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skessel1

Crazing With Commercial Glazes And Bisqueware?

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hi experts!  Please help!

 

One of my customers just sent me a picture of the plate that I hand-painted for them.  They said they never used the dishwasher or microwave and were very gentle with it when hand washing and only used it for cold desserts.  Please see the attached pictures.

 

The cracking that you are seeing is on a plain rim dinner plate from bisque imports with only the Duncan Pure Brilliance clear overglaze on it.  Why would this have happened?

 

I will say that I had not sieved my dipping glaze in several months and when I did it was seriously clumpy and the sieve got clogged really quickly.  Could that have been the reason?  We are going to do some testing on some glazed plates since we sieved it, but if you guys have any other ideas, please help!!!

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Glazes can come out of the kiln already crazed if the fit between the glaze and the clay body is significantly different. Coefficient of thermal expansion differences. If the glaze looks okay but the glaze isn’t a good fit it will still craze, sometimes within days, sometimes takes months, (delayed crazing). Are you sure your pots were fired to the correct cone and not underfired? that can cause crazing too. Another cause of glaze crazing in low fire work can be from moisture getting into a porous clay body which in turn causes glaze crazing. Doesn’t sound like the latter is the case with the customer who contacted you though. 

 

Lumps probably didn't help but no clue if they would cause crazing. Have to get the heavier materials back into suspension.

 

It’s tricky trying to troubleshoot commercial glazes, you don’t know what is in them, might be high expansion fluxes that you can’t alter. Adding silica might fix the glaze but if the crazing pattern is finely spaced that means the difference in expansion is fairly significant and adding silica might not do it. I would contact your bisque ware supplier and ask them for a recommendation for a well fitting, non crazing glaze.

 

I would suggest testing your pots for crazing and leaking prior to going into production and selling your work. I tend to be brutal with craze testing. 3 cycles of putting test pieces in a 315 - 320F oven for 20 minutes then putting them in a sink of cold water then repeating. Then I dry them off and brush sumi or calligraphy ink onto them, rinse them then check for crazing. Some people put test pieces in boiling water for 5 -10  minutes then plunge into ice water, then repeated 3 times. I’ve found this isn’t quite as severe a test. Both these tests are shock tests but simulate years of glaze wear on pots. Again, the closer and more crazed a glaze the greater the mismatch between glaze and clay.

 

Hope you can get it worked out. 

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Thank you so much for your responses.  I am going to do some serious testing AND call my supplier.  I have never seen this happen on any of my other pieces, so it's really confusing to me!!  I will let you know what I find out.  Thank you again for your help!!

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Duncan Pure Brilliance appears to be a low fire glaze. There was only one low fire clay body that I found that didn't craze with a clear over it, the clay was Laguna #10 and the glaze was an Amaco low fire clear. It crazed on terra cotta and the other 3 white talc bodies from Laguna. Since you're using commercial bisque, you won't have much control over changing clay bodies so your other option might be to try another clear glaze. With low fire, you're nearly always going to have some crazing. If it's functional ware to be eaten off of etc., stoneware or porcelain would be a better option, safety-wise.

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Since your clay body is import you may not know much about it.That in itself can be an issue.

Since you are painting on this ware maybe its not an issue as its not to be used for functional ware?

You will need to tell them not to eat off it.

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Now I can illustrate the importance of a mature body, covered by a mature glaze.

Clayglaze interface

 

Notice some of the white (clay) is pulled up into the blue (glaze). In other places, the blue (glaze) is pulled into the white( clay). That is called clay'glaze interface: where the two meet. If the clay is not mature: then the flux in the clay does not rise to the top to bond with the glaze. That lack of bond can cause grazing, just like the difference between COE values from clay to glaze can cause it. (as Min properly pointed out). The other issue from lack of bonding is even more hazardous than just checking cracks. If the bond between the clay and glaze is compromised enough: then shards of the glaze can come off (shivering). Potters tend to look only at the glaze and how to fix the cracks: but an immature body is even more hazardous in some cases.

 

Nerd

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Alright you guys!  I am at my whits end.  I ordered a new, different clear glaze and new plates and I did the shock testing that Min suggested.  Check this out.  I put it in the oven for 20 minutes at 300 degrees, then put it in a bath of cold water.  It IMMEDIATELY crazed.  I am so stressed!!  I will call the glaze and bisque manufacturer this week, but I have an order of 24 plates to deliver to this restaurant and don't know what to do!

 

I have never made my own glaze before but if this glaze is as worthless as it appears, I am willing to try.  I just need a food safe clear glaze.  Please help me!!!

 

THANK YOU!!!!!!!!

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Hmmm, couple thoughts if I may. Just so we are all on the same page here, could you post your firing schedule, confirm this is low fire bisque and if you know what cone to which it was bisqued. Have you measured the porosity of the clay after firing? That second pot looks like it has soaked up water.

 

It's very difficult to get a non crazing clear for low fire. If you have a porous body (just about all low fire is) it will just be a question of time before you get crazing. The oven/water test just sped up what would have happened eventually. Any chance you could use stoneware bisque and fire cone 6? Much easier to get glazes to fit without crazing. 

 

It takes time to get clay and glazes the way you want them. I can't help but think that it might be better to really get this worked out then make the restaurant order, if that means missing a deadline or a sale then so be it. You will sleep better at night knowing you have made a good product.

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Hi again!

 

Yes, this is low-fire bisque.  I'm not sure to what cone it was bisqued.  I will call the supplier to find out.  That is good to know about the fact that low fire clear glazes will craze over time.  Clearly that is not a product that a restaurant would be interested in.

