can i cover shivering glaze problems

[justin1287]

So, fired pieces twice cause i needed to add a colored foot that was a frit 3110, slurry, mason stain mixture. the coyote glaze shivered (not on the foots cause there was none there), or all have cracks and signs of shivering in the future.......100 pieces or so. i'm hoping i can fire again with a clear glaze over it to hold everything together. i originally fired at cone 6, then cone 6 again. Everything was fine before the first fire. Can i re-glaze with a clear over it......im thinking about possibly finding a cone 04 clear glaze to cover it and hope it holds the pressure in.....any advice....thanks all!

 

Justin

[bciskepottery]

You need to either find a clay body that works with the Coyote glazes, or find glazes that fit your clay body. Shivering is a fatal result, one that should not be patched over in an effort to hide the defect. Sorry, but you are better off doing a new load of pieces. But you first need to resolve the incompatibility between your clay body and glazes.

[Ben]

What bsickepottery said. The glaze doesn't fit the clay and it never will.

trash it and start testing.

You should be able to mix up a series of test glazes with varying Coefficient Of Expansion (COE). EVERY TIME you buy clay you should make test tiles and use these glazes to get a relative COE for the new batch of clay. Compare this with the COE that the clay manufacturer publishes. (if they don't publish this data, find a new clay supplier if possible).

You can use this method to test a bunch of clay bodies fairly quickly and get an idea of which clays have high, med or low COE.

When buying glazes you should get the published COE data for the glaze. If they don't publish a COE, find a new supplier.

There is a great cone 6 glaze book out there that can help you with this COE test glaze series. (don't know if I'm allowed to post the name)

Your glaze shivered so it is contracting LESS than the clay. This could mean that the clay and glaze simply don't match or it could mean that one or the other was made wrong. (This is where published COE data and your own tests come in handy. If you can show that the clay or glaze is out of range for its published numbers you might could get some refund from the supplier.)

 

[Mark C.]

In this case the trash can is your friend-any shivered pots need to be hammer tested-and tossed.

You have two options change clay body or change glaze-testing is key

good luck.

Mark

[Kabe]

Seeing we are talking about shivering. I have a glaze that shivers on porcelain but not my stoneware. I am working on a change. I use Digital Fire Insight for calculations. I know the figure for Thermal Expansion for the glaze is higher than most of my other glazes. Looking for a way to improve fit, I entered the ingredients of my claybody, as a glaze recipe. This gives me a Thermal Expansion figure. The premise being; If mix my claybody with enough water and add color, it would fix my pot about perfect. Might be ugly, but it would fit. I have not tried it yet. I am more interested in the calculations at this point. I want to use it as a way to check fit, comparing the claybodies number to the thermal expansion number of a glaze. Ballpark figure to get me close. Question, Is thermal Expansion the same as Coeffient of expansion or are they two different animals? One problem I have with using my claybody as a glaze recipe is that it contains a lot of grog. Grog isn't recognised as a glaze material so is not entered into the calculations. I'm sure it would effect the expansion. I do not know if I can get the COE # for the porclain or not to compare it to my stoneware, plus I'm not sure if the figure I have for my stoneware is correct. If this makes sence to anyone out there, please give a holler.

As far as your Pots go. I think the advise of starting over is sound. My shivered glazes are like razor blades. If the glaze looked all right after you refired it but later on down the road is still shivered and it went unnoticed, it might be hard for a customer to digest the splintered glass. Crazing can continue for along time after it is fired, I do not know about shivering I would think it could because there could still be tension. Bummer for sure!!! Happy firing, Kabe

[justin1287]

Ok, i understand all of that......the glaze fits my body just fine, and i have never had any problems with it.....it just seemed to get stressed out cause it fired it a second time......thats why i believe it should be fixable.

[Kabe]

I do not know enough about all that takes place when a glaze fired or why a second firing would cause it to shiver. I would think that the glaze on the pot that was fired the second time was not the same chemical composition as what it was in the first firing. A bunch of the stuff was vaporized the first time. I'm not sure if that would effect the outcome or not. I have refired things before and had no ill effects but it was not for the same reason. For me I would be afraid it might do it again. But I've been wrong before. I bet someone out there knows the reason. Happy firing Kabe

[Mark C.]

Once it shivers its not bonding with the body and can shiver later on as well. It Nothing worse than having someone later have the glaze come off as thats a real world hazard to be avoided at all costs and cannot be un-shivered if you will.

Firing twice effected the bond between body and glaze and now its toast. Firing it again will only weaken the body more.

I realize this is really bad news on 100 pieces but its the way it is. I have thrown away more than that so I know how that feels.

