Pottery Cracking In Glaze Firing

[Cynthia1]

Hi everyone!

I found this forum by accident, and really need your help because my plates have been cracked another time during glaze fire.  I guess they probably broke in the cooling process, which I let the kiln shut down after 1220 degrees c. What is a relatively gentle speed to cool down at 1220-1000 degrees?

Please feel free to leave comments or suggestions on how should I avoid another nightmare next time. Thank you soooo much!

[Cynthia1]

Some more photos. Heart broken.

[Bill Kielb]

It can be extremely cold on Mars (depending on location)  so when or what temperature did you open the kiln? For electric kilns generally everything is kept closed till it has cooled on its own to 100 - 300 f degrees or so.

These also look like fairly large platters so were they fired on a waster slab or at least grog or alumina  placed under them to make sure they shrink and grow easily without catching on the shelf?

[Babs]

What kind of kiln do you fire?

My gas fibre kiln dropped temp. like a rock off a cliff.

All bungs in?

Left to drop to ambient or just above before opening?

Unfooted pots?

In my brick kiln  takes a good  24hrs before I can open after shut down.

All plate load?

All plates cracked?

Need a buffer between shelf and pot imo.

The drop to 1000 deg not important imo it is the lower end  where damage is done if bungs lids etc are fiddled with

 

 

[Cynthia1]

12 hours ago, Bill Kielb said:

It can be extremely cold on Mars (depending on location)  so when or what temperature did you open the kiln? For electric kilns generally everything is kept closed till it has cooled on its own to 100 - 300 f degrees or so.

These also look like fairly large platters so were they fired on a waster slab or at least grog or alumina  placed under them to make sure they shrink and grow easily without catching on the shelf?

Hahahah I fired them in an electric kiln and opened a small gap at around 200 c degrees, then fully opened at 97c degrees.

You are right. I will try to put some alumina under a plate next time, and be more patient until the kiln is cooled enough.

[Cynthia1]

10 hours ago, Babs said:

What kind of kiln do you fire?

My gas fibre kiln dropped temp. like a rock off a cliff.

All bungs in?

Left to drop to ambient or just above before opening?

Unfooted pots?

In my brick kiln  takes a good  24hrs before I can open after shut down.

All plate load?

All plates cracked?

Need a buffer between shelf and pot imo.

The drop to 1000 deg not important imo it is the lower end  where damage is done if bungs lids etc are fiddled with

 

 

I used an electric kiln, which doesn't have a bung, but a lid on top.

I opened the top lid at around 250 c degrees, then a small gap of the opening at 200 c degrees. I fully opened the kiln at 97c degrees. Maybe I was a bit impatient?

Yes, there is no foot ring for my plates. I made them with a clay slab and bent the edge to form a plate.

It took me even longer time, around 35 hours before I opened after shut down.

All plates loaded. 5 in total, and all cracked unfortunately.

I will have a buffer like alumina next time and see it if helps! I put plates directly on stone board before. Maybe that's the problem.

Wow that is something new to me. I always assume if a crack happens during cooling process, it will do at a higher temperature for its fast drop. I know higher temperature is, faster it drops. So what is the most possible temperature range to cause these cracks, around 500c?

[Babs]

That is pretty drastic dunting. 

Your plates are pretty thin. 

Think you would be better sitting them on  biscuits which can be used again . So every shelf had dunted stuff?

That is cruel.

[Cynthia1]

9 minutes ago, Babs said:

That is pretty drastic dunting. 

Your plates are pretty thin. 

Think you would be better sitting them on  biscuits which can be used again . So every shelf had dunted stuff?

That is cruel.

Too cruel yes.

I will try to add alumina between a plate and stone board next time! See if it can avoid cracks. Thanks a lot!

[Babs]

I think two crucial stages but there are different clays which are more tolerant.

573degC and can't remember lower one , just over 200 I think. Really important on the way down so cracking the kiln then may be your prob

[Babs]

What is stone board?

