Problems with dunting

[birdypotter]

6 minutes ago, Min said:

@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.

Thanks Min, I'll try this. 

Assuming I did the absorption test correctly and got the correct figures (boiling for 5 hours, soak for 19, weighed before and after and calculated), how is the ink soaking in that much if the cylinder was fired to the same temp as the test bars? unless 2.1% is still far too much in terms of absorption?

[Babs]

Think lower firing programme too fsst.

Change to

After candling if ware is moist

100degC to 600

[Min]

6 hours ago, birdypotter said:

how is the ink soaking in that much if the cylinder was fired to the same temp as the test bars? unless 2.1% is still far too much in terms of absorption?

No, the ink shouldn't be soaking in as much as it did if the absorption is only 2.1%

I agree with what others have said about slowing the firing down. Also, don't unpack the kiln (either bisque or glaze) if the bottom of tall pots are still substantially hotter than the tops. Bottoms will naturally hold heat in for much longer due to sitting on the kiln shelves.  Cracking can occur if pots are removed while there is a great temp difference between the top and bottom of the pots, more so if the bases are thick.

[birdypotter]

7 hours ago, Min said:

No, the ink shouldn't be soaking in as much as it did if the absorption is only 2.1%

I agree with what others have said about slowing the firing down. Also, don't unpack the kiln (either bisque or glaze) if the bottom of tall pots are still substantially hotter than the tops. Bottoms will naturally hold heat in for much longer due to sitting on the kiln shelves.  Cracking can occur if pots are removed while there is a great temp difference between the top and bottom of the pots, more so if the bases are thick.

Could it be that the cylinders haven't reached the same level of vitrification as the test bars did? Perhaps because they are larger? I read yesterday on digital fire that sometimes bases in particular don't reach the same cone as the walls and can be underfed, therefore under vitrified. Could that have potentially happened here? Think I need to test some more cylinders but for absorption as well. 

8 hours ago, Babs said:

Think lower firing programme too fsst.

Change to

After candling if ware is moist

100degC to 600

Babs, would you suggest just slowing down only on the way up to 600 or including a further slow down on the cool down? I cool down from 1038 to 760˚c at 60 degrees an hour to achieve mattes, do I need to add an additional cooling ramp on to the end of this?

Just as a matter of interest, I was looking at a table of clays from supplier Potclays here in the UK - they give the firing range but also within that the vitrification range which is handy. None of the stoneware clays vitrify in the cone 6 or even cone 7 range...I imagine it might be a similar story for the clay I'm using - so how are people making functional ware the popular cone 6 (considering the John Britt book and Mastering Cone 6 glazes both talk about firing to 1220ish)? Am I missing a trick here? Or is there an accepted level of absorption for functional work that doesn't reach full vitrification? 

Anyway thanks for all of your suggestions and input. I'll test the cylinders and see if the glaze mismatch is the problem. Will also test the Pike's Oatmeal with a combo of zircopax and tin oxide to raise the COE to close of that to the inside and see if that helps without changing the quality of the white too much.

Will try to remember to update for any that find it helpful.

[liambesaw]

There are clays in the states that vitrify at cone 6 with less than 2%.  I can only assume there are the same in the UK.

[birdypotter]

1 hour ago, liambesaw said:

There are clays in the states that vitrify at cone 6 with less than 2%.  I can only assume there are the same in the UK.

The clays listed that vitrify at 1220 are listed as earthenware - I've been working with stoneware but perhaps need to change to earthenware. Would you think 2% is too high an absorption rate for functional ware? Am I looking more for 1% or lower porosity? I've read differing opinions so not sure!

[Min]

4 hours ago, birdypotter said:

Would you think 2% is too high an absorption rate for functional ware?

I would redo your absorption test. To save starting all over again from scratch you can dry out the original unglazed piece you tested, put it in the oven (150C or thereabouts, doesn't have to be specific) and really give it time to dry out, leave it in there for several hours. I would also check the calibration on your scale with a known weight, this needs to be precise. Weigh the test piece after drying it in the oven and get it down to the weight it was when it came out the kiln.