Sorry for being the messenger.

Mark

[Guest JBaymore]

Looking for a way to improve fit, I entered the ingredients of my claybody, as a glaze recipe. This gives me a Thermal Expansion figure.

 

 

Kabe,

 

While it seems to make intuitive sense, that will not work with the clay body to obtain a COE figure. You are getting the COE of the body if it was FULLY MELTED into a glass........ which is likely going to happen at about cone 20-30. The reason that you can calculate COEs for melted glasses is just that... it is assuming that they are fully melted glasses. In that case, the individual oxide's COEs can be calculated to give an overall COE by their individual contributions to that factor.

 

However no one has of yet been able to model the development of a COE for a clay body, because it is a mixture of melted glassy components as well as crystalline components. The variables involved there are immense. It will be a major breakthrough if/when someone figure out how to predict this. At the moment the ONLY way to come up with a clay body's COE is by using actual measurements. This is usually done using a device called a dialatometer. The dialatometer heats up a prepared sample rod and measures the actual physical expansion.

 

A technique I have my students do in the ceramic materials (glaze chemistry and related studies) is to do a series of GLAZE recipes with a run of steadily increasing COEs.... running from a very low one to a very high one. Than apply the glazes to the clay body in question. At one end of the spectrum the glaze will shiver badly on the body. At the other end of the spectrum it will craze badly. SOMEWHERE in the place in between.... you will find the glas that fits the body. Using this number you can then APPROXIMATE the COE of the given body.

 

best,

 

.........................john

[Guest JBaymore]

Question, Is thermal Expansion the same as Coeffient of expansion or are they two different animals?

 

Kabe,

 

Basically... yes. To throw "fuel on the fire" for the name..... I usually present it to students as the "Coefficient of (reversible) Thermal Expansion". The important concepts is that the situation is reversible. If you heat up the glass it expands by that factor... if you cool it down it contracts by that factor.

 

Delayed crazing and delayed shivering are very much products of that fact; it fits or it don't .

 

And remember ... in the intitial firing during the heating phase, you do not HAVE that formula of chemistry sitting on the outside of the pieces. What you have there initially is a coating of the various raw materials that will, with the application of heat energy over time, convert to their respective oxide forms, and then eventually involve themselves in a melt that will become that particular glass.

 

best,

 

....................john

[neilestrick]

Also think about this from a non-technical standpoint: If you apply a clear glaze over your shivered glaze and it all falls off during the firing before the clear melts, which is quite possible, you'll be melting all that glaze onto your kiln shelves. Not pretty.

 

Glazes do all sorts of funky things in refire. Some crawl, some shiver, some are fine.

[Kabe]

Looking for a way to improve fit, I entered the ingredients of my claybody, as a glaze recipe. This gives me a Thermal Expansion figure.

 

 

Kabe,

 

While it seems to make intuitive sense, that will not work with the clay body to obtain a COE figure. You are getting the COE of the body if it was FULLY MELTED into a glass........ which is likely going to happen at about cone 20-30. The reason that you can calculate COEs for melted glasses is just that... it is assuming that they are fully melted glasses. In that case, the individual oxide's COEs can be calculated to give an overall COE by their individual contributions to that factor.

 

However no one has of yet been able to model the development of a COE for a clay body, because it is a mixture of melted glassy components as well as crystalline components. The variables involved there are immense. It will be a major breakthrough if/when someone figure out how to predict this. At the moment the ONLY way to come up with a clay body's COE is by using actual measurements. This is usually done using a device called a dialatometer. The dialatometer heats up a prepared sample rod and measures the actual physical expansion.

 

A technique I have my students do in the ceramic materials (glaze chemistry and related studies) is to do a series of GLAZE recipes with a run of steadily increasing COEs.... running from a very low one to a very high one. Than apply the glazes to the clay body in question. At one end of the spectrum the glaze will shiver badly on the body. At the other end of the spectrum it will craze badly. SOMEWHERE in the place in between.... you will find the glas that fits the body. Using this number you can then APPROXIMATE the COE of the given body.

 

best,

 

.........................john

 

 

 

Thanks John For the information. I have put this question out there before. This will give me a way to find out. There seems to be so much information to absorb. I am in a process of setting up triaxial glaze trials. Making a set of tiles that can be pressed out so test results can be hung up, where people can get some use out of the tests. Most of them end up in a shoe box . I can run a 10 stage strip with a glaze that will show slivering at one end and crazing at the other. Thanks again I am in the procss of learning the glaze chemistry and terminolgy involved with the art. Challenging. Happy firing Kabe