glaze shivering define glaze thickness
Posted 10 February 2012 - 05:51 PM
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
Troubleshooting is so hard because there could be so many different reasons.
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Posted 10 February 2012 - 05:56 PM
Kiln Repair Tech
Owner, Neil Estrick Gallery, LLC
Posted 10 February 2012 - 06:10 PM
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.
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art
Posted 10 February 2012 - 08:12 PM
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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Posted 10 February 2012 - 09:14 PM
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.
Posted 10 February 2012 - 10:23 PM
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.
Posted 11 February 2012 - 09:39 AM