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Ginny C

pots cracking months later

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Any idea why some mugs and a bowl I gave to relatives for Christmas cracked after a month or so of use? Very fine cracks, but making the item probably unuseable. I use Laguna B-Mix stoneware cone 5-6 clay, bisque fired to 04 and glaze fired to cone 5.

 

I'd told them that the pots could be microwaved, but would that not be true?? I haven't heard of any cracks in the other pots I've made, unless the recipients are wary of telling me! But it hasn't happened with any of the items I've kept, and they've been zapped and repeatedly!

 

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Any idea why some mugs and a bowl I gave to relatives for Christmas cracked after a month or so of use? Very fine cracks, but making the item probably unuseable. I use Laguna B-Mix stoneware cone 5-6 clay, bisque fired to 04 and glaze fired to cone 5.

 

I'd told them that the pots could be microwaved, but would that not be true?? I haven't heard of any cracks in the other pots I've made, unless the recipients are wary of telling me! But it hasn't happened with any of the items I've kept, and they've been zapped and repeatedly!

 

 

 

 

Do you mean they cracked in two and now leak or the glaze cracked on top of the surface of the mugs. (Crazing) If it is on the surface it is crazing and it has to do with how well you glaze fits the clay body you are using. The two shrink at different rates so the glaze crazes. Is that what you mean? Kabe

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Guest JBaymore

Are you using B-Mix 5 or simply B-Mix? They are two different bodies.

 

Even fired at come 5 the B-Mix 5 has a 2.3% absorbtion figure. That means that the clay will absorb water over time in functional use. This water gets into the pores of the clay through the unglazed portions ....... like the foot area... as well as throuigh any crazed glaze.

 

Now put this into a microwave. Microwaves work by basically boiling water. So inside the micro-pores in the clay body that water is going from a small volume of liquid water to a much larger volume of steam. It is trying to get OUT of the clay. It can't readily do that, so it starts forming micro-cracks in the clay body. Over time the micro-cracks merge into slightly bigger cracks and those merge ....and those merge... and then BINGO.

 

For microwave use you need a body with a lower absorbtion rating.

 

If you are simply talking about CRAZING (cracks in the glaze layer) then it sounds like you have a case of delayed crazing.

 

That can simply be that the glaze is shrinking more than the clay body in the cooling phase of the firing. That puts the glaze under tension. Glass is not very strong in tension. If the amount of tension is slight, it is possible that at first the glaze looks just fine. But the stresses of heating and cooling (and microwaving) can cause that to finally "overcome" the stregnth of the glass... and it releases the tension by crazing.

 

Another cause of of delayed crazing can go back to the absorbtion figure. When a clay body absorbes water it can, over time, caus it to expand slightly. (As can the impact of steam created in the microwave.) So that the glaze that used to fit the body is now in tension... and bingo... crazing again.

 

Different glazes can have different coefficients of thermal expansion. So one glaze might stay fitting the B-Mix 5... and another of your glazes will not fit.

 

If you are firing Cone 5 glazes onto regular cone 10 B-Mix... everything mentioned above will basically be worse.

 

best,

 

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

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Pots are the same as people. If there is unresolved tension, they eventually crack up.

It is never a good thing if tension is not released soon, then the stress builds up.

Tension is caused by unresolved conflict.

e.g. clay and glaze shrink at different rates - result is cracking.

 

Sorry if that is not much help, but that is all I can offer today.:)

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Guest JBaymore

Pots are the same as people. If there is unresolved tension, they eventually crack up.

It is never a good thing if tension is not released soon, then the stress builds up.

Tension is caused by unresolved conflict.

e.g. clay and glaze shrink at different rates - result is cracking.

 

by

Fredrick -W; Ceramic Psychiatrist wink.gif

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Pots are the same as people. If there is unresolved tension, they eventually crack up.

It is never a good thing if tension is not released soon, then the stress builds up.

Tension is caused by unresolved conflict.

e.g. clay and glaze shrink at different rates - result is cracking.

 

by

Fredrick -W; Ceramic Psychiatrist wink.gif

 

 

Most potters are half cracked anyway-I know I am. (no drug reference implied.)

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When I test a new glaze I put it through the ringer. I scratch at the tests with forks and knives and pottery shards which I know will leave a mark in most glazes to check glaze durability. Once I've soaked it in water, thrown it in the freezer, microwave and oven, then dropped it on the floor and eventually broken it, I know what my customers can expect to receive. If the pots don't seem to hold up, to normal wear and tear like in a recent case I don't use the glaze for functional ware. I've got a beautiful glaze which is completely safe, however it's a bit soft when wearing on it with flatware. I go beyond what's considered normal use so that when a customer has a problem I know how to handle it. Generally it's much easier and cheaper to help a disgruntled customer by replacing ware that's been damaged or broken. The difficult part is figuring out whether you've got a real problem in your work or not.

