Problems with dunting

[birdypotter]

Me again! I recently posted a topic (here) about work cracking. A different issue (the absorption levels of the clay and possibly the use of a commercial liner glaze) was identified and so my focus was on fixing this and hopefully, in turn, my cracking problem would disappear. But it's back! After shock testing test pieces (freezer overnight and then boiling water), the image shows a hairline crack that wraps around at leats half the cylinder on the outside with smaller cracks coming off at right angles (highlighted with ink). The ink has been painted on to the gloss liner in the interior but the cracks haven't materialised on the inside. 

I am using Earthstone ES5 clay from Scarva that I tested in the post above and know that the absorption for this clay is on average 2.1% at 1230˚c/2246˚f   in electric oxidation. The inside is a high calcium gloss and the outside is a satin matte white, Pike's Oatmeal, from John Britt's Mid Range Glazes book (see below for recipes...I tried to attach insight images but to no avail). In the last post Min helpfully made me aware that particularly with calcium mattes, the CTE of glazes can be misleading so I don't want to rely too much on judging the outer glaze by its CTE values. Another cylinder, the same clay and inner glaze but with a different outer glaze in black, has odd ridges that have formed after firing, tried my best to show this in the photo. The glaze info for that particular glaze has also been included (see black satin matte glaze info).

My questions are - is this crazing? On either? The white exterior glaze shows a CTE of 5.9 and inner gloss of 6.3. I would have expected crazing from the gloss rather than the satin exterior. Is there too big a mismatch between the CTE of the inner and outer? If not crazing then by looking at the cracks and surface issues here is there another problem? Perhaps in the making process? How do I fix this? 

I'm going a bit craze-y here 

Any help appreciated!!!

Satin Matte White (Pikes Oatmeal):

f-4 feldspar	37.400
silica	22.700
kaolin	4.500
dolomite	6.400
talc	13.600
Gerstley Borate	12.700
zinc oxide	2.700

+tin oxide 10%

+rutile 0.2%

Black Satin Matte:

Soda Feldspar	5.900	5.41%
Silica	12.700	11.65%
Wollastonite	19.600	17.98%
China Clay	36.300	33.30%
Talc	13.700	12.57%
ferro frit 3124	11.800	10.83%

+ mason stain 6600 3%

+rutile 3%

+ red iron oxide 1%

High Calcium Gloss:

nepheline syenite - 4.000

quartz - 17.000

wollastonite - 29.000

china clay - 30.000

frit 3195 - 20.000

[liambesaw]

Doesn't look like crazing to me, looks more like dunting.

[birdypotter]

Would dunting show as a crack all the way through or not necessarily? Certainly the crack looks similar to the one here on pg 119...

[liambesaw]

2 minutes ago, birdypotter said:

Would dunting show as a crack all the way through or not necessarily?

Eventually the piece explodes.  You can get this result from glazing only the inside or only the outside of a piece, you can get it from using a liner and outside glaze that are too different in coe, you can also get it from cooling too quickly through quartz inversion.  

A long crack in the glaze like that is a ticking time bomb though.  The glaze and clay body are merged and a crack in the glaze is essentially a crack in the body.

[birdypotter]

7 minutes ago, liambesaw said:

Eventually the piece explodes.  You can get this result from glazing only the inside or only the outside of a piece, you can get it from using a liner and outside glaze that are too different in coe, you can also get it from cooling too quickly through quartz inversion.  

A long crack in the glaze like that is a ticking time bomb though.  The glaze and clay body are merged and a crack in the glaze is essentially a crack in the body.

Perhaps the outside glaze is too much of a mismatch in COE from the inner - it has a high percentage of tin oxide which really brings the COE down. May need to find a glaze more closely matched to the 6.3 of the gloss?

[liambesaw]

17 minutes ago, birdypotter said:

Perhaps the outside glaze is too much of a mismatch in COE from the inner - it has a high percentage of tin oxide which really brings the COE down. May need to find a glaze more closely matched to the 6.3 of the gloss?

I don't know, I'd wait til a member who knows a bit more comes in.  I'd do some tests to see if it happens when the piece is all the same glaze and such 

[Callie Beller Diesel]

4 hours ago, birdypotter said:

Would dunting show as a crack all the way through or not necessarily? Certainly the crack looks similar to the one here on pg 119...

If the piece is reasonably fresh out of the kiln (last few days or so), then yes the crack might not be visible on the other side yet. Hold the pot by the base and give it a flick. If it rings, the crack is only in the glaze. If it makes more of a clicking or thunking sound, the crack is through the clay, and is a dunt.

I agree that the crack in the white pot at least looks like illustration N, which is described as "The crack is a dunt caused by the difference in expansion/contraction rates of body and glaze. The stress between the body and glaze over most of the pot is satisfactorily absorbed by the body glaze layer. However at the roll there is an exaggerated stress which is sufficient to cause the dunt." 

