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Hi,

Hope this hasn't been asked anywhere else and I'm repeating a question but I've scoured the internet and can't find much of help...

Even though I have studied ceramics at 2 levels, during neither course were we taught about firing schedules... Over the past few years I've been using and tweaking schedules given to me by an amateur potter friend but they aren't cutting it anymore. I've recently been approached by 2 shops/galleries who want to sell my work but glaze faults are making it difficult for me to be happy with results as I can't seem to avoid miniature pinholing and am wondering if the firing schedule is the problem.

I have a nabertherm top loader, and the control panel allows 5 time segments. 1. Delay 2. Ramp One 3. Ramp Two 4. Soak 5. Cooling. 

Currently I fire 80oc an hour for 10 hours, 100oC for 2 hours (reaching 1000oC), 15minute soak, and finish for bisque. For glost I use 150oC an hour for 5 hours, 100oC an hour for 4.5 hours (reaching 1200oC), 15min soak, and off. 

From reading around online I now wonder if these schedules may be too fast? Or may it be something as simple as needing elements replaced? (Although an electrician has tested them and said they're in good working order). Any advice or suggested schedules would be greatly appreciated. Thank you.

P.S. I have tried multiple clay bodies so don't believe that clay is an issue. I also use an extraction fan so I don't think it is a buildup of gases within the kiln chamber either. The glazes I use are commercial and mixed by the manufacturers instructions including sieving before use.

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You should definitely have a look at schedules, there are many comments on this forum as well as common schedules explained.  so a search on  this forum should reveal many past Insightful discussions. Interestingly teaching basic firing schedules seems to be something many teachers avoid, not sure why. .

A few  questions strike me, do you use firing cones to verify your schedule has ended with the intended heatwork? What cone are you trying to fire to for your clay and glaze?  What cone is your clay and glaze rated to? You have 15 minute soaks in your schedules, why?

Pinholes can be caused by many things and even cured  in some cases for difficult glazes by a drop and hold firing schedule. My feeling though is before we go further  it might be best to understand what you understand or believe.

lots of schedules available in equipment manuals btw. One that comes to mind would be the genesis manual by Bartlet controls  which is free to download from their website. I would say all of these are based on cone theory though so understanding the importance of cones is almost mandatory.

Edited by Bill Kielb

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Like Bill said, you need to verify the heat work you're getting by using cones. Heatwork is important, not temperature. Heatwork is temperature over time. The only thing that really stands out to me is the final ramp of your firing schedule, the 100C/hr. Most firing schedules we recommend use 60C/hr for the last 100C degrees. That slower ramp may help with the issues you're having.

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On 7/31/2020 at 6:22 AM, Bill Kielb said:

You should definitely have a look at schedules, there are many comments on this forum as well as common schedules explained.  so a search on  this forum should reveal many past Insightful discussions. Interestingly teaching basic firing schedules seems to be something many teachers avoid, not sure why. .

A few  questions strike me, do you use firing cones to verify your schedule has ended with the intended heatwork? What cone are you trying to fire to for your clay and glaze?  What cone is your clay and glaze rated to? You have 15 minute soaks in your schedules, why?

Pinholes can be caused by many things and even cured  in some cases for difficult glazes by a drop and hold firing schedule. My feeling though is before we go further  it might be best to understand what you understand or believe.

lots of schedules available in equipment manuals btw. One that comes to mind would be the genesis manual by Bartlet controls  which is free to download from their website. I would say all of these are based on cone theory though so understanding the importance of cones is almost mandatory.

Hi Bill,

Thank you so much for your informative response, I really appreciate it. Admittedly I am not great at online research so will give another look through forums here to see if I come across more information that is helpful. 

I have never actually worked in cones, just in manually setting programmes in degrees Celsius. Looking at a cone table I would imagine cone 5 is the closest to what I aim for. Must do some research into them and will contact my pottery suppliers on Monday to order some cones to use in a firing.

The glazes I use fire between 1180oC and 1250oC and the clay bodies I use all fall within that range too. That is why I fire to 1200oC as a middle temperature within that range, thinking it would be a good temperature to go for. I actually don't know why the soak is in my schedules other than the fact that my friend said to have one to be honest... Would you advise to get rid of it altogether?

 

I will definitely look up the genesis manual by Bartlet controls and see what it has to offer. 

Thanks again for the help!

Edited by F Crowley

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10 hours ago, neilestrick said:

Like Bill said, you need to verify the heat work you're getting by using cones. Heatwork is important, not temperature. Heatwork is temperature over time. The only thing that really stands out to me is the final ramp of your firing schedule, the 100C/hr. Most firing schedules we recommend use 60C/hr for the last 100C degrees. That slower ramp may help with the issues you're having.

Hi Neil,

Thank you so much for your response!

