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Is occasionally firing an empty kiln good practice?


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8 hours ago, Mark C. said:

Thick glaze application will make this worse. The glazes look thick as others have said-try less glaze-with that much color it may not look any different  color wise but not pit.

I can appreciate why you might think that the glaze application is thick based on the photo, but I actually use it thinner than recommended by the manufacturer. Any thinner than applied in the pictured piece and this glaze turns out an awful brown colour...

Funnily enough the manufacturer actually suggested trying a thicker application! Needless to say this didn't resolve the problem...

Here is a photo of the underside of the piece showing that the glaze isn't thick and standing out from the clay (hope that makes sense...)

161504342285820210306_150944.jpg

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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 g

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 cal

This is really starting to look like a glaze that does not heal well. Does this glaze ever fire glossy with a smooth melt? Does it ever move off its glaze line at the bottom of the wares? Is it suppos

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5 minutes ago, F Crow said:

Here is a photo of the underside of the piece showing that the glaze isn't thick and standing out from the clay (hope that makes sense...)

Are there more, less or the same amount of pinholes in this one?  There might be a clue or trend here. Just sayin, worth a look.

Edited by Bill Kielb
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3 minutes ago, Bill Kielb said:

Are there more, less or the same amount of pinholes in this one?  There might be a clue or trend here. Just sayin, worth a look.

Sorry I didn't explain, this is actually the same piece as the one pictured previously above. Here is the underside in the light showing the pitting issue again.

161504411649720210306_152131.jpg

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2 minutes ago, F Crow said:

showing the pitting issue again.

This is really starting to look like a glaze that does not heal well. Does this glaze ever fire glossy with a smooth melt? Does it ever move off its glaze line at the bottom of the wares? Is it supposed to have a matte surface and, likely unknown but a I gotta ask do we know the recipe? The more I see of this the more I am thinking you are going to find a drop and hold schedule that makes these work almost every time.

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12 minutes ago, Bill Kielb said:

This is really starting to look like a glaze that does not heal well. Does this glaze ever fire glossy with a smooth melt? Does it ever move off its glaze line at the bottom of the wares? Is it supposed to have a matte surface and, likely unknown but a I gotta ask do we know the recipe? The more I see of this the more I am thinking you are going to find a drop and hold schedule that makes these work almost every time.

Thanks again for your reply.

The glaze is meant to be glossy alright. I have had success with it in the past, but thinking of it now that was on smaller pieces with texture so that may have hidden the defect?

None of my glazes move beyond the glaze line (with the exception of one crystalline  glaze but I think that is to be somewhat expected) as I use them slightly thinner than recommended.

Unfortunately I don't know the composition of the glaze as it is commercially bought.

This isn't the only glaze I've been having this problem with however. I'll attach a photo here of another piece with a different glaze showing the same issue.

Yes from what I've read a drop and soak schedule does sound like a potential solution! I found a cone 6 schedule on digitalfire but it contained too many segments for my controller so I had to edit it. It did help somewhat but ultimately didn't eradicate the issue. Would you know of a drop and soak schedule that would be worth a try that contains no more than 4 segments, around cone 5 1/2 or 6 temp?

161504542632620210306_154327.jpg

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18 minutes ago, F Crow said:

Would you know of a drop and soak schedule that would be worth a try that contains no more than 4 segments, around cone 5 1/2 or 6 temp?

I do not but ......... if you go back to the last 100c of the firing, this is the most important part to get right to fire to maturity (a specific cone) From there you are really free to use up your segments within reason pretty much as you like. The initial segment is about drying, after that folks fire as fast as 250c to 300c an hour for fast glaze always ending in a controlled final segment to hit their cone. (60c per hour for you)

Most folks find the best healing by varying the drop temp and time at that temp assuming the glaze has high fired surface tension and at some drop point it decreases enough to begin flowing together. So sneaking up on it, some minimal drop and reasonable hold on a test tile.  ....... better or the same. If better then maybe increase the hold .......... etc... at some point they zero in on the drop temp that gives best results and vary the time at that temp.

I would also be curious enough to take 300g of the mixed glaze and add 1,2,3 percent silica only (in 100g batches)  just to see if this becomes much glossier. If it did I would would then have an idea or two to make this melt better. Besides, you might end up with a high gloss version of this when ya want it.

