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Juli Long

glaze shivering

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I have some problems with glaze shivering/crawling and saw reference to the fact

that glaze could be too thick. Does this refer to the thickness of the glaze in

the bucket; viscosity, of how thickly the glaze was on the piece itself.......fingernail

thickness.?

Troubleshooting is so hard because there could be so many different reasons.

For those of you that love a good laugh, there is a great youtube video called "potter at an art fair"

google "youtube and potter at an art fair"

juli

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Guest JBaymore

There is ONLY ONE reason for shivering. If it is shivering, it means that the Coefficient of reversible Thermal Expansion (COE) of the glaze layer and the Coefficient of reversible Thermal Expansion of the clay body do not match. As the frozen glaze layer cools, it is shrinking less than the underlying clay body is. This puts the glaze in compression. Too much compression.

 

If this condition exists, even if it does not show up immediately on thinner layers of glaze, it is still there. Lurking. Waiting to happen. Just like you can have delayed crazing when the opposite clay and glaze relationship is true, you can have delayed shivering. Shivering is a much more significant defect than crazing... and as a functional ware maker.... would scare the heck out of me to know it might be happening to pieces that I have sold.

 

The fix is not to make sure that the glaze application layer is thinner.... it is to adjust the COE of the glaze to fit the clay body.

 

You need to increase the content of the oxides that have higher COEs in the glaze formula and/or decrease the low COE oxides. For example, if the glaze contains lithia (Li2O) that is likely the first place to decrease, since it has the lowest COE of all of the ceramic active oxides. Drop a little of the lithia and replace it with an equivalent molecular amount of Na2O. If there is a LOT of lithia relative to the typical Limit Formula , drop it a lot.

 

best,

 

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

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John ... I read what you wrote and at some point my eyes glazed over and my brain went south ... Arghh ...

Chemistry might as well be from Jupiter!

Is there any way of easily understanding glaze chemistry? Like not putting in a cup of salt instead of a cup of sugar in a cake recipe?

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From Hamer and Hamer: Shivering is known "as a glaze defect although the fault often lies with the clay body. It is a result of poor glaze fit and is the opposite of crazing . . . When a pot cools after firing, the glaze sets at approximately 500C / 932F. From this temperature down to room temperature the body and the glaze lie side by side as two solids which should be bonded to each other. The body and the glaze each contract at a different rate. This puts the glaze on the finished pot either under tension or under compression of the clay body. Some compression is desirable as an insurance against crazing but too much compression results in shivering. . . To overcome shivering, the glaze (or the clay body) must be altered in composition. The glaze must be made to contract more on cooling."

 

When the glaze and clay body fit together, with similar rates of contraction (COE), you get a nice unblemished finish. When the glaze contracts more than the clay body, you get crazing. When the clay body contracts more than the glaze, you get shivering or peeling. You can live with the first two, but the third is a fatal default. Shivering will manifest itself overtime regardless of whether you use a thin application or thick application.

 

One of the key differences between a clay body and glaze is that glazes have oxides. Oxides have different contraction rates. So, adjusting the percentages of oxides in the glaze recipe can produce a glaze that has a more compatible rate of contraction (COE) with the clay body. Which oxides you adjust depend on which ones are in the recipe. I'd start by using a glaze calculation software package to do some adjustments on paper, using the current glaze recipe as a baseline and seeing how small changes affects the COE, and then moving on to test tiles.

 

Also, if the glaze that shivers now did not before, I'd ask what is different. Is the glaze a recent mix? If it uses Custer Feldspar, would the new composition affect the glaze COE? etc.

 

 

 

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Makes sense to me. The glaze is a dry commercial glaze that I cannot change. I can change the clay. Yes I have had problems with it before and

have been plugging along changing little things. Hate it that I bought 25lbs of the glaze and have 100's of lbs of the clay that I enjoy. The shivering

looks like a fat persons cellulite on their legs, I don't know any other way to explain the look. Small areas pulling away. I won't be able to

look at someone at the beach with out thinking of my poor mugs and pitchers.

Thanks guys for the clear and complex information. I am glad it is something fixable.

Juli

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For crawling, you can try thinning the glaze, perhaps adding a bit of CMC or gum to hold the glaze in place. (When the glaze dries on the bisque, does it show cracks? Sometimes the glaze melts enough to heal the cracks, othertimes you get crawling.) Other possible causes of crawling include firing wet ware (glazes still damp, water from glaze has not had sufficient time to evaporate from the bisqued ware); glazes thickened by evaporation of water; bisque ware being too absorbent (try bisque firing one cone higher); dust/dirt/grease/oil on bisque surfaces. Very different from crazing and shivering.

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