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Infinite

Different Shrinkage Of Glazes In One Piece

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Infinite    9

Hi everyone,

 

I visit the forum quite often to browse through the topics for information, but I don't post a lot.

But now I've hit a problem and i could really be helped by your advise.

 

I'm new to ceramics, just stared about 1,5 years ago and I'm learning in the process.

Absolutely loving the material en I do sell some of my work at local fairs.

 

Now I'm making a new collection and I'd like to use white matt glaze on the inside of the plates/mugs and a matt colour on the outside.

So, I've made a test batch and pulled it out of the oven. there is a lot of tension difference in the glazes and some of the plates/mugs have cracked. a cups clearly shows the inside glaze pulls a lot on the bisc. some of the coloured glaze even 'jumps off' the bisc.

 

Is there any way to make this work? The simple solution obviously would be to only glaze in one colour, but i do like the colours and would like to work with two tones/colours of glaze.

 

These are some pictures of the test batch:

IMG 3036

 

IMG 3037

 

IMG 3038

 

IMG 3039

 

IMG 3041

 

Kind regards,

 

 

Carola

The Netherlands

 

 

 

 

 

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Another thing on digital fire saying you could try adding 10% nepheline syenite. Seems to be the temperature you are firing to. Not sure the glaze will stay so matt.

 

post-23281-0-24513700-1489677458_thumb.png

 

Without knowing the recipe it is hard to do much other than find a clay that fits the glaze or vice versa.

post-23281-0-24513700-1489677458_thumb.png

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Min    783

Couple thoughts, putting a matte glaze inside mugs or other functional pots is not really a good idea. It looks really nice but isn’t practical for use. Try take a fork or spoon and rub it back and forth across the surface. You will see cutlery marking on the glaze. Gloss glazes don’t do this. 

 

The cracking inside the pots could be from a mismatch in sizes of the glaze inside the pot versus the glaze on the outside. Glaze on the outside is shivering or spalling, so it looks to me you likely have 2 glaze fit problems going on here. The shivering of the outside glazes would likely happen irregardless of what glaze is on the inside of the pot, I really don't think that is from having a different coloured liner glaze. 

 

I would ask your clay supplier for glaze recommendations for this clay. I would also try the glaze(s) out on test pots before using on your real ones. Lovely design to your work!

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glazenerd    816

Think there is more than one issue going on here.

 

Would like to see extreme close ups of the yellow mug exterior and the yellow mug rim. Some things are not adding up.

What cone?  and you fired to what?

What clay.. and you bisque to what temp?

How was glaze applied, how thick,  dust, oils anywhere?

Was the cracks in the blue/rose bowls concentric or articular?

Cones used to verify peak temp?

Cream or sugar.. sorry... amusing myself..

 

Nerd

 

Think this is a case of chronic UK clay, compounded by glaze fit issues.

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oldlady    1,323

not on the exact subject, just a question.  are the black spots on the interior something you wanted or part of the problem?

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Think there is more than one issue going on here.

 

Would like to see extreme close ups of the yellow mug exterior and the yellow mug rim. Some things are not adding up.

What cone?  and you fired to what?

What clay.. and you bisque to what temp?

How was glaze applied, how thick,  dust, oils anywhere?

Was the cracks in the blue/rose bowls concentric or articular?

Cones used to verify peak temp?

Cream or sugar.. sorry... amusing myself..

 

Nerd

 

Think this is a case of chronic UK clay, compounded by glaze fit issues.

 

Glaze on the link in 1020-1080 so what, 04?

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glazenerd    816

Joel:

 

1. Rose dish: concentric ring is in exact alignment with shelf contact: so there is a cooling issue.

2. Yellow cup shows coarse texture/chalky: almost complete bond failure. (clay/glaze interface.)..way past shivering.

3. Yellow cup (rim pic) also indicated crawling.

4. Yellow cup ( rim pic) while the split does indicate glaze fit; notice however that the ends of the split are slightly misaligned. In addition, the top rim where the glaze is missing is very coarse, large particles are visible, and "appears" grossly under-fired. The sheen associated with a mature clay body is absent, and the exposed body does not follow the typical shivering pattern in that is highly undulated.