Edited by Bill Kielb
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a damp sponge does not reach every part of the pot.  just try dunking quickly once.      just for fun, let a flat piece of clay dry totally.   dip it quickly and wait a few minutes.   break it and notice how little the water penetrated.    that piece is not even bisque fired and it does not stay """WET'''''.   because it did not get wet.

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I'm seeing bisque ware/clay absorb water much more readily than raw dry clay. 

...haven't been paying close attention here; is OP single firing?

I'm seeing very dry raw clay pick up water fairly well - much faster than even slightly damp raw clay...

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1 hour ago, Hulk said:

...haven't been paying close attention here; is OP single firing?

@F Crow posted their bisque schedule so that would be a nope.

3 hours ago, F Crow said:

Would you know of a drop and soak schedule that would be worth a try that contains no more than 4 segments, around cone 5 1/2 or 6 temp?

There isn't going to be one drop and hold schedule that works for all  glazes. It depends on the fluidity of the glaze at the lower temperature. Somewhere to start would be

1) 60C / hour up to 120C   

2) 180C up to 1100C   

3) 60C up to 1200C  (when you get your cones this top temperature might have to be adjusted, you also have the option of having the top temperature go a little cooler (like 20C cooler) then add a 10 minute soak 

4) 9999C an hour drop to 1145C and hold for 20 minutes. (this last hold can be increased or decreased but start with 20 and see how the glaze does.)

Other thing I would do is try your problematic glaze(s) on a different claybody, preferably a smooth non-grogged white or porcelain body. Can you get a small amount of another clay to try this with? If not try screening the grog out of your current body (mix dry trimmings with water to make a slip), brush this on leatherhard and see what the glazes do over it.

 

 

 

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1 hour ago, Min said:

Somewhere to start would be

1) 60C / hour up to 120C   

2) 180C up to 1100C   

3) 60C up to 1200C  (when you get your cones this top temperature might have to be adjusted, you also have the option of having the top temperature go a little cooler (like 20C cooler) then add a 10 minute soak 

4) 9999C an hour drop to 1145C and hold for 20 minutes. (this last hold can be increased or decreased but start with 20 and see how the glaze does.)

Thanks for the suggestion however this is where my controller only allowing 4 segments becomes an issue. When inputting a schedule like this it would be (at least) 5 segments.

1-60oC/h to 120oC

2-180oC/h to 1100oC

3-60oC/h to 1200oC

4-9999oC/h to 1145oC

5-20min hold

This is why I've had problem coming up with a schedule as it seems I'd have to either go fast at first, to slow at top, drop, then soak, or else go slow at first, faster than desired at top, drop, then soak. Obviously neither option is great....

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3 hours ago, oldlady said:

a damp sponge does not reach every part of the pot.  just try dunking quickly once.      just for fun, let a flat piece of clay dry totally.   dip it quickly and wait a few minutes.   break it and notice how little the water penetrated.    that piece is not even bisque fired and it does not stay """WET'''''.   because it did not get wet.

Sorry I'm not sure I understand what the benefit of dipping the bisque in water would be? I have seen that wiping with a damp sponge will remove potential dust and help the glaze adhere to the bisque as water attracts water. Not sure what the advantage therefore would be in dipping it entirely in water? Could this not just waterlogged and saturate the ware making it difficult for it to receive an adequate glaze layer?

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1 hour ago, F Crow said:

Thanks for the suggestion however this is where my controller only allowing 4 segments becomes an issue. When inputting a schedule like this it would be (at least) 5 segments.

1-60oC/h to 120oC

2-180oC/h to 1100oC

3-60oC/h to 1200oC

4-9999oC/h to 1145oC

5-20min hold

This is why I've had problem coming up with a schedule as it seems I'd have to either go fast at first, to slow at top, drop, then soak, or else go slow at first, faster than desired at top, drop, then soak. Obviously neither option is great....

If the work is totally dry, you could do away with the first segment and jump right into the fast part of the firing. Here in the US, our controllers include a hold option at the end of each ramp segment, it's not a separate segment.