 

I would like to use earthenware bisque for this order, but i don't make pottery myself and don't know much about high fire.  Could I still use low fire underglaze under a high fire clear glaze and have it turn out?  What would be the best method to get the result in the above photo, i.e. hand-painted text under a clear glaze?

 

You guys are amazing!!!!

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I just found this.  Which method do you guys recommend.  The firing of the unglazed stoneware to cone 6, then applying low-fire underglazes and glazes and firing again sounds like the safest method.  What do you guys think??

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OOps.  Here it is: Can I use low fire glazes on Stoneware Bisque? Yes, except for clear glaze. Clear glaze will craze on Stoneware Bisque. You can apply some low-fire glazes to Stoneware Bisque and fire to Cone 6. Check your low fire glaze label for results at Cone 6. Alternately, you can fire the Stoneware Bisque to Cone 6 without any color, apply low-fire glazes, and fire the glazes to Cone 06. You will notice the color takes longer to dry with this method, as the Stoneware Bisque does not absorb any moisture from the low fire glaze. 

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one more question.  If I do the writing in a high fire black glaze, let it dry, then glaze the whole thing with a clear high fire glaze, then fire it to cone 6, will that work?  I'm sorry for all the questions.  High fire is very new to me.

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Cone 6 or 04 either way you need to test the glaze for “fit†on the clay you use. Cone 6 is less porous so crazing from the pot absorbing water is far less likely. For glaze fit think of a size 2 pair of jeans and a size 8 pair of jeans. If my daughter and I both try on the size 2 they will fit her but be too small for me and tear (craze) because the fabric (glaze) is under tension. The size 8 will fit me but be too big for her and may fall off (glaze shivering). Both pairs of jeans are good but need to fit the body they are going on. So the ideal situation is too find a glaze that fits the clay body you are using. I don’t use commercial glazes or bisque so can’t help you with suggesting a well fitting glaze/clay combo. 

 

I would contact Mayco or one of their distributors or whoever is available to source your bisque and ask which clear glaze they recommend then do your own testing with your kiln and your glaze / firing conditions. I would suggest just buying a few small pieces of bisque that you can use to run tests on. If I was doing a restaurant order I would go with cone 6 not low fire. 

 

Re your question: “The firing of the unglazed stoneware to cone 6, then applying low-fire underglazes and glazes and firing again sounds like the safest method†Problem with this is the pot is no longer porous so it’s harder to get the glaze to stick. It can be done but its far easier not to do it this way.

 

Re your question “If I do the writing in a high fire black glaze, let it dry, then glaze the whole thing with a clear high fire glaze, then fire it to cone 6, will that work†Glazes can move during the firing and lead to blurred edges. Underglazes or slips tend to stay where you put them. I used underglazes on the mugs pictured below, Spectrum brand, on greenware then bisque fired to 04 then clear glazed and fired to cone 6. The underglaze stays in place and can go on greenware or bisque. These are about 10 years old, the glaze fits the body, no crazing after years of daily use. (please excuse the lousy picture)

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Min,

 

You are amazing!  Thank you for explaining!

 

For the mugs, was it earthenware or stoneware?  I guess it had to be a clay meant for high fire, right?  Since you fired to cone 6?  And, was the clear glaze a high fire glaze?

 

I have learned a lot from this situation, but have basically learned how much I don't know (A LOT).  So, I appreciate everyone's willingness on this forum to share their years of experience and knowledge.  I am truly grateful!

 

I would like to experiment with some of my underglazes and then firing to cone 6, but I am worried about firing the bisque I have, identified as a low fire bisque, to cone 6.  What will happen?  Will it explode?  Melt?

 

THANK YOU SO MUCH!!

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I would like to experiment with some of my underglazes and then firing to cone 6, but I am worried about firing the bisque I have, identified as a low fire bisque, to cone 6.  What will happen?  Will it explode?  Melt?

 

 

 

Think of a volcano, spewing lava from it's cone.  The lava is molten rock.

 

That's what can happen to clay when it's fired hotter than intended.

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"For the mugs, was it earthenware or stoneware?  I guess it had to be a clay meant for high fire, right?  Since you fired to cone 6?  And, was the clear glaze a high fire glaze?"

 

Those mugs are cone 6 porcelain. The glaze is a cone 6 clear, the underglazes are from the Spectrum 500 series, they work at lowfire and most work at midrange. It’s common practice to call cone 04 lowfire, cone 6 midrange and cone 9+ high fire. There is leeway in each of those ranges. The places that sell the lowfire bisque oftentimes call cone 6 midrange “high fire†so be careful there.

 

"I would like to experiment with some of my underglazes and then firing to cone 6, but I am worried about firing the bisque I have, identified as a low fire bisque, to cone 6.  What will happen?  Will it explode?  Melt?"

 

I really would not fire lowfire clay to cone 6, you will probably have a huge mess to clean up, the pots will more than likely melt and fuse to the shelves and anything else they can drip onto. I don’t know which underglazes you use, so can’t comment on how they will do at cone 6 but try it and see.

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I would like to experiment with some of my underglazes and then firing to cone 6, but I am worried about firing the bisque I have, identified as a low fire bisque, to cone 6.  What will happen?  Will it explode?  Melt?

 

 

Maybe.  

 

I deliberately put low-fire slip-cast clay in a ^6 firing to see what would happen.  It melted and stuck to the bottom of the kiln.  Lesson to learn from this, is if you try something that you shouldn't, always put it on/in a bisque bowl/tray, so if it does melt it doesn't ruin your (or someone else's) kiln shelf.

 

Don't forget, experiment but protect.

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