Repeat the boil and soak and reweigh it. If you have more than one test piece then do them also.

2% should be okay for functional work but I'm questioning if it really is just 2.1% given how the ink soaked into the pot.

6 hours ago, birdypotter said:

Just as a matter of interest, I was looking at a table of clays from supplier Potclays here in the UK - they give the firing range but also within that the vitrification range which is handy. None of the stoneware clays vitrify in the cone 6 or even cone 7 range...I imagine it might be a similar story for the clay I'm using - so how are people making functional ware the popular cone 6 (considering the John Britt book and Mastering Cone 6 glazes both talk about firing to 1220ish)? Am I missing a trick here? Or is there an accepted level of absorption for functional work that doesn't reach full vitrification?

This comes up frequently with potters from the UK. There are wide firing claybodies on this side of the pond too but we have a better choice of claybodies that fall within a narrower firing range, ie: white lowfire and earthenware (up to cone 1 or 2 but usually cone 04-03) then midrange in the cone 4-7 range then highfire in the cone 9+ range. In the UK there are some bodies that are called earthenware but will go to cone 6 so there is a difference in terms between the UK and North America. I would double check your absorption figure and if you find it's higher than the 2.1% from your first test then look for a clay that will top out at cone 6 (1220C approx).

[birdypotter]

1 minute ago, Min said:

I would redo your absorption test. To save starting all over again from scratch you can dry out the original unglazed piece you tested, put it in the oven (150C or thereabouts, doesn't have to be specific) and really give it time to dry out, leave it in there for several hours. I would also check the calibration on your scale with a known weight, this needs to be precise. Weigh the test piece after drying it in the oven and get it down to the weight it was when it came out the kiln.

Repeat the boil and soak and reweigh it. If you have more than one test piece then do them also.

2% should be okay for functional work but I'm questioning if it really is just 2.1% given how the ink soaked into the pot.

This comes up frequently with potters from the UK. There are wide firing claybodies on this side of the pond too but we have a better choice of claybodies that fall within a narrower firing range, ie: white lowfire and earthenware (up to cone 1 or 2 but usually cone 04-03) then midrange in the cone 4-7 range then highfire in the cone 9+ range. In the UK there are some bodies that are called earthenware but will go to cone 6 so there is a difference in terms between the UK and North America. I would double check your absorption figure and if you find it's higher than the 2.1% from your first test then look for a clay that will top out at cone 6 (1220C approx).

What good timing Min, this pinged up just as I was reading this thread!

I think you're right, there is a little bit of a gap in clay availability made specifically for cone 6. Every clay body seems to have such a wide firing range, and the Potclays data sheet for a promising clay at earthenware was listed as having a firing range of 1100 to 1220 but a vitrification range of 1180 to 1250, which falls outside the firing range. The wide firing ranges given and lack of information readily available on other clays that Potclays don't sell, i.e. the lack of ACTUAL vitrification temp is a bit frustrating.

Soooooo slowly losing the will to live but back to testing to check the ink issue. When I washed the ink off the dunted crack I didn't just wipe off, I ran it under a tap at full blast....wondering whether I forced the ink inward. Either way it still absorbed so I will redo the absorption tests....using ohaus cs series and have just double checked to make sure they are calibrated with weights I bought for this purpose- they are. So at least it's not the expensive scales  

[Min]

@birdypotter, 

56 minutes ago, birdypotter said:

Soooooo slowly losing the will to live but back to testing to check the ink issue.

You'll get it solved, it just takes some trial and error. Better to get it sorted out now rather than after you've made a few kiln loads of pots. Get the claybody sorted out first then worry about the glazes. There is always something to test for or alter with ceramics, it's all part and parcel of working in this area. (that's probably the biggest understatement I've ever made!) Not saying it isn't frustrating but if you can accept that it's part of the craft then it's easier to move along.