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a damp sponge does not reach every part of the pot.  just try dunking quickly once.      just for fun, let a flat piece of clay dry totally.   dip it quickly and wait a few minutes.   break it and notice how little the water penetrated.    that piece is not even bisque fired and it does not stay """WET'''''.   because it did not get wet.

i know that you think it would be pointless to try this but i am trying to say that we all have ideas that are firmly implanted and are not true.  you mentioning "waterlogged" shows that you are not familiar with your clay.

wetting totally dry clay before wetting with glaze allows the estremely dry clay to accept a layer of glaze without sucking it up so quickly that holes form .

Edited by oldlady
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4 hours ago, neilestrick said:

If the work is totally dry, you could do away with the first segment and jump right into the fast part of the firing. Here in the US, our controllers include a hold option at the end of each ramp segment, it's not a separate segment.

Thanks for the advice, definitely seems that what is available to me in Ireland has more limitations than in the US from reading online.

Would picking a mid speed segment going up initially be a good option maybe? Something like;

- 100oC/h to 1100oC

- 60oC/h to 1200oC

- drop to 1145oC

-20min hold

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2 hours ago, oldlady said:

a damp sponge does not reach every part of the pot.  just try dunking quickly once.      just for fun, let a flat piece of clay dry totally.   dip it quickly and wait a few minutes.   break it and notice how little the water penetrated.    that piece is not even bisque fired and it does not stay """WET'''''.   because it did not get wet.

i know that you think it would be pointless to try this but i am trying to say that we all have ideas that are firmly implanted and are not true.  you mentioning "waterlogged" shows that you are not familiar with your clay.

wetting totally dry clay before wetting with glaze allows the estremely dry clay to accept a layer of glaze without sucking it up so quickly that holes form .

Sorry if I came across as dismissive, I was just trying to understand the reasoning behind your advice. There is so much to learn in ceramics and I certainly admit to having big gaps in my knowledge. It's just not a suggestion I've never seen elsewhere.

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Whether or not the water dipping method works depends a lot on your pots and your glazes. I could never get an adequate layer of glaze on my pots if I dipped them in water first, because I throw fairly thin and I need a slightly thick glaze application. Dipping in water would saturate the pot way too much. I have experienced pinholes due to the glaze being too thick in the bucket, but thinning out the glaze solved the problem for me, and I just dip longer or dip twice to get the thicker application. That said, the problem you're having doesn't look like what I experienced, or would expect from an application issue.

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1 hour ago, F Crow said:

Thanks for the advice, definitely seems that what is available to me in Ireland has more limitations than in the US from reading online.

Would picking a mid speed segment going up initially be a good option maybe? Something like;

- 100oC/h to 1100oC

- 60oC/h to 1200oC

- drop to 1145oC

-20min hold

100C/hr to 1100C is pretty slow. Just wasting energy there. If the pieces are dry there shouldn't be a problem with going faster with most pieces. Maybe slower for big platters and such, but for your typical pots they should be fine.

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20 minutes ago, neilestrick said:

Whether or not the water dipping method works depends a lot on your pots and your glazes. I could never get an adequate layer of glaze on my pots if I dipped them in water first, because I throw fairly thin and I need a slightly thick glaze application. Dipping in water would saturate the pot way too much. I have experienced pinholes due to the glaze being too thick in the bucket, but thinning out the glaze solved the problem for me, and I just dip longer or dip twice to get the thicker application. That said, the problem you're having doesn't look like what I experienced, or would expect from an application issue.

It is a similar thought I was having when contemplating dipping the pieces in water. Among other things I make bowls using a press mold and they are very fine (maybe 3mm thick when fired), so even after dipping them in the glaze they can sometimes take a while to dry depending on the glaze consistency.

I too have it in mind as a firing issue which is why I started this thread with a kiln question. Have been trying to come up with potential causes for weeks now and tried so many things (even bought a dehumidifier for the studio!) and the only varient that seems to cause any difference is the firing schedule.

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26 minutes ago, neilestrick said:

100C/hr to 1100C is pretty slow. Just wasting energy there. If the pieces are dry there shouldn't be a problem with going faster with most pieces. Maybe slower for big platters and such, but for your typical pots they should be fine.

