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mrcasey

Does Anyone Have A Simple Intuitive Analogy For Glaze Crazing

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I understand that the thermal expansion of the glaze being greater than that of the clay body results in crazing,  but I'd like to be able to give my students a visual model for what's going on.  What do you guys think about the analogy of a hot bowling ball with mud on it.  As the bowling ball cools, it will shrink just a little; as the mud dries, it will shrink a lot.  The mud will try to shrink down but pull away in some places creating little cracks.  I realize the analogy doesn't quite work because the mud is shrinking because evaporation and not cooling.  What do you guys think?  

 

 

        

Edited by mrcasey

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Yeah I don't know if that would be any more clear than saying the clay didn't shrink as much as the glaze so the glaze cracks (since it's thinner and weaker).

Although the physics are opposite, a good visualization could be when the hulk rages and his clothes rip to shreds because they can't contain his muscles any more. 

Maybe that could be a good addendum to the lesson to kind of help students visualize the science.  Dunno.  

You could do a demo in class too by taking a bone dry pot and putting flocculated slip on it.  As the slip dries it will crack on the pot as it shrinks.

 

Edited by liambesaw

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On 9/8/2019 at 8:24 PM, mrcasey said:

I understand that the thermal expansion of the glaze being greater than that of the clay body results in crazing,  but I'd like to be able to give my students a visual model for what's going on.  What do you guys think about the analogy of a hot bowling ball with mud on it.  As the bowling ball cools, it will shrink just a little; as the mud dries, it will shrink a lot.  The mud will try to shrink down but pull away in some places creating little cracks.  I realize the analogy doesn't quite work because the mud is shrinking because evaporation and not cooling.  What do you guys think?  

 

 

        

For crazing I like the slip idea it’s technically more accurate, very visual and as the glaze shrinks faster than the body it crazes or cracks. The other side of that is the opposite, or shivering. We have used the analogy of a buckle in the pavement with the roadway being the thin layer of glaze and the  roadbed or the earth being the Clay body..

As the pavement grows faster than the earth under it the pavement buckles. As the glazes grows faster than the Clay body under it the same thing happens, it buckles  or in Clay we call it a shiver.

What we notice is people get lost in tension and compression of two different substances. Limiting the discussion from the viewpoint of the glaze shrinking or growing is probably easiest.

The bowling ball seems fine and may appeal more to some than others. Our final attempt at this usually results in the students memorizing two basic rules so they can pass their test.

  • If a glaze is in excess tension, it crazes
  • If a glaze is in excess compression, it shivers

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