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Are you using B-Mix 5 or simply B-Mix? They are two different bodies.

 

Even fired at come 5 the B-Mix 5 has a 2.3% absorbtion figure. That means that the clay will absorb water over time in functional use. This water gets into the pores of the clay through the unglazed portions ....... like the foot area... as well as throuigh any crazed glaze.

 

Now put this into a microwave. Microwaves work by basically boiling water. So inside the micro-pores in the clay body that water is going from a small volume of liquid water to a much larger volume of steam. It is trying to get OUT of the clay. It can't readily do that, so it starts forming micro-cracks in the clay body. Over time the micro-cracks merge into slightly bigger cracks and those merge ....and those merge... and then BINGO.

 

For microwave use you need a body with a lower absorbtion rating.

 

If you are simply talking about CRAZING (cracks in the glaze layer) then it sounds like you have a case of delayed crazing.

 

That can simply be that the glaze is shrinking more than the clay body in the cooling phase of the firing. That puts the glaze under tension. Glass is not very strong in tension. If the amount of tension is slight, it is possible that at first the glaze looks just fine. But the stresses of heating and cooling (and microwaving) can cause that to finally "overcome" the stregnth of the glass... and it releases the tension by crazing.

 

Another cause of of delayed crazing can go back to the absorbtion figure. When a clay body absorbes water it can, over time, caus it to expand slightly. (As can the impact of steam created in the microwave.) So that the glaze that used to fit the body is now in tension... and bingo... crazing again.

 

Different glazes can have different coefficients of thermal expansion. So one glaze might stay fitting the B-Mix 5... and another of your glazes will not fit.

 

If you are firing Cone 5 glazes onto regular cone 10 B-Mix... everything mentioned above will basically be worse.

 

best,

 

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

 

 

I forgot to check for more answers! But now that I see this reply, John, I have to ask whether the Laguna B-Mix 5 clay (that IS the one I'm using) is just not suitable for functional pots...Do other clays have lower absorption figures?? I have loved the feel of this clay, but I sure don't want to worry about mugs cracking again! And yes, it was not just crazing, but actual cracks. Would it help to bisque fire it to a higher cone? OR, can someone recommend another nice clay that feels good to use but with less absorption?

Thanks!

Ginny PS...I did not get any notifications that replies had been posted, even though the options section says I AM receiving email notifications of replies.

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What absorption rates would be acceptable for microwave and functional use? its hard to understand what the range is appropriate for a water tight vessel.

Some clays I fire to Cone 6 have 1.9, 1.5, 2.0, and then a couple are equal to or less than 1%. Thanks for any additional information you can provide.

 

Are you using B-Mix 5 or simply B-Mix? They are two different bodies.

 

Even fired at come 5 the B-Mix 5 has a 2.3% absorbtion figure. That means that the clay will absorb water over time in functional use. This water gets into the pores of the clay through the unglazed portions ....... like the foot area... as well as throuigh any crazed glaze.

 

Now put this into a microwave. Microwaves work by basically boiling water. So inside the micro-pores in the clay body that water is going from a small volume of liquid water to a much larger volume of steam. It is trying to get OUT of the clay. It can't readily do that, so it starts forming micro-cracks in the clay body. Over time the micro-cracks merge into slightly bigger cracks and those merge ....and those merge... and then BINGO.

 

For microwave use you need a body with a lower absorbtion rating.

 

If you are simply talking about CRAZING (cracks in the glaze layer) then it sounds like you have a case of delayed crazing.

 

That can simply be that the glaze is shrinking more than the clay body in the cooling phase of the firing. That puts the glaze under tension. Glass is not very strong in tension. If the amount of tension is slight, it is possible that at first the glaze looks just fine. But the stresses of heating and cooling (and microwaving) can cause that to finally "overcome" the stregnth of the glass... and it releases the tension by crazing.

 

Another cause of of delayed crazing can go back to the absorbtion figure. When a clay body absorbes water it can, over time, caus it to expand slightly. (As can the impact of steam created in the microwave.) So that the glaze that used to fit the body is now in tension... and bingo... crazing again.

 

Different glazes can have different coefficients of thermal expansion. So one glaze might stay fitting the B-Mix 5... and another of your glazes will not fit.

 

If you are firing Cone 5 glazes onto regular cone 10 B-Mix... everything mentioned above will basically be worse.

 

best,

 

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

 

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Guest JBaymore

If under YOUR firing situation you are getting those absorbencies, they should be fine for microwave use. Are those your tests ... or are they the "manufacturer's typical specifications"?

 

My personal max. target for ware for use in a microwave is 2% Apparent Porosity.

 

best,

 

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

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