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Crack In Glazed Vase


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#1 Benzine

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Posted 25 January 2014 - 05:02 PM

I recently glaze fired this vase, and thought it came out of the kiln fairly well.

There was initially only some crazing on one of the glazes I used, which didn't concern me, as it was a decorative piece. 

However, several hours later, when I looked at the vase, these cracks had appeared.  Obviously, they are very much structural.

My first thought, is that they are stress cracks, that just didn't show up earlier in the process.  As you can see, one of them follows the curve, where the body meets the neck.  I had a bit of an issue with this area, when throwing.  It really wanted to "flop".  I'm guessing, this is where the initial structural problem arose.

I've just never had something crack this long after unloading it from the kiln. 

I plan to make a vase using a similar glaze combination in the near future.  So I'm trying to figure out if it was just a clay structure issue, or if the glaze fit, could have caused a crack this badly in the clay body?

Cracked Vase Top
Cracked Vase Side

 


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#2 Tyler Miller

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Posted 25 January 2014 - 07:05 PM

It looks like dunting.  Is it possible you threw some areas too thin or overworked them and the cooling stress pulled it apart once at room temperature?



#3 Benzine

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Posted 25 January 2014 - 07:37 PM

Is the entire inside of the vase covered with the same green glaze on the neck?

 

Yes Sir, it is.  Though I will say, there is at least one bare spot, that I missed, when glazing the interior.

 

It looks like dunting.  Is it possible you threw some areas too thin or overworked them and the cooling stress pulled it apart once at room temperature?

 

There is no doubt, that I spent some time, where the neck and body meet.  The clay was a little wet, so I'd let it dry, come back to it, carefully work it, maybe let it dry some more, etc.  As I mentioned, it really did want to collapse at around that spot.


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#4 JBaymore

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Posted 25 January 2014 - 08:01 PM

Sounds like a classic case of dunting. Do what Norm said on the glazes....... to see if they are the cause. Likely are.

 

best,

 

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#5 Benzine

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Posted 25 January 2014 - 08:49 PM

It could still be dunting, even though I let it cool before taking it out of the kiln?

 

How do I measure the COE?


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#6 JBaymore

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Posted 25 January 2014 - 08:58 PM

Dunting can happen days later or longer.  It is called "delayed dunting".... like "delayed crazing" that can show up later as the stresses on the body or glaze layer eventually catch up with it and something has to let go.

 

You can't measure it easily yourself without some sophisticated lab equipment........ but you can get a good prediction in most glaze chem software.  Try the free demo version of Insight at Digitalfire.com.  It is one of the best in predicting COE.

 

best,

 

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


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#7 Benzine

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Posted 26 January 2014 - 09:24 PM

So is it, that the expansion and contraction of the glaze, is different than the body, that caused the dunting?


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#8 JBaymore

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Posted 26 January 2014 - 09:37 PM

So is it, that the expansion and contraction of the glaze, is different than the body, that caused the dunting?

 

Most likely yes. Unless that clay body was a refire or was held at over 2000F temperatuires a LONG time and developed significant cristobalite due to inadequater fluxes in the body...... and then uneven temperatures on kiln cooling could have started off the scenario.

 

If the glaze puts the body in compression (body shrinks more than the glaze..... has a higher COE than the glaze) then it is sort of like a loaded mousetrap. The glaze is pressing on the clay, trying to make it smaller. But of course it can't do that. Slowly, over time the stresses on the body cause little tiny micro-fractures within the clay walls (like the mouse nibbling on the peanut butter on the release mechanism)....... until the wall becomes weak enough that the wall fractures (trap springs).

 

A thin spot in the wall section can be the place that the things lets got firrst... that section has less strength to resiast the loading.

 

It can happen in reverse also...... when the glaze pulls on the body.... but USUALLY the glaze in tension is weaker than the body and the glaze fractures first (crazes) and releases the tension. But sometimes not enough...and it still dunts.

 

The worst cases are a glaze on one side of the form putting it in compression, a glaze on the other surrface putting it in tension, and a physically strong body that can resist the forces a bit (for a while). These can let go dramatically ... like a bomb went off... sending shards everywhere.

 

Triggers for the dunting can be changes in temperature, physical shocks, and so on.

 

best,

 

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


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#9 Benzine

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Posted 26 January 2014 - 10:53 PM

Thanks John, that actually makes sense to me.  Normally, much that you (and Norm) talk about confuse me to no end.  I'm still at the point, that I believe that Ceramics are a combination of alchemy and magic.  I suppose since the fracture isn't the doing of an angry deity, I should probably scrub the ox blood off the kiln.

 

As I mentioned in my initial post, the curve from body to neck, to get worked a lot.  So this makes sense, that the stress affected this area.

 

Would it be reasonable to believe, that if this area, were a little strong structurally, the glaze would have not been able to cause the dunting, and simply crazed?  Some of the glaze, did indeed crazy, but not in the affected area.


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#10 nigich22

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Posted 26 January 2014 - 11:43 PM

you should never apply ox blood directly to a kiln glaze or actual essence of ox it voids the warranty. 



