Cups cracking in the final glaze firing!

[Rick Wise]

After several years of successful cup making I have recently had a rash of cups that come out of the glaze firing (electric, cone 6) with invisible cracks in them.  The cracks run generally top to bottom and are hard to see until you know they are there.  But if you thump them the flat sound gives it away.  Some are ok at first and crack only upon being filled with hot coffee for the first time.

I'm looking for an explanation.  My hypothesis is this:

I have recently been trying to get more weight out of them at trimming.  I start with 16 oz of clay and try to trim down to less than 10 oz at leather hard stage.  To do this I have been trimming more aggressively on the top half of the cup and not just at the base.  So, as a result, the wall thickness of the cups issomewhat thinner.

Does anyone have experience with this sort of thing?  Do you think the relatively thin walls could be the reason they are cracking in the kiln?  Comments and speculation welcomed.

[liambesaw]

I'm wondering if your clay body is defective since you have been having trouble with stuff cracking when fired.  Seems like a reasonable area to explore

[Rick Wise]

Doubt it as it is the same body I have been using for some time.  Same clay, same glazes, same techniques other than more trimming of the upper walls. 

[liambesaw]

Just now, Rick Wise said:

Doubt it as it is the same body I have been using for some time.  Same clay, same glazes, same techniques other than more trimming of the upper walls. 

But is it the same batch?

[Babs]

Been discussions in this forum re glaze fit.  Thinner pot making it more evident?

What are they sounding like after the bisque?

All you glazes or just one?

Same glaze inside as out?

Can yo throw a few thin ones to see if stressing on trimming?

[Min]

3 hours ago, Rick Wise said:

Do you think the relatively thin walls could be the reason they are cracking in the kiln?

Sharp edge to the cracks, not rounded into the crack? If the glaze is on the borderline of putting too much compression on the glaze with the wall thickness you used to make them and now they are thinner it could be enough to push them into a cooling dunt. Clay can't take the compression the glaze is now putting it under. Break one open and see how thick the glaze is (especially on the bottom) compared to the clay thickness. I'ld take your remaining mugs and freeze them overnight then put them in the sink and pour boiling water into them. Clay:glaze mismatch of COE's can show up days/weeks after unloading, freezer/boiling water test will stress them so (hopefully) if they are going to dunt they will do so with this test.

I'm currently working on porcelain casting slip recipes with clear glaze. Little test pot below came out the kiln in 2 pieces, ill fitting glaze.

[Babs]

@Min anyctip on "loosening" a tight glaze.

last batch of clay doesn't like my glaze

[Min]

Increase the expansion of the glaze. What's in the glaze?

[Babs]

Will post later. Not in shed.

[liambesaw]

Sodium

[Bill Kielb]

5 hours ago, Rick Wise said:

Do you think the relatively thin walls could be the reason they are cracking in the kiln?  Comments and speculation welcomed.

It’s certainly possible. Why not make a couple the old way and a couple the new way and see if it’s the clay or the trimming. Personally when I want very light thin wares I try and throw them that way, then no trimming at the top  or excess stress on it trimming inverted. It’s too easy to introduce really minor cracks  or make it go out of round. My thin stuff needS to be mostly free of any major trimming as thrown. Based on the outcome of your test above, you might then try experimenting with  throwing thinner and pre trimming the bottom as much as possible before removing from the wheel.

[Babs]

9 hours ago, liambesaw said:

Sodium

Chloride???:-)))

[liambesaw]

2 minutes ago, Babs said:

Chloride???:-)))

That would be a mess!  More like substituting some potash Feldspar for neph sy or soda spar.  Easiest way I can think to add expansion.  

[Babs]

51 minutes ago, liambesaw said:

That would be a mess!  More like substituting some potash Feldspar for neph sy or soda spar.  Easiest way I can think to add expansion.  

ThankThankss. Will get to the recipe tomorrow and see what you guys come up with

[Mark C.]

Glaze fit on that clay body . Thin pots should not crack-but improper glaze fit can crack them.

[Babs]

@Min

Here is my recipe.

Cornish stone   65

Frit3110              20

Calcite                 10

Zinc oxide dense  5

Addition:

Tin oxide                   4

Good majolica but now a bit tight for clay company sources from different region.

Thanks

[Min]

@Babs, so for sure the pots are getting dunting with the new clay and this glaze? Easiest way to increase the COE without adding any new materials would be to increase the 3110 frit. It's going to throw the flux ratio off a bit but should be okay still. Glaze some test cylinders (thinly thrown or slabbed) on the inside only, apply the glaze thickly, you want to create the perfect storm for dunting here. Take X amount of the glaze and add 5% more 3110, glaze a test cylinder and repeat. I'ld keep doing this until you get up to an additional 20% of 3110. After firing freeze them then pour boiling water into them and see if they dunt. If you don't want to do such an extreme test then glaze both the inside and outside with the glazes. Adjust the tin once you figure out how much frit you need to add.

[Babs]

Thanks Min. Have a time coming up to play so will throw a few cylinders for this purose. May take glaze just over the lip to make beakers for grandkid drinking if they survive.

[Min]

Other approaches to take also if this doesn't fix it but I think it's worth testing. Not knowing what spars, frits etc you have in Australia makes this a bit tricky.

[Babs]

@MinThanks Min.

Most Ferro frits.

But will start as you advise.

Will also try anoter liner glaze as don't always need the maj. factor just love the eyeballing of this glaze

[Rick Wise]

I have concluded to my own satisfaction that I got a bad batch of clay from Standard.  After changing clays all the problems disappeared as quickly and as mysteriously as they had arrived.  I also noticed that the un-used clay in the bag had, after sitting a while, begun to develop fissures in it.  Weird -- but glad to know this is possible.

[oldlady]

rick, are you now going to continue to trim the tops or try to throw and trim at the same time to make the walls even?

[Rick Wise]

Hi Oldlady!

I may have been unclear earlier.  It was never a matter of uneven-ness.  I always trim (or turn as the Brits say) my pieces both to get the shape I want as well as to reduce weight (I lack the skill to throw as thin as I would like).

So, in trying to get weight out of the cups I would trim up and down the sides of the cup to make the wall thinner.  I will still do this as I like them to be as light as possible.

So .... you single fire everything?

[Rick Wise]

On 9/15/2020 at 2:01 PM, liambesaw said:

I'm wondering if your clay body is defective since you have been having trouble with stuff cracking when fired.  Seems like a reasonable area to explore

You win the prize liambesaw!  It was BAD CLAY!

[oldlady]

i only bisque if i am in florida making things to sell later.  the florida studio is small and i need to harden the pots to drive them home a thousand miles.  single firing is so easy for the slab based  things i make and  because i do not have to do liner glazes in mugs.    i leave throwing to the rest of you folks.   i only throw bowls for the empty bowl supper.

my plans are to widen out in future to make what i really want, undersea coral-like formations as sculpture.