Tea Pots Cracking Violently

[hitchmss]

    Here in the last two years I've made three batches of teapots (totalling around 35 seperate vessels). The first batch, two years ago, suffered no issues, and the last two I've been having problems, specifically the most recent (1 week ago) batch of 8 tea pots.

     Allow me to provide pertinent information; teapots are thrown using Bmix ^10. Bodies are expanded using a rib from inside and compressed using a rib outside. I use a thrown hollow handle, which is attached during leatherhard or softer, along with a thrown spout. Pots are dried slowly under plastic for a week or more until bone dry, then bisqued to ^06 with a 30 minute hold. Up to this point no signs of cracking or failure.

    Pots are glazed with a number of glazes that I use on almost all of my work. Two dipped/poured coats (liner, mainly redart, and base exterior coat of strontium crystal), and then three sprayed glazes (ash glaze, oribe, and tenmoku). Then they are fired to ^11 gas oxidation using a fairly rapid cycle (room temp to maturation in 8 hours), then cooled naturally for 12 hours before opening kiln to cool completely.

    Heres where the issues arise; this last batch of 8 teapots came out of the kiln looking gorgeous and had absolutely no damages. When they were pulled from the kiln they were barely above room temp, and set on a table. Within 1 hour 3 of the teapots had cracked (see photos) starting with barely visible hairline fractures (pots didnt "ring" true), and as time progressed they worsened. After 6 hours, 4 of them had cracked leaving me with only 4 good pots left. I boxed the remainders up, and took them to a show where when I opened the box I found that the remaining 4 had cracked sometime between 12-24 hours after coming out of the kiln.

   The previous batch (about three months ago), had suffered losses of similar issues (not nearly the severity of cracking) of around 30%. 4 of 12 died. The first batch, over a year ago, I had no problems. And now this last batch, a complete loss.

    Heres my thought process of trying to deduce culprit.

1. Same construction process and technique

2. Same bisque, and glaze firing schedule

3. Same glazes that are used on thousands of pots annualy with no issues relating.

4. I throw pots as thin as these tea pots, with similar decorative styles with no issues.

5. Wall thickness in between handle and body are very close in thickness

6. Handles have thorough glaze inside hollow cavity

    My only thoughts to culprit would be change in my clay, or a change in glaze materials. However, pots glazed from the same batch of glaze show no similar troubles, and likewise, only 1 or 2 other pots (so far) made with the same clay are showing same problems.

   Cracks tend to occur radially around the pot, but also occur in vertical lines, most do stop cracking below the horizontal line of the handle, but some go all the way up to the neck of the body.

   I know these are not dunting cracks as you can tell by the visible clay body shown at the fracture, and I know they are not caused by thermal shock during cooling as I was diligent in checking as they came out of the kiln.

  Any help would be greatly appreciated as this is costing me thousands in losses, let alone the few bits of sanity I have left.

 

[hitchmss]

This is reposted from the Clay and Glaze Technical Section as this area may be more appropriate:

Here in the last two years I've made three batches of teapots (totalling around 35 seperate vessels). The first batch, two years ago, suffered no issues, and the last two I've been having problems, specifically the most recent (1 week ago) batch of 8 tea pots.

     Allow me to provide pertinent information; teapots are thrown using Bmix ^10. Bodies are expanded using a rib from inside and compressed using a rib outside. I use a thrown hollow handle, which is attached during leatherhard or softer, along with a thrown spout. Pots are dried slowly under plastic for a week or more until bone dry, then bisqued to ^06 with a 30 minute hold. Up to this point no signs of cracking or failure.

    Pots are glazed with a number of glazes that I use on almost all of my work. Two dipped/poured coats (liner, mainly redart, and base exterior coat of strontium crystal), and then three sprayed glazes (ash glaze, oribe, and tenmoku). Then they are fired to ^11 gas oxidation using a fairly rapid cycle (room temp to maturation in 8 hours), then cooled naturally for 12 hours before opening kiln to cool completely.

    Heres where the issues arise; this last batch of 8 teapots came out of the kiln looking gorgeous and had absolutely no damages. When they were pulled from the kiln they were barely above room temp, and set on a table. Within 1 hour 3 of the teapots had cracked (see photos) starting with barely visible hairline fractures (pots didnt "ring" true), and as time progressed they worsened. After 6 hours, 4 of them had cracked leaving me with only 4 good pots left. I boxed the remainders up, and took them to a show where when I opened the box I found that the remaining 4 had cracked sometime between 12-24 hours after coming out of the kiln.

