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Is occasionally firing an empty kiln good practice?


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I've been having problems with pitting  in glazes lately and am wondering is it good practice to fire an empty kiln to burn off any potential lingering fumes to help glazes?

Have been firing non-stop with about 2 months now, and in the last few weeks a lot of work is coming out of the kiln with pits. Have been in touch with my clay and glaze supplier and they are stumped!

Have tried many firing schedules, from slow to fast, with up to 45min soaks at top temp and tried a drop and hold schedule but nothing seems to work. Have also tried bisque firing high to 1020oC with a 45min soak at 780oC on the way up and 45min at top temp but no success.

It leaves me wondering should I fire an empty kiln, and would that "freshen up" the atmosphere within? Could the bricks be holding onto residual fumes from firings and could this be causing the glaze issue?

I have an extraction fan that runs for the full firing until the firing is finished. And allow the kiln to cool naturally with the fresh air valve closed (as per operational instructions).

Any advice would be much appreciated. Thanks in advance.

Edited by F Crowley
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  • F Crow changed the title to Is occasionally firing an empty kiln good practice?

Do you use cones to verify what you are firing to?  Firing with holds at the top often makes pinholes worse. Do you have a last segment speed you always use, say in the last 100c of the firing? My first impression is your elements are wearing and your firings are taking longer and as a result your work is now firing 1-2 cones higher. Just a guess though but easy to confirm by measuring your element resistance and comparing with new.

Not much survives 1200c firings so the contamination thing seems very unlikely to me. That said, not sure what cone you do glaze fire to. A bisque firing to cone 04 at 12-14 hours is normal to me to remove contaminants in a ventilated kiln.

Edited by Bill Kielb
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Not at all necessary (freshen up")

Just a waste of energy

(Could the bricks be holding onto residual fumes from firings and could this be causing the glaze issue?). not this will not be the case

More likely the clay /.glaze /firing schedule and soak /end temps

 

Maybe speak to those details so folks can help you out on this

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10 hours ago, Bill Kielb said:

Do you use cones to verify what you are firing to?  Firing with holds at the top often makes pinholes worse. Do you have a last segment speed you always use, say in the last 100c of the firing? My first impression is your elements are wearing and your firings are taking longer and as a result your work is now firing 1-2 cones higher. Just a guess though but easy to confirm by measuring your element resistance and comparing with new.

Not much survives 1200c firings so the contamination thing seems very unlikely to me. That said, not sure what cone you do glaze fire to. A bisque firing to cone 04 at 12-14 hours is normal to me to remove contaminants in a ventilated kiln.

Thanks very much for your reply.

I actually don't use cones and it doesn't seem to be common practice where I live because when I ask other ceramicist about using them no one seems to. Read so much about them online though, so will order some soon and see what info they give.

The firings finish within 10-15min of schedule so would guess the elements are still okay? I fire to 1204oC with the temp range for the clay being 1200oC-1290oC and the glaze 1180oC-1250oC so would presume that the work isn't being over fired either.

 

My latest bisque is;

- 60oC/h to 780oC

- 25min hold

- 100oC/h to 1020oC

- 45min hold

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3 hours ago, F Crow said:

The firings finish within 10-15min of schedule so would guess the elements are still okay? I fire to 1204oC with the temp range for the clay being 1200oC-1290oC and the glaze 1180oC-1250oC so would presume that the work isn't being over fired either.

 

Pretty much spot on with temp. Cones are great equalizers in that they are made of glaze and melt by way of the same flux aided reaction so they are a good prediction of the actual heatwork done as opposed to finishing at a temperature.  

Potters will often fire to a lower cone with a hold to get more heatwork without actually reaching the top temperature. So for instance firing to cone 5 with a 15-20 minute hold actually makes it to cone 6 heatwork  wise.  Your comment about adding  a 45 minute hold gives the impression you likely fired 1 to 2 cones higher.

Cones can be reasonably approximated by referring to a cone chart and firing at the published speed in the last 100 c of the firing. Turns out this is where most of the material heatwork is done. A popular final segment speed for you would be 60c per hour in the last 100c. Something you might try and see if it improves your glaze finish.

Your escalating issue had earmarks of a firing that was taking longer as in elements degrading over time. Definitely just a guess though.  As far as contamination, very unlikely so something has changed whether clay, glaze or firing.

If you can post some pictures, I believe folks here will have ideas on things to try after viewing the results of the firing.

Edited by Bill Kielb
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16 hours ago, Bill Kielb said:

Firing with holds at the top often makes pinholes worse.

Interesting observation. I find doing a short hold just below cone final position to bring the heatwork to the same level as directly firing to the desired cone doesn't have this effect. What I do find greatly helps reduce pinholes is dropping 100F and doing a second short hold there. (in addition to having an appropriate clean bisque firing)

edit: @F Crow, could you also post which clay and glaze(s) you use that you are having issues with.