Roll out clay size of plate, dry and place under your plate for firing. The top surface of your plate when it sits on the shelf with no foot will be cooling more rapidly that the bottom.

How close to the elements are your plates?

[Callie Beller Diesel]

@Cynthia1 You can actually crash cool at the top end, although it’s not advisable for a lot of very good personal safety reasons. You want to be careful and drop slowly around quartz inversion (around 550 C) and crystobalite inversion, which happens at a broader temperature range, between 220* and 280* C.  Not every clay has issues with crystobalite inversion, but if it has a lot of free silica, it can. I’d venture from these pictures, your clay is in that category.

If your pyrometer is reading 200 C, that’s just from the atmosphere in the kiln. Your kiln furniture is still at a hotter temperature, and because your plates are in contact with a lot of shelf surface area, they probably are too. 

Large flat platters with no foot ring are notorious for needing to be babied. The alumina/grog/sand/waster slab idea is a good one, and I would maybe not crack the kiln until it’s under 200 C. Under 100 would be better, but we all know what the waiting is like! Another thing that can help is to place other items around the rim of the platter, like small cups, test tiles or even overturned kiln posts. They’ll act as more thermal mass, and let the platter cool and heat more evenly. 

Another possibility is that if your pieces are only glazed on the inside and your glaze is one that is well fitted to your clay, that can contribute to pulling apart pieces like this. It looks like it’s nice and smooth and uncrazed, and like it would put tension on the piece if it was glazed inside and out, exactly like you’d want for good functional ware. If you’ve tested this glaze on cup forms and it resists crazing even under an ice bath test, your glaze is part of the problem with your plates. If you don’t have a cup, you could take one of those shards and shock test it to confirm. If the glaze is contributing to the cracking, you could try glazing the rim and the outside, or we might have to look at the chemistry.

 

[Cynthia1]

23 hours ago, Babs said:

What is stone board?

Roll out clay size of plate, dry and place under your plate for firing. The top surface of your plate when it sits on the shelf with no foot will be cooling more rapidly that the bottom.

How close to the elements are your plates?

Stone board is just the shelf in kiln. I call it in this way by its material.

No I didn't have a clay slab under the plate. I'll try to use alumina instead. Thanks for your advice!

[Cynthia1]

17 hours ago, Callie Beller Diesel said:

@Cynthia1 You can actually crash cool at the top end, although it’s not advisable for a lot of very good personal safety reasons. You want to be careful and drop slowly around quartz inversion (around 550 C) and crystobalite inversion, which happens at a broader temperature range, between 220* and 280* C.  Not every clay has issues with crystobalite inversion, but if it has a lot of free silica, it can. I’d venture from these pictures, your clay is in that category.

If your pyrometer is reading 200 C, that’s just from the atmosphere in the kiln. Your kiln furniture is still at a hotter temperature, and because your plates are in contact with a lot of shelf surface area, they probably are too. 

Large flat platters with no foot ring are notorious for needing to be babied. The alumina/grog/sand/waster slab idea is a good one, and I would maybe not crack the kiln until it’s under 200 C. Under 100 would be better, but we all know what the waiting is like! Another thing that can help is to place other items around the rim of the platter, like small cups, test tiles or even overturned kiln posts. They’ll act as more thermal mass, and let the platter cool and heat more evenly. 

Another possibility is that if your pieces are only glazed on the inside and your glaze is one that is well fitted to your clay, that can contribute to pulling apart pieces like this. It looks like it’s nice and smooth and uncrazed, and like it would put tension on the piece if it was glazed inside and out, exactly like you’d want for good functional ware. If you’ve tested this glaze on cup forms and it resists crazing even under an ice bath test, your glaze is part of the problem with your plates. If you don’t have a cup, you could take one of those shards and shock test it to confirm. If the glaze is contributing to the cracking, you could try glazing the rim and the outside, or we might have to look at the chemistry.