[birdypotter]

3 minutes ago, Min said:

Better to get it sorted out now rather than after you've made a few kiln loads of pots.

Absolutely. So easy to feel defeated with ceramics but the feeling of getting it right is worth the frustration of getting it wrong. I'll soldier on!

[Babs]

Only on way up unless your kiln plummets downward or you are firing an almost empty kiln imo.

An older potter was stunned when I asked her to put her red/  coloured  clay coffee mugs she'd made since day one pf her potting life, filled with water,  on a piece of paper overnight. We were having a conversation about how high we could fire before losing the attractive terracotta colour.

She was shocked when the paper was damp the next morning.

Never had any complaints in 50+ years of potting.

( after a few cups of milky tea  I wonder if it would seal??????) Do not follow this practice at home:-/

However a mug left on the grandpiano or a vase of blooms , well....

Not trivialising this matter just adding to conversation,  observations.

[neilestrick]

I don't think it's a firing speed issue. If it survived bisque without cracking it should be able to survive a relatively quick firing and cooling, especially since it's not a super tight body. In my experience, horizontal and diagonal cracks in a cylinder point to either cristobalite issues or glaze COE issues. Because we're seeing it on two different glazes, and on pots that aren't all that thin, I'm blaming the clay. I don't think changing the firing speed is going to improve the situation. I'd first try firing hotter. Even just 1/2 cone may help, which you can accomplish with a 10-15 minute hold at the end of the firing. It'll tighten up the body, but hopefully it won't be enough to mess up how the glazes look.

[birdypotter]

4 hours ago, neilestrick said:

Because we're seeing it on two different glazes, and on pots that aren't all that thin, I'm blaming the clay.

On reflection, the ridges on the black glaze I think are caused by the slight groggy texture of the body - I think they're the throwing/trimming lines as the pot still rings when flicked, there's no ink settling in to the cracks permanently (I tried with white ink on the black outside) and its started since I changed to this clay but have only noticed it now. They're more dents where the glaze is settled rather than cracks. I think I can fix that just by taking more care to burnish the surface after trimming with a rubber kidney and not leave the obvious scars of trimming in. However I think your suggestion of raising the firing temp is a good idea regardless @neilestrick. When I did my absorption tests on this clay it came about between 1.4% and 2.7% at 1230 but on average about 1.2% at 1240 which was way better and tells me more about where this clay is likely to vitrify - though my glazes were too affected at 1240 so I stuck to 1230. I already have a 10 min soak but think I might increase it to 20 and see what difference that makes. Eventually I might have to go to 1240 and start thinking about changing the glazes if it turns out not to be COE mismatch.

Seeing as I think the issue is only on one glaze now the COE mismatch is still a contender, so will still do all the tests mentioned above. And I don't think slowing the firing a little on the way up can hurt to try @Babs so I'll give it a go!

[Callie Beller Diesel]

@birdypotter forgive me if this is a dumb question, but are you doing your absorption tests on your glazed or unglazed pieces? Something about the way you worded one of your posts in that other thread and how much that ink is soaking in is making me wonder.

[Babs]

Is it soaking in or moving into an existing crack?

[birdypotter]

7 hours ago, Callie Beller Diesel said:

@birdypotter forgive me if this is a dumb question, but are you doing your absorption tests on your glazed or unglazed pieces? Something about the way you worded one of your posts in that other thread and how much that ink is soaking in is making me wonder.

Not a dumb question, I might not just have been clear enough. I do my absorption tests on unglazed bars of my clay that have been fired up to my glaze temp  I followed the Digital fire instructions - is that the correct way to do it? I'm wondering whether Babs is right it's moved in to an existing crack as there was another smaller dunting crack around the other side that I put ink in to and when I broke that portion apart it didn't have the same issue. But yes, it could definitely be an absorbency issue.

However I won't know for sure until I've finished redoing absorption tests just to double check my figures! I will be sure to update when I've done these and the tests Min suggested. 