   The previous batch (about three months ago), had suffered losses of similar issues (not nearly the severity of cracking) of around 30%. 4 of 12 died. The first batch, over a year ago, I had no problems. And now this last batch, a complete loss.

    Heres my thought process of trying to deduce culprit.

1. Same construction process and technique

2. Same bisque, and glaze firing schedule

3. Same glazes that are used on thousands of pots annualy with no issues relating.

4. I throw pots as thin as these tea pots, with similar decorative styles with no issues.

5. Wall thickness in between handle and body are very close in thickness

6. Handles have thorough glaze inside hollow cavity

    My only thoughts to culprit would be change in my clay, or a change in glaze materials. However, pots glazed from the same batch of glaze show no similar troubles, and likewise, only 1 or 2 other pots (so far) made with the same clay are showing same problems.

   Cracks tend to occur radially around the pot, but also occur in vertical lines, most do stop cracking below the horizontal line of the handle, but some go all the way up to the neck of the body.

   I know these are not dunting cracks as you can tell by the visible clay body shown at the fracture, and I know they are not caused by thermal shock during cooling as I was diligent in checking as they came out of the kiln.

  Any help would be greatly appreciated as this is costing me thousands in losses, let alone the few bits of sanity I have left.

[Crusty]

Sorry to see this, very beautiful pots.. someone will chime in soon..

[neilestrick]

The glaze is pulling them apart. It's either way too much glaze for the thin pot to handle, or the difference in glaze thickness between the outside and inside is too great. Either throw thicker, use less glaze, or put more glaze on the inside.

[mregecko]

I'm inclined to say glaze fit problems, judging by the pattern of the cracks and when they are occurring.

 

Try throwing a handful of bowls with equivalent thickness and composition, then doing the standard post-firing freeze / boil / freeze process....

[neilestrick]

This question is double posted, but here's my reply from the other post:

 

The glaze is pulling them apart. It's either way too much glaze for the thin pot to handle, or the difference in glaze thickness between the outside and inside is too great. Either throw thicker, use less glaze, or put more glaze on the inside to balance it.

[hitchmss]

The glaze is pulling them apart. It's either way too much glaze for the thin pot to handle, or the difference in glaze thickness between the outside and inside is too great. Either throw thicker, use less glaze, or put more glaze on the inside.

Neil, you think that a double coat of my liner glaze would supply a more equal surface tension to the glazes that are on the outside? Not to say you're not right, but I also use the same amount and same glazes on other pots with no issues, on pots that are equally as thin. Maybe in conjunction with the tension from the handle though.....??

[neilestrick]

 

The glaze is pulling them apart. It's either way too much glaze for the thin pot to handle, or the difference in glaze thickness between the outside and inside is too great. Either throw thicker, use less glaze, or put more glaze on the inside.

Neil, you think that a double coat of my liner glaze would supply a more equal surface tension to the glazes that are on the outside? Not to say you're not right, but I also use the same amount and same glazes on other pots with no issues, on pots that are equally as thin. Maybe in conjunction with the tension from the handle though.....??

 

 

Worth a try. I usually see this on pots that aren't glazed at all on the inside, but I have seen it like this, too. The other possibility is that there's some weird stress due to the handle and/or spout that makes this form especially susceptible to this issue.

[Min]

I agree with Neil, stress between body and glaze. Small body with a larger glaze which is thick enough to pull apart the thin pot by putting the body under extreme tension. I think that given the pots cracked after being removed from kiln this is the most likely scenario. Sorry to see such beautiful pots destroyed.

[hitchmss]

 

 

The glaze is pulling them apart. It's either way too much glaze for the thin pot to handle, or the difference in glaze thickness between the outside and inside is too great. Either throw thicker, use less glaze, or put more glaze on the inside.

Neil, you think that a double coat of my liner glaze would supply a more equal surface tension to the glazes that are on the outside? Not to say you're not right, but I also use the same amount and same glazes on other pots with no issues, on pots that are equally as thin. Maybe in conjunction with the tension from the handle though.....??

 

 

Worth a try. I usually see this on pots that aren't glazed at all on the inside, but I have seen it like this, too. The other possibility is that there's some weird stress due to the handle and/or spout that makes this form especially susceptible to this issue.