My kiln only allows for two ramps. Would you suggest keeping the ramp at 150oC per hour until the last 100oC and then slowing the last hour to 60oC per hour? Or would this make the firing too fast?

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1 hour ago, F Crowley said:

The glazes I use fire between 1800oC and 1250oC and the clay bodies I use all fall within that range too. That is why I fire to 1200oC as a middle temperature within that range, thinking it would be a good temperature to go for.

Are you making functional pots? Reason I ask is that by using a broad firing range claybody and underfiring it by 50C it could remain fairly porous. For non utilitarian ware this isn't an issue but for functional pots it can be. Moisture and oils etc can enter the claybody which in turn cause issues for the clay and glaze. It seems to be a common topic on this forum that potters in the UK have a far broader number of wide firing bodies available to them than ones that mature at midrange. I've noticed some of the earthenwares can go to cone 6 in the UK whereas over here earthenware is generally below cone 1 or thereabouts.

If you are making functional pots then I'ld suggest doing some absorption tests on your claybody to save some aggravation down the road. If you are not familar with how to do this here's a link explaining it. (about 1/2 way down the page)

There can be issues with using immature glazes also.

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It's easy to stay focused on firing schedules when that's the title. So we don't get caught up in that phenomenon.....

I'm reading "aren't cutting it anymore" as an element problem, of course, cones will quickly verify that.

If it remains unelemental, I would try adjusting or changing glazes before schedule.

What type of glazes? Commercial? (Bin them!) I found home mixed glazes to have a much larger success window than the frigging quarter cone window you get with Amost Every commercial glaze.

You can fire a whole kiln full of different glazes mixed to match the schedule.

You can't fire different schedules in one kiln load.

So you will always have more flexibility matching glazes to one schedule.

Of course, if you use just one glaze, or can fill a kiln reasonably with one glaze and fire one schedule, then fill a whole other kiln with a different glaze and fire a different schedule, adjust the schedule.

 

Sorce

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10 hours ago, Min said:

Are you making functional pots? Reason I ask is that by using a broad firing range claybody and underfiring it by 50C it could remain fairly porous. For non utilitarian ware this isn't an issue but for functional pots it can be. Moisture and oils etc can enter the claybody which in turn cause issues for the clay and glaze. It seems to be a common topic on this forum that potters in the UK have a far broader number of wide firing bodies available to them than ones that mature at midrange. I've noticed some of the earthenwares can go to cone 6 in the UK whereas over here earthenware is generally below cone 1 or thereabouts.

If you are making functional pots then I'ld suggest doing some absorption tests on your claybody to save some aggravation down the road. If you are not familar with how to do this here's a link explaining it. (about 1/2 way down the page)

There can be issues with using immature glazes also.

Hi Min,

Thanks for your reply.

I hadnt actually considered this at all. I do both functional and non functional work, so will definitely carry out an absorption test with the clay bodies. I had assumed once I was within their firing range it would be safe and gave it no more thought until now.

Can I ask what you mean by immature glazes?

Thanks

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23 minutes ago, Sorcery said:

It's easy to stay focused on firing schedules when that's the title. So we don't get caught up in that phenomenon.....

I'm reading "aren't cutting it anymore" as an element problem, of course, cones will quickly verify that.

If it remains unelemental, I would try adjusting or changing glazes before schedule.

What type of glazes? Commercial? (Bin them!) I found home mixed glazes to have a much larger success window than the frigging quarter cone window you get with Amost Every commercial glaze.

You can fire a whole kiln full of different glazes mixed to match the schedule.

You can't fire different schedules in one kiln load.

So you will always have more flexibility matching glazes to one schedule.

Of course, if you use just one glaze, or can fill a kiln reasonably with one glaze and fire one schedule, then fill a whole other kiln with a different glaze and fire a different schedule, adjust the schedule.

 

Sorce

Hi Sorce,

Thanks for your reply.

I had also assumed elements so was surprised when the electrician said they were in good order... I had even bought replacements ready to have them changed out but he said to hold off! Think I will get back in touch and have new ones fitted anyway though.

I use Terracolour High Fire Powder Glazes. Knowing nothing about the chemistry behind glaze making etc I am very hesitant about making my own... All the glazes I use have the same range however so it isn't an issue with picking a temperature to fire to.

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11 minutes ago, F Crowley said:

electrician

Oh, I forgot to mention....

Electricians will read "working" as an electrician, and be correct.

A kiln technician will have the different individual ohm ratings each should read, verify it with the manufacturer, and know they aren't up to par.

 

Sorce

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@F Crowley Until you get the cones dialed in I wouldn't worry about the elements. If it's getting to temp and doing it on schedule, then they're probably working fine. Clay and glazes need to be fired to full maturity, so work on dialing in the accuracy of your firings. If your elements look good- the coils are standing up straight and even- then they're likely still good.