[Callie Beller Diesel]

No, you’re doing it right.  Carry on. 

[Pres]

Cooling dunt to me also. Considering the effort you are putting into this, I would do a few things, fill the next load to full, try firing down (using the controller or switches), and make certain not to open kiln early. Much of the advice above is useful, and should be followed especially when considering the testing of COE, and your absorption. 

There have been several threads on this site stressing the problems with wide range clay bodies. I have had my own problems with a body that was meant to be the "do all". . . . it did nothing well!  Because of that I would consider changing clays if you are not too invested in this clay body. I could understand hesitancy to change, as I have just purchased a ton, and would not want to change if I found I was having problems.

I had some problems years ago with cooling dunts that you would not believe, bisque pieces that cracked in spirals, glazed pieces that had base cracks both vertically and horizontally. In the end, I changed up firing and down cooling so that it did not happen again. Experimentation can be expensive and frustrating, but in the end you learn a ton of stuff about kiln firing and temps. Remember also that a kiln that has fired can still be monitored by a color temp chart. This will help to approximate the cooling speed. Testing is probably your only way out.

 

best,

Pres

[birdypotter]

5 hours ago, Pres said:

try firing down (using the controller or switches)

 

5 hours ago, Pres said:

I changed up firing and down cooling

Hi Pres, thanks for the advice. May sound dumb here but unsure as to what you mean by firing down and up firing?

Though I do have a fair amount of this clay, I wouldn't be averse to changing if it is indeed the issue - stubbornness won't solve my problem! It's difficult to find a clay here that is clearly rated for cones 4-7, for example. Almost all clays here are advertised as having a wide firing range and vitrification ranges aren't usually advertised or recommendations are that 'the clay matures at 1200 and above' which is obviously totally inaccurate when you're looking at specific applications for the clay such as durable dinnerware.

If my absorption tests come out as the same as last time - which is on average 2%, do you think it would be necessary to change? I know the aim is for 0% porosity with porcelain but I have read this is much less likely with stoneware and 1-2 is to be expected.

I tested Earthstone ES10, which is a smooth, lower fire version of my current clay, for absorption when I first tested ES5 (my current clay) and its absorption at 1230 was about 0.3% ...much more appropriate. The only reason I didn't choose this was because I slab press my plates in plaster slump moulds rather than throwing them and wanted the slight groggy tooth of ES5 to aid with warping and lifting of the plates in the centre. I dry my plates on plaster bats to evenly absorb the moisture, wax the rims immediately after making to avoid them drying out before the centre and weight with rice bags to try and prevent lifting when drying them under plastic over 1-2 weeks. I've not been with this clay very long but that seems to have worked with the few I've made this far...so it may work with the ES10 - as you said Pres, testing testing testing!

 

[Pres]

birdypotter, Firing up and firing down is to fire to cone target using a your normal firing, and then using your kiln switches control the cool down of your kiln. Most newer kilns now have a controller that will allow them to fire up and fire down thus slowing the cooling process. If you only have sitter, you can raise the brace use a twisty tie to hold it in place, and then use one, two or three switches on low to fire down. . . slowing the cool down. This will often help with dunting if your kiln is cooling too quickly. Older kilns had a tendency to be thinner walls, and cooled faster. Most newer kilns are thicker. . .2 1/2 to 3", at the same time lid thickness matters, where I used to have two inch lid, when I replaced it, I bought a 2 1/2 inch lid. These are some of the ways I have overcome cooling dunts. If you have not read any of my earlier posts about firing, my kiln is completely manual. . . . no sitter, and no controller. All my firings are done by hand with careful changes of switches to keep a balanced firing. Cone tests indicate I am with in 1/2 cone temp from top to bottom on most firings.

Most problems with clay bodies may be overcome, and you may need to call the manufacturer and querry the problems you are having and ask them for solutions. Their tech people should be able to help you out.