#11 Babs

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Posted 27 January 2014 - 05:10 AM

 

So is it, that the expansion and contraction of the glaze, is different than the body, that caused the dunting?

 

Most likely yes. Unless that clay body was a refire or was held at over 2000F temperatuires a LONG time and developed significant cristobalite due to inadequater fluxes in the body...... and then uneven temperatures on kiln cooling could have started off the scenario.

 

If the glaze puts the body in compression (body shrinks more than the glaze..... has a higher COE than the glaze) then it is sort of like a loaded mousetrap. The glaze is pressing on the clay, trying to make it smaller. But of course it can't do that. Slowly, over time the stresses on the body cause little tiny micro-fractures within the clay walls (like the mouse nibbling on the peanut butter on the release mechanism)....... until the wall becomes weak enough that the wall fractures (trap springs).

 

A thin spot in the wall section can be the place that the things lets got firrst... that section has less strength to resiast the loading.

 

It can happen in reverse also...... when the glaze pulls on the body.... but USUALLY the glaze in tension is weaker than the body and the glaze fractures first (crazes) and releases the tension. But sometimes not enough...and it still dunts.

 

The worst cases are a glaze on one side of the form putting it in compression, a glaze on the other surrface putting it in tension, and a physically strong body that can resist the forces a bit (for a while). These can let go dramatically ... like a bomb went off... sending shards everywhere.

 

Triggers for the dunting can be changes in temperature, physical shocks, and so on.

 

best,

 

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

 

would this occur if the pot was of a less defined shape?  eg a cylinder or has it been encouraged by the sharp curve of Benzine's pot?



#12 neilestrick

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Posted 27 January 2014 - 10:53 AM

That curve is probably the thinnest part of the part, and also the least compressed, so it was the natural place for it to crack. It may not have happened on a perfectly even cylindrical form, but clearly there are some stresses at work there just waiting for a weak point. I assume Benzine as used this glaze combo before? This is a good lesson in how the quality of construction is important beyond just getting it to stand up long enough to get it off the wheel.

 

I had this happen on a pair of lamps I made last year, with a glaze combo I had used many many times. I decided on that pair that I didn't need to glaze the inside since it would never show, and three days after firing the glaze ripped them apart.


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#13 JBaymore

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Posted 27 January 2014 - 11:34 AM

 I'm still at the point, that I believe that Ceramics are a combination of alchemy and magic. 

 

Ahhhhhhh Don't let me mislead you into believing that that it is NOT.  Sorry.  I'm firmly convinced of that fact also.

 

best,

 

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


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#14 Rebekah Krieger

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Posted 28 January 2014 - 08:01 PM

Your green glaze is to die for! I Have been trying to find a recipe for a green that is a "faux celadon" but in that emerald shade.  It reminds me to stay on mission! <3 (i am not yet at the point of creating my own, I plan to order the jon britt video within a month or two before I get started) 


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#15 jrgpots

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Posted 29 January 2014 - 01:55 PM

[quote name="Benzine" post="51087" timestamp="1390794819"]Thanks John, ...I'm still at the point, that I believe that Ceramics are a combination of alchemy and magic.  I suppose since the fracture isn't the doing of an angry deity, I should probably scrub the ox blood off the kiln.....

Unless it's a big kiln, ox blood may be overkill...try two turtle dove. They are smaller and you can always use the feathers for you raku ware. You could say you get "two birds with one stone.". It's easier clean up too.

Jed

#16 Benzine

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Posted 29 January 2014 - 10:58 PM

Your green glaze is to die for! I Have been trying to find a recipe for a green that is a "faux celadon" but in that emerald shade.  It reminds me to stay on mission! <3 (i am not yet at the point of creating my own, I plan to order the jon britt video within a month or two before I get started) 

 

Not really "mine", just one I use.  Amaco Chrome Green.  It's a classroom favorite.

[quote name="Benzine" post="51087" timestamp="1390794819"]Thanks John, ...I'm still at the point, that I believe that Ceramics are a combination of alchemy and magic.  I suppose since the fracture isn't the doing of an angry deity, I should probably scrub the ox blood off the kiln.....

Unless it's a big kiln, ox blood may be overkill...try two turtle dove. They are smaller and you can always use the feathers for you raku ware. You could say you get "two birds with one stone.". It's easier clean up too.

Jed

Dang, why didn't I think of that?  But it might be hard to find turtle doves off season.  They've all been gifted to "True Loves".


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#17 Norm Stuart

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Posted 30 January 2014 - 02:50 AM

Notice that Chrome green glazes are a lighter green and tend to be opaque rather than translucent.

 

Your green glaze is to die for! I Have been trying to find a recipe for a green that is a "faux celadon" but in that emerald shade.  It reminds me to stay on mission! <3 (i am not yet at the point of creating my own, I plan to order the jon britt video within a month or two before I get started) 



#18 cathgill

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Posted 15 February 2014 - 01:36 PM

after many years of using the same glazes  I have suddenly had a large number of pieces simply break/dunt upon opening the cold kiln

I believe I need to make some changes to my glazes  

Looking at comments on other sites from Ron Roy I am trying to figure out if I increase the custer spar will that help????






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