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Crazing On Mugs Only!

crazing mugs cracks slip casting

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#1 PotteryLodge

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Posted 25 March 2014 - 04:10 PM

I have been using the same clay,slip and glazes purchased from a local ceramic shop. Everything has turned out beautifully but I got a new mug mold that will NOT stop giving me issues!

The mug is slip cast using low fire slip. I fire to bisque..all good. Then I apply my glaze and fire..I let the kiln cool for at least 12 hours and still around the rim and handles I keep getting light crazing. No matter what I do..less glaze, more glaze, longer firing, soaking on high, the mug will still craze. Keep in mind, I haven't changed anything from the other ceramics I'm glazing, which all turn out without crazing, just the mugs are crazing. It's weird and I can't seem to figure out why or what I can do to keep this from happening. 

If you need any other info just let me know. I have a ceramic shop and really need to fix this issue asap!

 

-Casi



#2 Benzine

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Posted 25 March 2014 - 05:56 PM

Crazing tends to be a fit issue.  The clay body and glaze, are not expanding and contracting, at the same rate, leading to the crazing.  

 

I can't really think, that a new mold, would affect anything.  I would have to say, that something is different with the glaze and/ or clay body.  

 

Commercial glazes manufacturers do change their formulas.  I can say, for a fact, that I have a couple different bottles, of the same type of glaze in my classroom, that look completely different.  So they no doubt altered the formula, which could affect the fit.

 

Beyond that, I don't know what to think.  The more learned folk here, will be of better help.


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#3 PotteryLodge

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Posted 25 March 2014 - 06:18 PM

I'm using the same clay and glaze on other ceramics and they don't craze..just the mugs. I have thought about a fit issue as well, but if I'm using the same on other ceramics, then I would expect them all to have craze, not just singling out my mugs : (



#4 mregecko

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Posted 25 March 2014 - 06:38 PM

Do you apply the glaze differently to the mugs than to your other pieces? Maybe brushing instead of pouring or dipping, etc?

 

Or is the clay a different thickness with the mugs?

 

I'm assuming your other, good pieces are slip-cast too.

 

The reason I ask is if either the glaze or clay thickness is different, it can provide a different magnitude of tensile force if there are fit issues. 



#5 PotteryLodge

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Posted 25 March 2014 - 07:02 PM

I've used the same glaze on both slip cast and hand built pieces. All are fine, no crazing. The mugs are no thicker than other pieces. Should I try a different slip?



#6 PotteryLodge

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Posted 25 March 2014 - 08:23 PM

Okay I have now narrowed it down to being my clear glaze. I have re-checked previous work and all the pieces that I've used this clear glaze with have some crazing on them, yet pieces from the last clear glaze I used (from a year ago), is craze free. 

 

I fire to 06 for my glaze and my clear glaze says to fire to 05. Now when I first bought this clear glaze, my local ceramic shop was out of the 06, but told me there would be no issue in using the 05 at 06. I guess they were wrong. Does this sound like this is the issue to everyone else reading?



#7 Babs

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Posted 25 March 2014 - 09:14 PM

How are you glazing the mugs?

 

may make subtle difference to the thickness at rims and handles

Pics would help.

There have been other posts on crazing perhaps reading these will help spot the issue. May have to play with the glaze or body to sort this.



#8 PotteryLodge

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Posted 26 March 2014 - 10:35 AM

It's too difficult to get a picture because it's so lightly crazed. I will try later to take some though...once the sun comes out a little more to help highlight the crazing.

 

I fire my bisque to 04 in an electric skutt. I fire on low for 1.5 hours medium 1.5 high until it shuts off. Am I underfiring?



#9 PotteryLodge

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Posted 26 March 2014 - 11:03 AM

how do I add a picture?



#10 Chilly

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Posted 26 March 2014 - 11:47 AM

how do I add a picture?

 

Click "More Reply Options" \at the bottom right of the reply box, then scroll down again, and click "Choose Files..."


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#11 PotteryLodge

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Posted 26 March 2014 - 12:02 PM

Okay here are two pictures of the crazing(I stained the crazing with tea so its a little darker and easier to see the cracks). I have concluded it is the clear glaze that is causing it. I am using Amaco LG-10 clear glaze. I have bought a new clear glaze by Duncan(ordered online so it should be here in a few days), so I hope this fixes the problem. I have a ceramic store online that sells a lot daily and can't afford to keep ruining pieces. What else can I do?

 

I fire bisque to 04---- low with lid prop for 1.5 hours. Then on medium for 1 hour with lid prop, then 30mins with lid closed (peep hole still open), then on high until shut off. Usually takes 5hours for a bisque fire. I have a small 120v kiln. 

 

I fire glaze to 06-----low with lid closed (peep hole open) for 1.5 hours. Medium with lid closed,peep hole closed for 1.5 hours, then high until shut off. Usually takes 4-5 hours. 

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#12 PotteryLodge

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Posted 26 March 2014 - 12:29 PM

This is the slip I am using http://www.dickblick...s-casting-slip/

 

This is the clear glaze I am using http://www.dickblick...d-gloss-glazes/

 

Though both say to fire on bisque 05 I was told by my local ceramic supplier that I could fire using my cone 04? I trusted her knowledge, being she's been a potter for 20+years...So could this be my issue? I went ahead an bought from a different shop a new clear glaze that says 04 bisque..should receive this new glaze in a few days. Should I also buy some 05 cones to test with? http://www.clay-king...brilliance.html



#13 Babs

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Posted 26 March 2014 - 05:35 PM

I bet that the glaze is crazing in other places, just not evident at this point. Soak a mug in a food dye for some time or Indian ink if you hafve an amount.