 

I have 4 large teapots that are bone dry that did not get fired with this last batch. Ill bisque and try doing a second glaze coat on the interiors of two, and doing lighter glaze coats on the other 2 exteriors to see if any success can be had. I'm not inclined to think that the stress from handle and spout are causing these to fail as Ive made these forms for a long time with no issues. Granted Ive been using this new glaze combination for the last three batches only, but have had some survive and others not. Ive talked to some other potters who had issues similar, who used a different clay, and found clay company changed feldspar which led to their issues. I have been having issues with laguna clay lately, as have a vast number of other potters in area have been too. Im not inclined to point a finger and lay blame as I'm not experiencing it amongst all of the work, but maybe all these issues in conjunction is causing the failures.

[hitchmss]

I appreciate the sentiment; tis' a shame to lose so much work (especially when it looks so nice, if I do say so). I'll try some thicker bodies in the next batch, and try some different glazing with the remaining 4. I originally thought that glaze compression might be the cause, but having so many others with same glaze patterns not fail like this, I dismissed it. Unless I can find information which points elsewhere, I think Ill have to restructure my glazing...unfortunately.

[Marcia Selsor]

I agree with Neil about the glazes but also what about the wall thickness and variation. If you are pushing out with the rib is it thick at the base or are you trimming it to a similar thickness of the belly of the pot. I do think the glazes are the main issue. Any way you can check the COE and compare the inside glaze with the outside combination? If the inside is mostly RedArt, then it is a slip glaze and shouldn't have tremendous COE. The outside glazes may be shrinking more and pulling the pot apart.

 

Marcia

[hitchmss]

I agree with Neil about the glazes but also what about the wall thickness and variation. If you are pushing out with the rib is it thick at the base or are you trimming it to a similar thickness of the belly of the pot. I do think the glazes are the main issue. Any way you can check the COE and compare the inside glaze with the outside combination? If the inside is mostly RedArt, then it is a slip glaze and shouldn't have tremendous COE. The outside glazes may be shrinking more and pulling the pot apart.

 

Marcia

There is some variation in wall thickness from foot to belly to rim. I trim the foot and maybe 1" up from foot to a very close thickness. Without having pot in hand, and a micrometer, id say the relationship is as follows; Foot 3/16", belly 1/16", rim 3/16". A better way of saying it is that it is not going from very thick, to very thin, to very thick again.

    I learned at one point how to calculate COE, but never used it in application. Without the aid of a glaze calculation software it would take me some time to get back to you with COE's on all my glazes. I do not apply even coats, covering the entire surface of the pot that is, with any one single sprayed glaze (3 sprayed layers), so potentially the change in surface tension between an area that does have a higher build, vs one immediately next to it, that has less, is causing a lot of the over compression and cause for the irregular fracturing.

[Benzine]

Sad indeed.  That is a great pot.

 

I've had some dunting issues before, but never that bad.  My issues usually arise, with previously untested glaze combinations.

[bciskepottery]

I originally thought that glaze compression might be the cause, but having so many others with same glaze patterns not fail like this, I dismissed it. Unless I can find information which points elsewhere, I think Ill have to restructure my glazing...unfortunately.

I'm thinking it's a combination of glaze compression and form . . . you are getting the cracking on a form that is basically closed and, it appears, at a point equidistant or near equidistant from top and bottom; are the other non-cracking forms similar (closed) or different (open, like bowls or cups)?

[Pres]

I have moved the In the Studio post of this topic to here and combined the two. I hope this makes things easier to deal with. Even though John is out of touch recently, I am sure he will probably horn in later on as this is part of his broad area of expertise. 

 

My own solution to the problem would be

• Throw the pieces with more consistent thickness from top to bottom.
• Try to get a more even inside outside coat of glaze to balance stresses.
• Cool the kiln a little slower, just in case, as I still feel this is part of the issue.

 

 

Best,

Preston

[Mark C.]

Neil pegged it

I would add another different liner glaze inside may cure this.

Mark

[Wyndham]

I'm wondering about the SCM glaze as a possible problem area. It has nearly 3 % Lithium and that might affect the other glazes and the body adversely. .

I also think I saw that the handle is hollow, do you have an air release hole somewhere on the handle? If you don't the air pressure from firing might be an issue.

Hope you find the answer soon

Wyndham

[Mark C.]

Setting the body issue aside for a moment-the many layers of glaze would be the main cuase unless its s body issue.