 

best,

Pres

[birdypotter]

Ah I see, thanks for clarification Pres. I'm in awe of how you fire, I feel frankly amateur with a controller doing it all for me! 

I am able to program some extra cooling cycles in to my firing program. I drop the temp fairly rapidly from peak down to 1038 and then from there down to 760 I cool very slowly to achieve the semi matte surfaces, but perhaps I will consider slower cooling from 760 down too by adding in some more segments.

Lots to try!

[birdypotter]

Quick update for anybody who followed this and might have the same problems:

A mixed bag of results!

I tested the cylinders with different glaze combinations as follows:

Black in/out
Black out/clear in
White in/out
White out/clear in
Clear in/out
Grey (almost identical to black) in/out
Grey out/clear in

I changed my firing schedule to allow for a slow fire up as suggested, adding in a 100˚c per hour to 600˚c. Cool down was kept the same. I froze all cylinders overnight and poured boiling water in them straight from the freezer the next day. Not one has dunted! Which I'm actually a little disappointed with  as I was hoping for a conclusive result to my problem. However, the additional segment when warming up may have solved the problem, or I may still have the problem and by chance it hasn't revealed itself in this current batch. So I will be making more cylinders and glazing all of them in the potentially problematic white/clear glaze combination to be absolutely sure it won't happen again. If it happens in one of these then I'll know there will always be a potential problem in a batch.

However I have adjusted the white glaze and substituted some of the tin for zircopax...it has brought the COE up to match the inside glaze more closely and hasn't massively changed the surface quality...it has just made the white slightly creamier and little more yellow. I might try to work out how to bring up to COE another way so that I can stick with tin oxide.

In terms of absorption of my current clay, ES5 Earthstone Original for UK potters, I redid these tests with 14 bars as opposed to 4 this time and absorption came back on average at 1.7%. Scales were checked and calibrated so pretty confident in this figure being correct and happy to stick with this clay for the current temps.

So...nothing conclusive and a little more testing to be done to be sure! 

[Min]

On 9/11/2019 at 5:25 AM, birdypotter said:

So I will be making more cylinders and glazing all of them in the potentially problematic white/clear glaze combination to be absolutely sure it won't happen again.

Make them as thin as you can, if necessary slab build them. 

"However I have adjusted the white glaze and substituted some of the tin for zircopax...it has brought the COE up to match the inside glaze more closely" If this is the Satin Matte White aka Pikes Oatmeal the coe figure won't be valid as calculated coe values only work for a fully melted gloss glaze. Does the clear gloss glaze craze when you test it? 

[birdypotter]

8 minutes ago, Min said:

Does the clear gloss glaze craze when you test it? 

No, not at all! With an inaccurate COE for glazes that aren't fully melted, is the true COE value likely to be higher or lower or does that depend purely on the individual chemistry? Or is it that the COE value just cant be relied on regardless cause it's not fully melted? Just wondering whether the 5.9/6.5 figures of the matte vs the glossy are even further apart than that which would explain the problems, or whether they are more likely to be closer and therefore not as much of an issue.

[Min]

Good to hear the clear is craze free.

Matte glazes are fully melted when done properly. Sometimes though people are using underfired higher temperature glazes that are immature and under melted to achieve a matte surface. Either way the coe figures don't work. In the former, some of the matting agents precipitate out of the melted glaze matrix. If you look at what's in your white glaze you see both dolomite plus talc so basically this is a magnesium matte (with some calcium which can also be a matting agent), and we know magnesium is a low expansion flux. So in theory some of the magnesium isn't going to be in the remaining glaze melt, does this mean the theoretical melt will then be a higher coe than indicated by the math, perhaps, but there isn't a way to quantify it. This is why I suggested doing the cylinder tests. Reason I asked about your clear is if it had crazed then you could try dropping the expansion of that one fairly easily instead of the possibility of needing to alter the matte recipe(s). If you're not confident with your last round of testing then I would do one more round, make the cylinders thin, glaze heavily and see what happens.