Edited by Min
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1 hour ago, Min said:

Interesting observation. I find doing a short hold just below cone final position to bring the heatwork to the same level

It definitely can be counter intuitive for sure as most folks assume that more liquid / fluid  = better healing. The drop and hold seems to be a more successful solution with the holding at peek seeming to be a less successful solution to pinholes.

I am not convinced that holding at peak really evens things out either as anything that can see the radiation always gets more heatwork. So the argument that we are waiting for the heat to get to the center of our kiln, center of a pot ........ is interesting as we change our thinking of heatwork and revert to one more of  a single temperature.  I like to be cautious of this as folks start adding 10, 15, 20, ......... 45 minute holds without realizing they are moving to a higher cone which can be verified  of course with cones. Many new potters seem to not realize this as a useful technique in their toolbox of tricks.

In the end, if it works for their glazes and firing schedule then that is the final say though.  The reason it works, probably not as relevant other than their repeated success.

Holding just short of would seem to be the best of all worlds as the anticipation that heat transfer will just even this out, so sort of a cool advanced idea in my view.

Edited by Bill Kielb
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14 hours ago, Mark C. said:

Not at all necessary (freshen up")

Just a waste of energy

(Could the bricks be holding onto residual fumes from firings and could this be causing the glaze issue?). not this will not be the case

More likely the clay /.glaze /firing schedule and soak /end temps

 

Maybe speak to those details so folks can help you out on this

Thanks for your reply Mark. Sorry, I had responded earlier but seems to have disappeared!

I use a white stoneware ball clay based clay with firing range of 1200oC to 1290oC.

The glazes are commercial dipping glazes with a range of 1180oC to 1250oC. I pass them through a 100mesh sieve as per the manufacturer's instructions before use.

My bisque is stated in another reply.

Some of the high firings I've tried are as follows.

My kiln controller allows for only 4 segments. A firing that was good but didn't eradicate the issue was;

- 80oC/h to 600oC

- 100oC/h to 1204oC

- 500oC/h to 1148oC (this just allows a controlled cool to the temp. It doesn't actually cool this rapidly).

-30min soak.

The manufacturer's suggested the following schedule but alas it made no improvements;

-140oC/h to 1020oC (bisque temp)

- 75oC/h to 1140oC

-60oC/h to 1200 (1oC per minute for the last hour)

-20min soak

A previous schedule was; 

-60oC/h to 780oC

-120oC/h to 1150oC

-60oC/h to 1210oC (again 1oC her minute for the last hour).

-15min soak

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14 hours ago, Mark C. said:

Not at all necessary (freshen up")

Just a waste of energy

(Could the bricks be holding onto residual fumes from firings and could this be causing the glaze issue?). not this will not be the case

More likely the clay /.glaze /firing schedule and soak /end temps

 

Maybe speak to those details so folks can help you out on this

Thanks for your reply Mark. Sorry, I had responded earlier but seems to have disappeared!

I use a white stoneware ball clay based clay with firing range of 1200oC to 1290oC.

The glazes are commercial dipping glazes with a range of 1180oC to 1250oC. I pass them through a 100mesh sieve as per the manufacturer's instructions before use.

My bisque is stated in another reply.

Some of the high firings I've tried are as follows.

My kiln controller allows for only 4 segments. A firing that was good but didn't eradicate the issue was;

- 80oC/h to 600oC

- 100oC/h to 1204oC

- 500oC/h to 1148oC (this just allows a controlled cool to the temp. It doesn't actually cool this rapidly).

-30min soak.

The manufacturer's suggested the following schedule but alas it made no improvements;

-140oC/h to 1020oC (bisque temp)

- 75oC/h to 1140oC

-60oC/h to 1200 (1oC per minute for the last hour)

-20min soak

A previous schedule was; 

-60oC/h to 780oC

-120oC/h to 1150oC

-60oC/h to 1210oC (again 1oC her minute for the last hour).

-15min soak

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3 hours ago, Bill Kielb said:

Pretty much spot on with temp. Cones are great equalizers in that they are made of glaze and melt by way of the same flux aided reaction so they are a good prediction of the actual heatwork done as opposed to finishing at a temperature.  

Potters will often fire to a lower cone with a hold to get more heatwork without actually reaching the top temperature. So for instance firing to cone 5 with a 15-20 minute hold actually makes it to cone 6 heatwork  wise.  Your comment about adding  a 45 minute hold gives the impression you likely fired 1 to 2 cones higher.

Cones can be reasonably approximated by referring to a cone chart and firing at the published speed in the last 100 c of the firing. Turns out this is where most of the material heatwork is done. A popular final segment speed for you would be 60c per hour in the last 100c. Something you might try and see if it improves your glaze finish.

Your escalating issue had earmarks of a firing that was taking longer as in elements degrading over time. Definitely just a guess though.  As far as contamination, very unlikely so something has changed whether clay, glaze or firing.

If you can post some pictures, I believe folks here will have ideas on things to try after viewing the results of the firing.

Oh I thought it was 60oC for the last hour not 100oC, thanks for that. Will definitely give it a try!