What I suspect might be happening in the white piece because there's no glaze "roll" or drip, is that there's a variation in the wall cross section that put additional stress on a missmatch between the coe of the two glazes and the clay. This would be confirmed if you were to break the pieces in half to see what's going on. If this theory is correct, we have some more information to work with. If I'm wrong, then we look at glaze coe's.

Firing and cooling cycles are important to fixing dunting too. What are you using for your firing cycle, and how tightly packed is your kiln? is there any thermal mass to slow cooling?

[birdypotter]

Hi Callie, thanks for your response.

I did the flick test on it today and it rang rather than clunked. It has been out of the kiln for about 2 weeks, and the freezer to boiling water test was done today, though the crack very well could have been there prior to this as I didn't paint ink on to reveal before I did the freezer test. The kiln for this particular load was pretty light as it was mostly tests, evenly distributed across 3 shelves in my Skutt top loader but definitely far from tightly packed.

My firing cycle is a slow cooling cycle to allow for the matte glazes and is as follows (all in ˚c) taken from John Britt book and adjusted slightly:

83˚c up to 104˚c

278˚c up to 1081˚c

83˚c up to 1230˚c with a 10 min soak

275˚c down to 1038˚c

60˚c down to 760˚c

natural cooling from then on.

(I slow bisque to cone 05 just for reference)

I will break it into bits tomorrow and report back on wall thickness but hopefully in the meantime the above info might reveal more about what might be going on!

 

[liambesaw]

Since it was lightly packed, do you know how slowly it cooled around the 573c mark?

In a lightly packed kiln you aren't getting the slow cooling once the elements are off because there's not a bunch of stuff in there radiating heat like there would be in a nicely packed kiln.  So if there was a chance it cooled a little too quickly through quartz inversion (573c), that can cause dunting by itself regardless of the glazes on it.

[birdypotter]

I wouldn't be able to tell you how quickly or slowly it cooled through quartz inversion unless I packed the kiln to a similar standard and sat with it to time its cooling through this point...the link above with the info on different crack types mentioned the potential for dunting through quartz inversion so you may have hit on a further contributing factor. Perhaps I need to add an additional cooling ramp to take the kiln load down slowly through QI? Ahhhh, so many variables! I will enjoy smashing them apart tomorrow and post some pictures.

[Callie Beller Diesel]

You’ve also sped quite rapidly through quartz inversion on the way up. That won’t do anything any favours either.

I’m probably the only weirdo who doesn’t own John Britt’s book (yeah, yeah, it’s on the list!), so I have no idea what the original cycle looks like. I’m assuming you’ve edited some steps out on the climb.

Slowing things down so that pieces move evenly through quartz inversion would be a  good place start.

How big is your kiln? Having a sparse load on 3 shelves in a 3 cu ft kiln is different than in a 7. How many hours did this particular kiln take to cool down take after your kiln shut off?

[birdypotter]

The only parts I changed in the original firing schedule were that I fired to 1230 rather than 1218, and I cooled the kiln from peak temp to 1038 at 275/hr rather than the suggested 555. No steps missed out on the climb or the cool! 

Original cycle is:

83˚c up to 104˚c

278˚c up to 1081˚c

83˚c up to 1230˚c with a 10 min soak

555˚c down to 1038˚c

56˚c down to 760˚c

Granted the kiln is quite big (277l or 9.7 cu ft) but I do try to absolutely pack it with tests, and this had 4 levels with several cylinders on each (and some plates and test tiles) if you include the bottom shelf. Though definitely not rammed cheek to cheek with pots, so for sure there wasn't enough work in there to adequately hold on to and release the heat slowly. I turned it on at 8.30 in the morning, it had reached temp about 5-6pm and was able to open it about half 5 the following day when it was at a bout 50˚c - that's a rough estimate of times as far as I can remember without having my notebook here - its in the studio and I'm where I should be at midnight in the UK - tucked up in bed!

As well as investigating the potential problems of uneven wall thickness tomorrow or mismatched glaze COEs, perhaps slowing down the firing schedule at quartz inversion on the way up and the way down might help? How slow would you suggest? 

[birdypotter]

I have broken the cylinder! See attached picture. I'm not the most proficient of throwers - not been throwing all that long and self taught so I'll leave it up to the experts to assess. I think it might be too thick in the corner. Whether or not this is enough to cause a problem I'm not sure....it may be a combination of this and mismatched glaze COEs + too rapid firing through quartz inversion?  

[Bill Kielb]

Thickness looks decent,  It’s hard to zoom in on the picture and see the crack so I am wondering if this was a micro crack before the firing.

As far as quartz inversion clay and glaze go through it every day thousands and thousands of times with every bisque and glaze firing. Not only that,  different parts of your pot go through this at different times as a pot is rarely the same temperature from top to bottom, front to back, inside to out in your kiln. Quartz inversion just happens, on the way up and on the way down so it would be low on my list of likely having caused this.

any chance you can get a close up of the cracked section. Just trying to see if it extends into or through the body.. It also appears that there is a corner crack (Bottom corner) and if I  am right there is a hole in the bottom of this so it is not necessarily a mug. Might just be the picture though.

