cones, next time I'll put them both where I usually do and move the ware  around instead.   Since you fire dark bodies, do you stack them?  I use Standard 266 which is a dark brown.  No stacking.