Interesting, hope you get results soon.



#14 High Bridge Pottery

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Posted 27 March 2014 - 08:05 AM

That is some interesting crazing. Sounds like it could be the glaze though if you have changed recipe.

 

Is it crazing where the seam is on the mould?



#15 PotteryLodge

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Posted 27 March 2014 - 09:50 AM

That is some interesting crazing. Sounds like it could be the glaze though if you have changed recipe.

 

Is it crazing where the seam is on the mould?

There is no crazing down the seam, just down the handle, around the handle, and around the top rim. I am using commercial glaze, therefore changing the recipe is not something I can do. I have ordered a different clear glaze, hoping this will solve the issue. 



#16 JBaymore

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Posted 27 March 2014 - 11:22 AM

Potential hypothesis here:

 

Crazing happens when the COE of the glaze is greater than the COE of the body.  BUT..... even if the COEs of the two differ, there has to be enough strees induced in the glaze layer so that the glass can't "stand" being in that amount of tension anymore (glass tensile strength is exceeded) and the glass cracks to relieve the stress.

 

When the COE of the body and glaze is close to the same, but still mismatched......... the glaze WILL eventually craze.  It just takes time and other stressors to precipitate the crazing.  The common ones are thermally induced... as in when hot liquid is poured into cups, hot food is placed on plates, sunlight is impacting on exterior tile installations, etc. .

 

So pieces that have this COE mis-match can still come out of the kiln looking just fine.  No crazing.  But the glaze is still in tension.....and is waiting to let go.  The customer will be the one to see the crazing eventually.

 

Now when the glaze layer is very THIN, the stress between the body and glass is a bit lessened.  There is less glass to be in tension when compared to the body.  The stress is still there...... but the magnitude relative to the fracturing point of the glass is below the threshold.  So it holds together (for a time).

 

So now we look at the cast mugs. 

 

The wall cross-section for the general mug is pretty thin.  So at a given bisque temperature the absorbency of that thickness of section will cause a certain thickness of glaze to adhere for a given dip / pour of liquid glaze slurry for a given amount of contact time.  BUT... the HANDLE area is far thicker in cross section, being a solid cast and being in addition to the wall section at the attachment points.  So it is possible (highly likely) that the thickness of the glaze layer in that area is greater than that on the rest of the overall mug.  If you are holding the mugs upside down as the glaze drains, then the lip area would be a bit thicker deposit also.

 

In that area the thickness allows the crazing that is going to happen to show up first.  So it is noticeable to the potter. 

 

For making functional ware....... here's a suggestion to test the fit of your glaze to your claybody for crazing.  This is a bit of a standardized stress test.  Take a few fired sample pieces and pour boiling water into them.  Empty them out after a couple of minuites.  Immediately place them in your freezer for a couple of hours.  Take them out and pour boiling water into them.  Repeat this at least 10 times.  Use the india ink test to check for crazing.  If they haven't crazed by then......... the glaze and body fit each other.

 

Firing a glaze to a higher or lower level of heat work (cone) can change the COE of the glaze.  Could be part of the issue.  All cone X glazes are not of the same chemical formulation... so the COEs can vary a lot.  So that too can be part of the issue.  Rapid cooling has NOTHING to do with the fit issue.  If the glaze fits the body...... shy of raku-ing the pieces out of the kiln........ a slightly fast cooling cycle will not "cause" crazing.

 

best,

 

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


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Professor of Ceramics; New Hampshire Insitute of Art

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#17 JBaymore

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Posted 27 March 2014 - 12:57 PM

FYI: 

 

http://www.astm.org/Standards/C554.htm

 

http://www.astm.org/Standards/C424.htm


John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

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#18 PotteryLodge

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Posted 31 March 2014 - 11:39 AM

Well the new clear glaze isn't working either. The crazing is not has severe, however it is still crazing and therefore unsellable. I'm guessing my next step is to try a different slip company? If this doesn't work I have no idea what else to do. : (



#19 Min

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Posted 31 March 2014 - 04:38 PM

Well the new clear glaze isn't working either. The crazing is not has severe, however it is still crazing and therefore unsellable. I'm guessing my next step is to try a different slip company? If this doesn't work I have no idea what else to do. : (

 

Having tried only 2 clear glazes on 1 body and expecting a perfect fit is a bit of a long shot. Might be an idea to follow John's excellent post about matching body and glaze to fit. Your current glazes have too high a rate of expansion and contraction, you need a lower expansion glaze to fit your slipware without crazing. 



#20 PotteryLodge

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Posted 31 March 2014 - 05:07 PM

 

Well the new clear glaze isn't working either. The crazing is not has severe, however it is still crazing and therefore unsellable. I'm guessing my next step is to try a different slip company? If this doesn't work I have no idea what else to do. : (

 

Having tried only 2 clear glazes on 1 body and expecting a perfect fit is a bit of a long shot. Might be an idea to follow John's excellent post about matching body and glaze to fit. Your current glazes have too high a rate of expansion and contraction, you need a lower expansion glaze to fit your slipware without crazing. 

 

I've actually used 3 different clear glazes, but the first (which worked), was discontinued. I'm using commercial glaze so the ingredients are not noted, therefore it's more difficult to find a low expansion glaze. I'm switching slips tomorrow and will see if this helps. 







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