 

Yes, there is a glaze issue here: but I do believe way more than that. T4 materials are used in the Netherlands; which I suspect is the large particles I am seeing. There is also a nearly complete failure at the clay/glaze interface: which means it is either lacking flux or grossly under-fired: or both. The cooling segment was too rapid: concentric crack in rose bowl suggests that. Clay is in a highly expansion state from 1800 to 2100F  ( 1000C to 1100C or so) : which is the real cause for the glaze blowing off when it cooled.  Because of the crawling / pooling; the glaze might be actually over-fired.   Let's call it a cone 06 glaze on a cone 10 body for generalities sake.

 

The offset in the rim crack is very revealing: the biggest indicator the body is grossly under-fired. The coarseness and graininess makes me think it was fired not too much above bisque. When the body is in a highly expanded state, a glaze will move it around and deform it: rather than just splitting it as a normal COE fit issue would do.

 

Reason I asked for close ups and more info.... but my first look assumptions at this point.

Nerd  

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glazenerd    816

Corala:

 

I do not want to rain on your parade: but I would strongly advise you do not sell anymore work until you get these issues corrected. You are showing us extreme problems: but I would expect to see the invisible problems to show themselves in a very short period. Glaze shivering is an extremely hazardous problem for the end user. Sorry, not trying to be an alarmist: but this is a serious problem for functional ware.

 

Nerd

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neilestrick    1,381

 In addition, the top rim where the glaze is missing is very coarse, large particles are visible, and "appears" grossly under-fired. The sheen associated with a mature clay body is absent, and the exposed body does not follow the typical shivering pattern in that is highly undulated.

 

Yes, there is a glaze issue here: but I do believe way more than that. T4 materials are used in the Netherlands; which I suspect is the large particles I am seeing. There is also a nearly complete failure at the clay/glaze interface: which means it is either lacking flux or grossly under-fired: or both. The cooling segment was too rapid: concentric crack in rose bowl suggests that. Clay is in a highly expansion state from 1800 to 2100F  ( 1000C to 1100C or so) : which is the real cause for the glaze blowing off when it cooled.  Because of the crawling / pooling; the glaze might be actually over-fired.   Let's call it a cone 06 glaze on a cone 10 body for generalities sake.

 

The offset in the rim crack is very revealing: the biggest indicator the body is grossly under-fired. The coarseness and graininess makes me think it was fired not too much above bisque. When the body is in a highly expanded state, a glaze will move it around and deform it: rather than just splitting it as a normal COE fit issue would do.

 

Reason I asked for close ups and more info.... but my first look assumptions at this point.

Nerd  

 

It's a low fire white body, probably a white talc but could be a calcium body, so it's not going to be matured to anywhere near vitrification. Shivering is not at all uncommon on low fire white bodies, although is definitely an extreme example. Of course, I'm assuming she's using a low fire body. If she's using a cone 6 or 10 body then that could definitely be contributing to the issues.

 

Infinite, what is the cone/temp rating of your clay body?

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glazenerd    816

Remember Neil; we are talking UK clay here. Notorious for claiming " fires from cone 06 to cone 6.

 

Which is why I placed a disclaimer... :blink:

 

Nerd

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curt    117

At 1020 to 1080 Celsius this is an earthenware glaze. Earthenware (aka "chipware") glazes do not intermingle with the underlying clay body like stoneware glazes do. Rather, they sit on top of the clay almost as a seperate layer, which is why earthenware glazes chip off so easily in functional use compared to stoneware.

 

However, putting aside the occassional earthenware ding, I agree that this kind of massive chipping and shivering suggests a severe mismatch between body and glaze. Body does look somewhat underfired to me, even after accounting for the fact that it may actually be an earthenware body, which to my stoneware eyes always look underfired anyway.

 

Assuming for a second that both clay body and glaze are actually both earthenware by design, is it possible that one or both are just a bad batch? Maybe send pictures to vendor and see if they think there is a batch problem?

 

I agree that adding some flux like nepheline syenite might help create a better bond.

 

But as others have said more info needed.

 

 

Nerd, I do not see glaze pooling, where do you see this?

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Infinite    9

Hi everyone!

 

Thanks to all of you allready for the helpful replies.