 

Ah ha,  you are right. I will pay more attention to let it cool more slowly! I have used this clay for some time, there is no problem working with thicker pieces, maybe it is because my plates are too thin. They are just 3-4 mm thick.

I will also try to put small pieces around a plate!

Yes, I only glazed inside of plates. However it's kind of weird to me. Because in a previous glaze fire, all small plates cracked but a bigger one survived. I used the same clay and glaze as this time. So it is still mysterious to me. Eager to find out why.

I'm a bit confused with the ice bath test you mentioned. I've never done this before, but you said if a cup with inside and out glaze resists crazing under an ice bath test, the glaze is a problem. Is it? Because I thought the opposite way. Isn't no crazing good instead?

[Bill Kielb]

On 1/18/2022 at 6:27 AM, Cynthia1 said:

I opened the top lid at around 250 c degrees, then a small gap of the opening at 200 c degrees. I fully opened the kiln at 97c

Maybe first and foremost, try not to open the kiln till 100 c or so. 250c is a bit hot.

[Cynthia1]

40 minutes ago, Bill Kielb said:

Maybe first and foremost, try not to open the kiln till 100 c or so. 250c is a bit hot.

I will be more patient this time! Hopefully everything will be fine. 

[Pres]

On 12/23/2019 at 12:02 PM, Bill Kielb said:

Popular methods include grog smooth and evenly under the pot or alumina neatly under the pot. 

One easy trick for evenly spreading grog is a common salt shaker. I used to keep one handy near the kiln at the HS when ever firing pieces that were flat. We did a lot of slab construction, and it worked great.

 

best,

Pres

[Callie Beller Diesel]

3 hours ago, Cynthia1 said:

I'm a bit confused with the ice bath test you mentioned. I've never done this before, but you said if a cup with inside and out glaze resists crazing under an ice bath test, the glaze is a problem. Is it? Because I thought the opposite way. Isn't no crazing good instead?

You are correct, crazing isn’t desirable in most food ware you want to last.  I worded that a bit funny. Let me explain.

A well fitted glaze actually puts the clay underneath under a bit of tension. The coefficients of expansion of the clay and glaze aren’t identical, and the glaze is a bit smaller, but not too much smaller. When a piece is glazed all over with this glaze, that evenly distributed tension makes the whole unit a lot stronger than it would be otherwise. 

If that same tension is distributed unevenly somehow, it winds up giving the piece stress points where it’s more likely to break. So if the piece is only glazed on one side, or glazed with a fitted glaze on one side and an unfitted glaze on the other, or if there are 90* corners, or the piece is too thin, the tension of the fitted glaze can inadvertently weaken the piece. Curved surfaces will distribute the tension more evenly, and slightly thicker clay will absorb the stresses, which is why it can cause problems with some pieces but not others. Yes, you seem to have the perfect storm of things going wrong with these plates!

If you want to confirm if your glaze is a nicely fitting one, you have to put it under some extra stress to bring out any delayed crazing. It’s a good idea to do for any glaze you want to go on kitchenware, imo. 

You add this stress via thermal shock tests. The mildest version is to put a piece in the freezer overnight, and pour boiling water over it in the sink the next day. Safety glasses are a good idea, just in case. If your glaze is a light coloured matte or semi-matte, you might have to colour a section with a marker and wipe away to find any craze lines.

The boiling water thing is only a temperature difference of about 100-120*C though, depending on your altitude and freezer settings. If you really want to see how well your piece holds up, you can increase the temperature variance by going from hot to cold. Just heat the piece in the oven for an hour at your desired temperature, and dunk the piece in ice water. You can start at a 200* oven and keep going up until it fails.

Ideally you use some thin-ish cups with flat bottoms for this test, but if you happen to have some flat shards handy, they’ll work for initial information. If those shards do show crazing, the glaze tension probably isn’t an issue, and you don’t have to continue down that line of thought. If there’s no crazing, the good news is you probably have a good liner glaze for your clay, but you probably have to adjust your design.