Thanks again for the advice, it is very appreciated! Wish I had learned these things when studying ceramics, but from reading around online it seems tutors skipping over theory about firings etc isn't uncommon practice. Try to learn as much as possible reading various sources but there are so many variables in the world of ceramics it's hard to weed out what is relevent/reliable and what is not.

All my work is hand built, either slabs or press molds, and never thicker than 6mm (or approximately 1/4inch) and I leave ware at least a day after glazing to dry out before packing into the kiln, so hopefully a faster first segment shouldn't cause any harm.

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3 minutes ago, F Crow said:

All my work is hand built, either slabs or press molds, and never thicker than 6mm (or approximately 1/4inch) and I leave ware at least a day after glazing to dry out before packing into the kiln, so hopefully a faster first segment shouldn't cause any harm.

It should not, especially if dry.

I am not sure what you have read about speeds but one of the things I have noticed over the years is in general,  firing speeds are not taught anywhere with any consistency. Not sure why as In industry making tiles one can go from room temperature to cone 10 to room temperature in hours.

200 c per hour generally is medium in a glaze fire. 300c is fast but generally fine for most normal wares. Many folks glaze fire in four to  eight  hours successfully and regularly. Their glazes and their ware construction are the final say on speed.

I have read many texts emphasizing quartz inversion  etc.... for the most part clay goes through quartz inversion each and every time up and down. Clay is tough it can take it, especially something that has been bisqued. For bisque firings time and time at a given temperature become very important.

In a glaze firing, until the fluxes begin dissolving the silica and alumina pretty much nothing has been done. All that work is sort of reversible regardless of how fast or slow it went. That last 100c to say 150c is super important though.

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11 hours ago, F Crow said:

available to me in Ireland has more limitations than in the US from reading online

@F Crow Out of interest , what controller do you have?  My Stafford controller also has the hold as part of the segment.

Edited by Chilly
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15 hours ago, Bill Kielb said:

It should not, especially if dry.

I am not sure what you have read about speeds but one of the things I have noticed over the years is in general,  firing speeds are not taught anywhere with any consistency. Not sure why as In industry making tiles one can go from room temperature to cone 10 to room temperature in hours.

200 c per hour generally is medium in a glaze fire. 300c is fast but generally fine for most normal wares. Many folks glaze fire in four to  eight  hours successfully and regularly. Their glazes and their ware construction are the final say on speed.

I have read many texts emphasizing quartz inversion  etc.... for the most part clay goes through quartz inversion each and every time up and down. Clay is tough it can take it, especially something that has been bisqued. For bisque firings time and time at a given temperature become very important.

In a glaze firing, until the fluxes begin dissolving the silica and alumina pretty much nothing has been done. All that work is sort of reversible regardless of how fast or slow it went. That last 100c to say 150c is super important though.

Thank you so much for the informative response. Definitely seems to be more variety in approaches to firings out there than inconsistencies. Makes it a hard field to navigate and figure out!

The manufacturers actually ran a test of their own with one of the troublesome glazes using a firing schedule of;

-500oC/h to 1030oC

-75oC/h to 1200oC

-15min soak 

And they claim to have had no issue... However they didn't recommend going as fast as they did in the initial segment. So I will definitely give a go with a faster first segment leading into a slower rate at the end of 60oC/h for the last 2 hours.

Putting a bisque on today so will be a few days before I get to try it out.

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7 hours ago, Chilly said:

@F Crow Out of interest , what controller do you have?  My Stafford controller also has the hold as part of the segment.

I have a Nabertherm top 80 kiln with a Nabertherm B400 controller. Wish I could upgrade to the C440 controller as this allows up to 10 saved programmes with up to 20 segments in each, but it doesn't seem that it can be purchased without buying a kiln at the same time...

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1 hour ago, F Crow said:

I have a Nabertherm top 80 kiln with a Nabertherm B400 controller. Wish I could upgrade to the C440 controller as this allows up to 10 saved programmes with up to 20 segments in each, but it doesn't seem that it can be purchased without buying a kiln at the same time...

Silly that they won't sell it. What voltage does your controller run on, and what are the dimensions? You may be able to swap out for a different brand without much modification. If you can't get one the same size, it's easier to go to a smaller controller, by making a sheet metal panel to make it fit.

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