Crackle glaze


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Crackle glaze

Postby Maneki Neko » Feb 12th, '13, 07:47

I have a question about crackle glaze :)

How is it created? Does it always come out of the kiln that way, or do the crackles sometimes develop with use and/or age?

For example, could I expect my Arita rice bowl to develop crackles with continued use for hot rice?
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Re: Crackle glaze

Postby JBaymore » Feb 12th, '13, 11:37

"Crackle" glazes are technically "defects". The other name for this effect is "crazing". One person's defect is another person's feature :wink: .

Whern the molten glaze freezes (glass and glaze is actually a "super-cooled liquid".... not a solid) as the kiln cools down, you have a coating of glass on the surface of a body of clay. While there is some bonding of the two (forming what is known as the "interface zone"), particularty at the higher stoneware firing ranges, they are somewhat two distinct different substances.

As the work cools, it all contracts in physical size. If the layer of glaze shrinks slightly more than the underlying clay body does, then the stress in the glaze puts it under tension. Glass is weak in tension...and so the glass cracks.... causing the "crackle" you see.

This can be "deliberate"....... and you can formulate glazes that have high expansion and contraction rates (officially called "Coefficient of Reversible Thermal Expansion). These can be rather spectacular.

Or it can be a bit incidental to the production of the work. Part of it.... but not a hugely deliberate and stressed feature. Meaning that the glaze and the clay body do not exactly fit each other...... and the crazing simply happens due to this. And the ceramist has decided that this is OK.

Some glaze formulations, because of their inherent chemical nature to get the REST of the look of the glaze, cannot be made to "fit" chay bodies...... and crazing/crackle is just "part of the deal." Anything depending on high sodium oxide and potassium oxide contents to flux (lower the melting point) of silica (SiO2) will always craze over clay.

If the fit between the clay body and the glaze is close....... often it initially emerges un-crazed from the kiln. But over time and usage, including the thermal stresses that hot tea (or hot rice) place on the wares, the glass can slowly relieve the stresses by crazing.

If the glaze actually fits the clay, no amount of hot tea or rice will cause it to ever craze.

There you have it.

best,

..................john
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Re: Crackle glaze

Postby paul haigh » Feb 12th, '13, 11:57

Picture a pot cooling down in the kiln. As it cools, it contracts. The glaze is solidified on the pot and it's contracting as well.

If the glaze contracts more than the pot, it will craze to reduce stress between it and the clay.

It can be under stress and then craze years later with heating/cooling in normal use- but its due to that inherent stress.

The chemistry of the glaze can be altered to make it craze more or less.

On woodfired pots it's common to see the side that faced the fire craze more because the wood ash increases the expansion/contraction of the glaze. The inside opposite lip of the piece will get hit as well- so some glaze on one side in the piece may craze more.

Picture- inside lip of a cup. Picture fire going over the cup- it hits one side of the cup, and then can dip into the cup and hit the opposite inside lip. Without ash, this glaze is gold with crystals, with ash to flux it more it develops a look like a blue flame-job, and the ash causes crazing.
craze.jpg
craze.jpg (30.93 KiB) Viewed 240 times
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Re: Crackle glaze

Postby Maneki Neko » Feb 13th, '13, 01:35

Interesting! Thanks for clarifying it for me guys :D
I think we actually have a few little spots on the bricks of our house that were glazed by accident, and they also display crackles :wink:

Those blue flames are awesome!
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