[birdypotter]

The picture above doesn't show the original crack, only the cross section to reference thickness. 

I found it pretty difficult to get close ups but I've done my best (also I really did smash it to smithereens so its harder to photograph)!

The crack in the corner was created when I dropped it, it wasn't there before and there is no hole in the bottom, just a simple cylinder, so I think its just the picture! It broke right along the hairline crack when I smashed it - not sure if this indicates a dunting crack or whether, if it was a crack in the glaze, that this was enough to cause an area of weakness.

[Bill Kielb]

Fascinating picture! My guess at this point is the material and shape (Claybody) could not withstand the test. Not necessarily crazing although a body that’s glazed with a glaze that keeps it slightly in compression is considerably stronger than one without.  Up to 20% stronger actually. This is true right up to the point of crazing. Since you have two different glazes the test likely is affected by that  as well.

still thinking, but I would say at this point not traditional crazing as most know it.

[Roberta12]

If I read the website for this clay correctly, it is rated mature up to cone 9.  Would that have been an issue??  I have had problems with clays that have that wide maturation.  You know, like 5-9 or 5-10?    

just wondering.

 

[birdypotter]

17 minutes ago, Roberta12 said:

If I read the website for this clay correctly, it is rated mature up to cone 9.  Would that have been an issue??  I have had problems with clays that have that wide maturation.  You know, like 5-9 or 5-10?    

just wondering.

 

Hi Roberta

Yes with my previous clay I had similar issues - it was absorbing too much water which definitely could have led to cracks. However though the firing range for this clay is pretty wide which does cause problems, I did some absorption tests and found that this absorbed about 2.1% on average at 1230˚c, which is fairly good in comparison to my previous clay. I'm hoping that this isn't an issue or I'll be looking at changing again!! (I've linked the thread that had those issues at the top of this thread if you want to have a look, might shed some light). Hopefully it fired to 1230 gives it enough vitrification, but who knows!

[Bill Kielb]

9 minutes ago, birdypotter said:

Hi Roberta

Yes with my previous clay I had similar issues - it was absorbing too much water which definitely could have led to cracks. However though the firing range for this clay is pretty wide which does cause problems, I did some absorption tests and found that this absorbed about 2.1% on average at 1230˚c, which is fairly good in comparison to my previous clay. I'm hoping that this isn't an issue or I'll be looking at changing again!! (I've linked the thread that had those issues at the top of this thread if you want to have a look, might shed some light). Hopefully it fired to 1230 gives it enough vitrification, but who knows!

Interesting in that I wonder what the lower temperatures do to its ultimate strength and finished body coe? Likely not as strong as fully vitrified.

[liambesaw]

1 minute ago, Bill Kielb said:

Interesting in that I wonder what the lower temperatures do to its ultimate strength and finished body coe? Likely not as strong as fully vitrified.

Not to mention formation of extra cristabolite in clay bodies with underfluxed silica.  This can result in dunting in itself.

[Mark C.]

The whiote pot in photo is dunted in my eyes

[birdypotter]

A frustratingly wide pool of potential issues and solutions. Firing higher would mean changing the glazes, firing to 1230 would mean changing the clay. Unsure as to whether the COEs of outer and inner glazes are too mismatched either and that has caused dunting. I'm at a loss!

[Roberta12]

1 minute ago, birdypotter said:

A frustratingly wide pool of potential issues and solutions. Firing higher would mean changing the glazes, firing to 1230 would mean changing the clay. Unsure as to whether the COEs of outer and inner glazes are too mismatched either and that has caused dunting. I'm at a loss!

I have switched clays.  I do not use clay with a wide firing temp.  There are a lot of choices out there.  And while I will be on the search for the perfect clay to the end of my days, it was easier for me to switch clays.   However, I have tried a couple of new clays recently and I use clear for a lot of my work, I found I did have crazing issues.  And I am now in the process of trying different clear recipes (thanks @Min)  

Roberta

[Bill Kielb]

I feel your pain but like most of clay, testing is the final decider. We are at  a point where full vitrification is a must along with  glaze durability and then testing for our functional wares.

All very time consuming though.

[Min]

@birdypotter, re your photo of how far the black ink has soaked into the clay, looks like the ink has soaked into the clay wall about 1/2 the way through the wall at least.  With the absorption figure you had of 2.1% this shouldn't happen.

Looks like dunting to me too but if you want to be sure then to confirm if the glaze is causing the dunting make 5 cylinders. Glaze #1 inside and out with just the liner glaze. Glaze #2 inside and out with the Satin Matte White (Pike's Oatmeal) and glaze #3 both inside and out with the Black Satin Matte.  Glaze #4 with a thick heavy layer of the liner on the inside and Satin Matte White outside and glaze #5 with thick heavy layer of liner on the inside and Satin Black outside.

Fire them together with a cone next to them to verify cone reached then do your freeze / boiling water test with all 5 cylinders. 

If #1, #2 and #3 don't dunt and the others do then it's a glaze mismatch causing the dunts.