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2 hours ago, F Crowley said:

I had assumed once I was within their firing range it would be safe and gave it no more thought until now.

This is why cones are used.  If you look at a cone chart, you'll see final temperature for a given cone number can vary as much as 60* C from lowest to highest, depending on how quickly that final temperature is reached.  The  faster the temperature climbs, the higher it has to go to produce the same amount of 'heat-work'.

For example:  1200*C could be anywhere from a Cone 5 to Cone 7, depending on how fast it climbs the last 100*C  - and the range of 1180* - 1250* could be anywhere from ^4 up to ^10.

 

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2 hours ago, F Crowley said:

Can I ask what you mean by immature glazes?

Glazes melt over a range of temperatures, some of the materials will start melting before others then as the heat/time increase other materials within the glaze get pulled into the melt. What you want is a fully melted glaze matrix so the glaze is both durable and non leaching (for functional wares). Think of glaze maturity like making caramel on the stove, you start off with butter, sugar and cream and turn on the heat. The cream is already fluid, next the butter melts and lastly the sugar dissolves. If you stop the process part way the sugar crystals won't be melted and won't be incorporated within the caramel. There will be a range where the glaze is fully melted but not over fired (which can have other issues). 

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3 hours ago, Min said:

Think of glaze maturity like making caramel on the stove, you start off with butter, sugar and cream and turn on the heat. The cream is already fluid, next the butter melts and lastly the sugar dissolves. If you stop the process part way the sugar crystals won't be melted and won't be incorporated within the caramel. 

Oh, yes, I love this description. Much better than my baking a fruit cake. 

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10 hours ago, Sorcery said:

A kiln technician will have the different individual ohm ratings each should read, verify it with the manufacturer, and know they aren't up to par.

Thanks again for your response.

I did reach out to the kiln manufacturer and they explained what the electrician was to test for. When I passed on the results of his tests they said it was all as it should be thankfully. Unfortunately I live remotely and having enquired about getting a kiln technician come to visit I found out the cost would be €200 just to get one here before any work is done! Hoping because of this that I will be able to resolve the issue with a better firing schedule instead of having to do this...

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8 hours ago, Rockhopper said:

This is why cones are used.  If you look at a cone chart, you'll see final temperature for a given cone number can vary as much as 60* C from lowest to highest, depending on how quickly that final temperature is reached.  The  faster the temperature climbs, the higher it has to go to produce the same amount of 'heat-work'.

For example:  1200*C could be anywhere from a Cone 5 to Cone 7, depending on how fast it climbs the last 100*C  - and the range of 1180* - 1250* could be anywhere from ^4 up to ^10.

 

Thank you so much for your reply. It definitely made things a bit clearer for me as I have no idea about cones... Had always assumed there was just a temperature they were to reach and that was that, I hadn't realised the ramp speed changes the cone! Will definitely acquire some cones and do tests!

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8 hours ago, Min said:

Glazes melt over a range of temperatures, some of the materials will start melting before others then as the heat/time increase other materials within the glaze get pulled into the melt. What you want is a fully melted glaze matrix so the glaze is both durable and non leaching (for functional wares). Think of glaze maturity like making caramel on the stove, you start off with butter, sugar and cream and turn on the heat. The cream is already fluid, next the butter melts and lastly the sugar dissolves. If you stop the process part way the sugar crystals won't be melted and won't be incorporated within the caramel. There will be a range where the glaze is fully melted but not over fired (which can have other issues). 

Thank you so much for this reply, it really makes such easy understanding of what you are talking about. Had just assumed that once I was firing within the glaze range stated by the manufacturer that it would be alright but may this not be the case?

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Thank you all for your replies thus far.

It is becoming more and more clear to me as this topic develops that I didn't know how much I didn't know about firing!!! 

Does anyone know of any good theory based book out there that I could get which would help me gain a better understanding of the firing process as a whole?

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3 hours ago, F Crowley said:

Does anyone know of any good theory based book out there that I could get which would help me gain a better understanding of the firing process as a whole?

My list would be very short as in I would suggest ceramic materials workshop by Matt Katz.  At least the glaze course but the beginner clay is great as well for a simple but technical - if ya want it, explanation. When folks ask me what first, I usually tell them this as then you will be able to weed out a bunch of let’s say misleading info that has been printed over the years. Just my opinion, but in the end saves money and lots of confusion.

https://www.ceramicmaterialsworkshop.com/

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Tony Hansen's website is a trove - recipes, articles, tests, glossary... https://digitalfire.com/

Local JC instructor lent me Susan Peterson's The Art and Craft of Clay (I have my own copy now!) - lots of info, and pics too; I particularly appreciate her explanation of the Unity formula.

 

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