Here are some images showing the defect I'm talking about. One of the piece looking okay from afar, one of the piece tilted in the light highlighting the problem, and a third of the glaze taken using a macrolens. See how the defect doesnt seem to penetrate down to the clay body which is why I call it pitting as opposed to pinholing.

161498006234720210305_212122.jpg

161498006305020210305_212129.jpg

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Why are you using holds for heatwork and not just firing hotter? Glazes will respond to holds, but only once they've gotten hot enough. I'd a try a firing going to 1220C, no hold, 60C/hr for the last 2 hours. Anything less than 1200C and your clay body is going to be underfired.

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26 minutes ago, neilestrick said:

Why are you using holds for heatwork and not just firing hotter? Glazes will respond to holds, but only once they've gotten hot enough. I'd a try a firing going to 1220C, no hold, 60C/hr for the last 2 hours. Anything less than 1200C and your clay body is going to be underfired.

Hi Neil,

Everything I've read online, from places like digitalfire, lakeside pottery, and even in an article on this website seems to suggest a soak to burn off whatever may be causing the pitting?  That's why I've been using the soaks at top temp, or after a drop, to try remedy the issue.

My go to book, "the ceramics bible" by Louisa Taylor suggested  to soak the bisque longer which is why I've tried soaking it for 45min lately. Unfortunately that doesn't seem to be the solution either though.

Yeah I never fire less than 1200. The manufacturer did recommend this as a possible solution but I wouldn't do it for the very reason of under firing the clay.

Would you suggest a schedule something like this instead?

- 80oC/h to 600oC

-120oC/h to 1100oC

- 60oC/h to 1220oC

Thanks

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1 hour ago, F Crow said:

Oh I thought it was 60oC for the last hour not 100oC

Sorry if I stated that wrong. 60c for the last 100c degrees. Ends up under two hours.. Pick your finish fire temp from the chart (60c) column, subtract 100 and the speed of that segment is 60c per hour.

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1 hour ago, F Crow said:

Would you suggest a schedule something like this instead?

- 80oC/h to 600oC

-120oC/h to 1100oC

- 60oC/h to 1220oC

Yes, give it a try and see what happens. I'd also try firing to the high end of the recommended range of the glaze.

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Grog in a body can cause dimples like this, especially on flat pieces if it's a viscous glaze. Also, have you checked the specific gravity of the glaze recently? What can happen as a bucket of glaze gets used up the specific gravity increases as the water content in a glaze gets pulled into the bisque faster than the solids plus from just evaporation. A thick glaze layer is going to be more prone to dimples than a thinner layer. 

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2 hours ago, Bill Kielb said:

Sorry if I stated that wrong. 60c for the last 100c degrees. Ends up under two hours.. Pick your finish fire temp from the chart (60c) column, subtract 100 and the speed of that segment is 60c per hour.

Thank you so much. It  is similar advice to that given by Neil so will definitely try a more extended slow ramp at the end of my next firing.

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2 hours ago, neilestrick said:

Yes, give it a try and see what happens. I'd also try firing to the high end of the recommended range of the glaze.

Brilliant thanks so much will give that schedule a go next firing.

I actually prefer the results of the glazes at the lower end of their firing range as they are more vibrant, I think they're called reactive glazes? They are the same glazes I used in school when learning and we all had work that was blue, white, or brown because they all ended one of those colours when fired to 1250oC. It's through my own experimenting with the firing range that I've achieved a much wider colour range.

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2 hours ago, Min said:

Grog in a body can cause dimples like this, especially on flat pieces if it's a viscous glaze. Also, have you checked the specific gravity of the glaze recently? What can happen as a bucket of glaze gets used up the specific gravity increases as the water content in a glaze gets pulled into the bisque faster than the solids plus from just evaporation. A thick glaze layer is going to be more prone to dimples than a thinner layer. 

Thanks for the advice. The glazes I use are actually reactive glazes. What I do is pour off excess water and then add some back in depending on the desired consistency. This allows me to achieve a range of finishes using the same glaze.

However having said this, the manufacturer says to use 800ml per 1kg of glaze and then adjust as desired and I use 1litre per 1kg when mixing the glazes. With this, I don't expect that glaze thickness is the cause of the issue.

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not a chemist but just a word or two.  like the square plate and your use of the bubble glaze technique on only half of it.   are you getting the bisque wet before applying the glaze?   it might help to just quickly dip the entire piece into a bucket of clean water before you start glazing.  it will suck up the water and dry a little before you apply the glaze.    trying it won't hurt anything and may help.

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9 hours ago, oldlady said:

are you getting the bisque wet before applying the glaze?   it might help to just quickly dip the entire piece into a bucket of clean water before you start glazing.  it will suck up the water and dry a little before you apply the glaze.    trying it won't hurt anything and may help.

I haven't done this but have tried using a damp sponge to wipe down the bisque pieces before glazing. Unfortunately no joy with it though

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