Now in my 40 years of Laguna business I have seen a fe of their bodies go bad as well. Talking to John Paccini (Laguna Clay body guyon west coast) they are going thru some material issues from some larhger companies conglomerating smaller ones and closing some mines-so some clay issuess will come out of this.

Was this new clay or old stock?-you heating cycle is very short but since thats never been a factor I assume thast not it .

You will never get Laguna to say its the clay-I know this from past experience-but B-mix issues have gone on for years with my fellow potters and I know many who switched from using it long ago.

I think some tests are in order.

Please let us know how this shakes out?

Mark

[Callie Beller Diesel]

Forgive me if I seem to be reiterating what Neil said, but I'm willing to bet your liner glaze fits that clay body like a glove, and that the outer glaze combos aren't quite so perfect. I had a batch of very voluptuous mugs crack similarly using Bmix, a liner that fit, and an outer that didn't. A few vertical lines down, and cracks at the widest point, and all the cracking happened well after they had cooled. In some cases they didn't crack until weeks later.

[hitchmss]

 

I originally thought that glaze compression might be the cause, but having so many others with same glaze patterns not fail like this, I dismissed it. Unless I can find information which points elsewhere, I think Ill have to restructure my glazing...unfortunately.

I'm thinking it's a combination of glaze compression and form . . . you are getting the cracking on a form that is basically closed and, it appears, at a point equidistant or near equidistant from top and bottom; are the other non-cracking forms similar (closed) or different (open, like bowls or cups)?

 

The only other form that Ive had crack in this similar manner was an casserole. From one rim, across the bottom, and up the other side to the other rim. Definitely not an S crack. Crack occured in same manner; well after cooled.

[hitchmss]

I'm wondering about the SCM glaze as a possible problem area. It has nearly 3 % Lithium and that might affect the other glazes and the body adversely. .

I also think I saw that the handle is hollow, do you have an air release hole somewhere on the handle? If you don't the air pressure from firing might be an issue.

Hope you find the answer soon

Wyndham

While the handle is hollow, at both attachment points there are air release holes into the body cavity. I do not have my SCM glaze recipe here at my home studio, but off the top of my head it is:

Custer

Strontium

Lithium

Titanium

6 Tile

Bentonite

Proportions of which I am not too mentally gifted to remember.

[hitchmss]

Setting the body issue aside for a moment-the many layers of glaze would be the main cuase unless its s body issue.

Now in my 40 years of Laguna business I have seen a fe of their bodies go bad as well. Talking to John Paccini (Laguna Clay body guyon west coast) they are going thru some material issues from some larhger companies conglomerating smaller ones and closing some mines-so some clay issuess will come out of this.

Was this new clay or old stock?-you heating cycle is very short but since thats never been a factor I assume thast not it .

You will never get Laguna to say its the clay-I know this from past experience-but B-mix issues have gone on for years with my fellow potters and I know many who switched from using it long ago.

I think some tests are in order.

Please let us know how this shakes out?

Mark

The clay used was new stock, at least new to me, and my distributor. How long ago the factory made it....not sure. Ive had issues recently with metal, needle like, shavings in my clay, chunks of plaster, and almost as usual incredibly inconsistent moisture contents (from incredibly dry to super soft). Local companies have had X-acto blades, cigarrette butts, chunks of other plaster/unmixed raw materials. A good friend, and handmade tile maker has recently had pressed tiles that should be 4"x6" straight edged tiles. Here recently the tiles are bisque fired and coming out in the shapes of hour glasses. Doubtfully cause would be a part of his making process as nothing has changed.

    My rapid heating cycle is one that I have used personally on over 50 firings, and my business partner has used at least 30 firings a year for the past 10 years. I have never suffered issues like this with any other forms

[hitchmss]

Ok, so it seems to be a resounding opinion that improper glaze fit, or more aptly, uneven COE from my liner to exterior glazes is the most likely culprit. In combination with thin bodies, and a possibly issue compounding handle form.

     Not that I'm trying to disagree with free and sound advice, but would some(one) be able to offer advice why this same glaze combination used on other forms(both closed vessels, and wide forms), with equally thin walls have never suffered issues of similar failure?

[bciskepottery]

Try another clay body and see if the problem recurs . . . then you will have narrowed it down to a bad supply of B-Mix or the glazes. From the sounds of your comments on clay body, something different there might be enough to cause previously compatible intrior/exterior glaze combinations to become incompatible.