@Nerd, thanks for your concern  ^_^ . i'm definitely not going to sell any of this until the problem is fixed.

@ Min, happy to hear you like the design B)

 

the clay i use is a earthingware 'pouring' clay to use in plaster molds/casts (K150 bij Sibelco) 1000 - 1140 degrees Celcius (I'm not working with cones, it's not used so much in the Netherlands I believe).

Clay body is fired to 1020 so could definitlely be underfired. glaze is fired at 1050

 

I allready have a lot of bisc on the shell ready for glazing. all bisc fired at 1020. should/could I re-fire some bisc (maybe a stupid question :ph34r: ) to a higher degree (like 1100?) then glaze and fired at 1050. Or should/could I glaze some underfired bisc and fire at 1100 directly?

 

 

I also got the feeling that there are several problems happening. I never had an item chipping so badly as the little yellow pitcher but in the past i did some tests with a white shiny glaze on the inside of a cup and a coloured glaze on the outside ans this too burst as a result of tension. I aborted the mission and only used the coloured glaze all over to avoid the problem... Now i'm trying to tackle it again...

 

about the circular crack in the bottom of 2 plates/flat bowls: the plates where placed onto 'triangles*' so they did not touch the surface of the kilnshelf

 

* https://www.keramikos.nl/stapelmateriaal/1146-triangels-nr-01-ptp-22-mm-per-stuk.html?search_query=stapel&results=68

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glazenerd    816

 

Nerd, I do not see glaze pooling, where do you see this?

Last pic of yellow cup rim: @ 1 o'clock position. Can see barren white (not shivered) and a thick area of  glaze next to it. Trying to figure out if the micro pinholes I see in it is from over-firing or gas.

 

Carola:

The recommendation of adding 10% Nep Sy to the slip is probably the easiest and most direct approach. Been awhile since I have seen clay this immature that it basically "let go" of the glaze. I personally would be dealing with the clay issues first: and glaze secondly because the clay is causing the vast majority of the problems. Possible that once the clay issues are fixed, glaze may be just fine with some minor tweeks.

 

If the clay is this immature at 1050C, trying to rationalize just how much higher you need to go? Even with adding 10% Nep Sy, 1050C is still not hot enough to really incorporate a melt IMO> Being a little more radical in my approaches to clay: personally I would be testing in the 1150C to start: and I have my doubts that will fully resolve it. Deal with the clay first, then the glaze.

 

Nerd

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Infinite    9

Hi Nerd,

 

Thanks for your advise,

 

so than, can I re-fire my immature unglazed bisc.

Or should i throw away all my ready-made bisc and start from scratch and fire upto 1100/1150?

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Min    783

Looks like the glaze is rated to ^03 (using 60C/hr climbing rate) and the clay is rated up to ^2 (same rate) so 4 cones difference in temp. I would pour a test piece, using the mug form, as thinly as possible. Bisque it to 1140C, with the last couple hours of firing at 60C/hr. I’m guessing the bisque will still be somewhat porous as it’s earthenware. Then glaze the mug the same way you want to glaze real pots but put the glaze on as thickly as you can then fire it to mature the glaze at 1080C (rising 60C/hr for the last couple hours). If it comes out of the kiln without shivering/dunting then put it in the freezer overnight. Next day put it in the sink and fill it with boiling water. If it survives this then repeat it 2 more times. 

 

 

Still don’t think it’s a good idea using the matte glaze as a liner glaze though. I would contact your supplier and ask for a gloss glaze that fits this clay and do the shivering test using that as a liner. If it cracks the pot apart then try a different gloss liner glaze or a different matte glaze on the outside. Your supplier should be able to point you in the right direction here. One last question, could you post your firing schedule and how quickly you cool the kiln down and open it up?

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Infinite    9

Hi Min,

 

I'm going to send the supplier an e-mail this evening. Hopefully he can give some directions as well :-)

 

For the firing scedule:

i have a kiln with a simple automatic computer

the kiln will slowly rise in temparatuur (certain # degrees per hour) until it hits 600 degrees celcius, from then on it will go full on till it hits the final temp.

