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Nir Dvash

Porcelain plates breaking on glaze firing

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Hello All!

I wonder if someone here has the knowledge to solve a very annoying problem I have been facing lately:

I create Porcelain plates, about 9" in diameter. The wight of each plate after glaze firing is about 200 to 250 grams which makes them pretty thin.

When bisque firing them, all is well - Always.

When glaze firing them (1230 C) , many many of the plates are broken WHEN TOUCHED, that means that when I open the kiln, they all look fine but when I come to pick them up, they break up in my hands. Those that do make it in one piece, have a hollow sound when knocked and break soon after. 

A few facts to add to the mystery:

1. I do not use a slab roller.

2. Other plates that have a wavy edge style  also made from the same Porcelain and with the same method do not break at all IN THE SAME FIRING,

3. Some plates do come out OK.

4. Am using an electric Kiln.

5. The plates are only glazed on their top side.

6. Some plates have a supporting ring element (made also from Porcelain) underneath their circumference to keep their shape (raised edges) and some don't. They all break.

7. The plates are bone dry when entering the glaze firing - the glaze is air dried for a few hours before I fire up the kiln.

Any help on this issue will be greatly appreciated!

Thank you,

Diana.

 

 

 

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If you only glaze one side, and use a tight glaze, it will pull the form in the direction of the glaze. If the same glaze also crazes, it will pull in that direction and cause cracks to migrate into the clay.  Try glazing both sides or firing one without glaze to see how it does

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@Nir Dvash, would you elaborate on this part of what you wrote: "Those that do make it in one piece, have a hollow sound when knocked..."  Sounds like something is off here, porcelain should "ring" when tapped when it's fired to maturity, not sound hollow. Could you tell us more about the clay you are using, specifically it's firing range.  If it's a "thunk" when you tap then there's probably a crack in it or it's underfired or if it's vastly over fired it can thunk also.

Welcome to the forum.

Edited by Min

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Thin porcelain pieces can be ripped apart by the glaze when only one side is glazed. Are you using alumina wax on the supporting ring to keep it from sticking to the plate? If not, it could be that they're binding up and causing the cracking. 

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@MinThank you. Please find attached a couple of pictures of the Porcelain i'm using.

@neilestrickThe supporting ring is not glazed so it does not stick to the base of the plates which are also not glazed. Unsupported plates also crack.

@liambesaw Thank you.  I assume you are right. I can not glaze both sides since the bottom side of the plate is flat with no base/legs.  I read about CTE which I think is what you are talking about. and I understand I somehow need to match the CTE of the glaze to the CTE of my Porcelain. The glaze does not craze -  it comes out perfect (on a  broken plate ;)).  In the beginning we use to fire to a higher temperature (1250) which produced good plates but some of them use to flatten out completely so we (I assume wrongly) lowered the firing temperature thinking this is the cause and now we broken plates - Please see attached image for an example.

All - I appreciate your help !!

Diana.

 

 

 

 

IMG-2639.JPG

IMG-2640.JPG

IMG-2642.JPG

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I think you have a couple things going on. When I looked up your claybody I found it's rated at 1250 - 1280C, so in the cone 8-10 range. By firing it only to 1230C you are firing to approx cone 6 1/2 so the body won't be mature which would account for the hollow sound when the pieces are tapped.

Also, how fast does your kiln cool? Looks like a cooling dunt, perhaps exacerbated by an ill fitting glaze. If there is no crazing then the CTE (or COE) of the glaze might be too low for the clay, combine this with either a thick glaze layer or only glazing one side of the plates and this alone would account for the breaking. If you add in a fast cooling kiln that would add another layer of stress on the plates. If the glaze is sharp on the edges of the cracks then they broke on the cool down part of the firing.

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@MinThank you for the extra info. I will  use a thinner glaze, fire higher and let the kiln cool down slower. Can you recommend a firing  scheme? will something like this work:

1.  0 --> 200c in 2 hours.

2. 200 c- ->1260c in 5 hours

3. Soaking at 1260? for how long?

4. Off.

 

 

 

 

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1 hour ago, Nir Dvash said:

@MinThank you for the extra info. I will  use a thinner glaze, fire higher and let the kiln cool down slower. Can you recommend a firing  scheme? will something like this work:

1.  0 --> 200c in 2 hours.

2. 200 c- ->1260c in 5 hours

3. Soaking at 1260? for how long?

4. Off.

 

 

 

 

I don't know if that would work or not. There are several moving parts to this.

If it was me working with this clay and glaze I would start off by sorting out what's the best temperature/cone to fire it to to and determining if the glaze actually fits before trying out some more plates. I would suggest glazing the inside only of a thinly made cylinder, apply the glaze thickly to the inside but no glaze on the outside. What you are trying to simulate is a worst case scenario with thick glaze on the inside and none on the outside. After firing if the cylinder is split then I would work on getting a higher CTE glaze to fit. If the cylinder survives the firing then put it in the freezer overnight then in the morning put it in the sink and carefully fill it with boiling water. If it survives this then try it on just one plate and don't put it on too thickly. At the same time I would be testing the absorbency of the clay. Just fire an unglazed test piece along with your glaze firing, making note of the final temp / cone. Do a boil test and re-weigh the sample to see what the absorbency is. If it's over about 2% then I would fire hotter if you need vitrified pots.

If the glaze smooths out without pinholes or blisters and your kiln fires evenly then you might not need a hold, just take it to the cone you are aiming for. As the kiln cools you might need to add a ramp from 600C down to 525C at a rate of 40C / hr to get through the quartz inversion safely. If the glaze doesn't fit this won't help but if your cracking is from cooling dunts this would help. Also, don't open the kiln above 200C. I would also suggest you slow the firing down at the top end, something like 40C per hour for the last 100C. If you can put the larger plates in the middle of the kiln so the shelves above help create a heatsink.

What Neil said about alumina wax is really good advice. It's common practice on this side of the world to use alumina wax with porcelain because of its inherent nature of sticking and plucking. It's used that between any parts of the unglazed pots that are in contact with either the shelves or lids on pots etc. (1 cup of cold wax resist to a couple teaspoons alumina hydrate, stir it up often as it settles quickly)

 

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Agree with Min and Neil.  That kind of cracking (right across the middle of a wide flattish form from side to side) is the classic glaze-on-one-side-only problem having to do with the glaze having a higher CTE than the clay and “pulling” the clay on the glazed side putting the clay body under stress until it gives up and cracks to relieve the stress.  The thinner the clay object and the thicker the glaze layer the more likely it is to happen.

 

 

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Hello All,

Wanted to update you that my current fire was much much better. Out of 23 plates, I had only 1 broken! The kiln was fired to 1250 C , I used a  thinner glaze and also let the kiln cool down very very slow.  The only problem I am still facing is that some of the plates flatten out and some do not (I fired this kiln without any plates being supported from underneath).

I would love to hear your opinion on this issue.

Thank you,

Diana

 

 

 

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Glad to hear you are having some success!

do you want the plates to flatten out or not?  Not quite clear if that is a good thing or a bad thing....

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Porcelain becomes pyroplastic (“melty and saggy”) at high temps.  Nature of the beast. The more cantilevered a form is the more likely it is to sag.  You can try firing a bit lower - say 10 or 15 degrees (but your glaze may not like this)  - or making your forms a bit thicker so they stand up better.  Or try another porcelain that is less fluxed.

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