 

the kiln can rise in the following temperatures:

60 degrees per hour

120 degrees per hour

260 degrees per hour

360 degrees per hour

 

for cooling; the kiln does not have regulated cooling. it just cools down. I open the vent when the temperature is 200degrees or lower and the door when the temperature is 80degrees or lower.

 

for bisc i use the '60degrees per hour, until 600degrees, than full on till final temp'

for glaze i use the '360degrees per hour, until 600degrees, than full on till final temp'. final temp is kept for 20minutes.

 

hope this helps..

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Min    783

Once it hits 600C then the only option is full power until finished? 360C/hr seems pretty fast for the start of the glaze fire. Is there any chance your supplier has a slip and glaze(s) that are both rated for close to the same temperature?

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glazenerd    816

Min is on the right track- good advice.

 

If the clay is rated to cone 2: then I would make that my peak temp. This means you need to find a glaze that will fit the clay, and mature at that temp. Firing bisq to 1000-1040C is fine: it is the glaze fire that will ultimately mature the clay.  In the glaze fire: the most important firing schedule in this case would be the last 120F: which I would slow down to 60F an hour with a short hold. Getting the clay mature is the most important issue in this case: and then find a glaze that matches the peak temp and the body.

 

Nerd

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Min    783

  In the glaze fire: the most important firing schedule in this case would be the last 120F: which I would slow down to 60F an hour with a short hold. Getting the clay mature is the most important issue in this case: and then find a glaze that matches the peak temp and the body.

 

Nerd

 

Unless I misread what Infinite said I don't think it can be fired at that speed. Sounded like the only option was full on after hitting 600C. Before then only 4 ramp options, 60C being the lowest. I was suggesting slowing down towards the end of both the bisque and glaze but I don't think it can be done with her kiln. 

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Infinite    9

Hi Min,

 

you are correct.

 

(I'm sorry to all of you if my messages aren't as clear as they should be due to my not-so-perfect English -_-)

 

Yesterday I also contacted my local supplier from which I bought the clay. They said that I bisc fire too high and that 985 should be the best temperature for the glaze to stick onto the bisc...

 

I also contacted my former teacher from art school and he advised also to rise the final temp, and said that most commercial glazes will hold when overfired (his experience) but that the colour can change.

 

So being the stubborn curious Dutch woman I am, I've made a testbatch with several mugs and glazes combined with the yellow matt glaze, just to see what happens (learningprocess) but now i'm firing upto 1120 (1140 is the limit temp of the clay) degrees celcius.

 

to be continued....

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What the supplier is saying isn't related to your final glaze problem. You can bisque to a lower temperature so the clay is more porous and sucks up more glaze. If you have no problems with applying glaze then keep doing the same thing. 

 
The glaze may be fine up to 1120, the odds are probably on your side but I don't know if it will fix the issue that it doesn't fit the clay.

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Min    783

Did your supplier recommend another glaze to fit that clay or did they say your current glazes should fit when fired to clay maturity temp?

 

I would put the test pieces on scraps of clay and/or don't glaze within a few cm's of the bottom outside of the pots. You might get running glazes so the scrap saves glaze melting onto your kiln shelves. The matte glazes might be okay as I think they are underfired gloss glazes since they are matte without a slow cooling kiln. May see blisters. Also, if any survive without shivering I really would do the testing I mentioned a few posts ago with freezing/boiling.

 

"So being the stubborn curious Dutch woman I am..."  

 kinda think you have to be stubborn to succeed in ceramics, so many things that can go wrong. If you aren't stubborn it would be so easy to just give up. I know I can definitely be stubborn, obstinate, headstrong, willful, pigheaded, contrary, strong-willed, well you get the idea. ;)

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Infinite    9

Just opened the kiln and to my surpise and relieve everything was still in one piece and looks good zo far! I'm so happy ^_^

No cracks so far, colour looks ok, no runny glaze on the kiln shells. I've used a white shiny glaze on the inside of the cups as reccomended in this topic (thanks) and added 'rutiel/rutile'.

 

 

IMG 3046

IMG 3047

IMG 3043

 
The mugs are currently in the sink, undergoing a boiling-water-tea-test.  :)  fingers crossed :)
 
Big kiss on the cheek to you guys!

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