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Crack In Finished Bowl ?

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bciskepottery    925

The thinness at the top of the bowl could be a cause -- and my guess.  If the pot is much thicker at the bottom than the top, you have unequal tension that affects the pot while it is expanding/shrinking during heating and cooling in a firing cycle.  The crack could have been there all along -- a latent stress crack; because your glaze temperature is higher than bisque, the crack was not visible after bisque but became pronounced after the higher glaze firing.  Latent/stress cracks are often caused by handling while still in the greenware stage.  Uneven glaze applications could contribute -- thinner at the top, thicker at the bottom -- affects how the pot cools. 

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Marcia Selsor    1,301

If it was in a gas kiln, the uneven flame may be the cause on something that thin. If there is not much of a foot, the stress from slumping may has caused it. There are many variables.

 

Marcia

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PotterGrl    3

Just want to say that it's a beautiful bowl ;o) I'd still use it in my home, even if I couldn't sell it or give it away. I have a square plate that I made and the corner cracked. It's on display in my kitchen, one of my favorites!

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PotterGrl    3

All of my stuff is fired at the studio where I take classes as well (stuff I make at home I have to take there).  I find it difficult to not be in control of the kilns. Sometimes a piece doesn't turn out and my teacher/owner of the studio says "Oh, that was in the kiln that didn't get up to temperature" or "That was in the kiln that underfired" or "Someone elses pot exploded".  Many variables, as you know. I guess the good part is that the entire kiln load wasn't mine, just one or a few pieces ;o)  So that is the upside.  

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Min    778

I agree that the crack was probably there but invisible at the greenware stage. Did you rest the pot on the rim while working on the bottom of the pot? It kind of looks like the cracks in the first 2 pots are on the high point of the rim, flipping the pot onto the rim would cause stress there. If that's the case then using a chuck to suspend the pot on while working the underside would alleviate the stress as would working on a piece of soft foam. 

 

Min

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cgb    6

If it was in a gas kiln, the uneven flame may be the cause on something that thin. If there is not much of a foot, the stress from slumping may has caused it. There are many variables.

 

Marcia

 

All of my stuff is fired at the studio where I take classes as well (stuff I make at home I have to take there).  I find it difficult to not be in control of the kilns. Sometimes a piece doesn't turn out and my teacher/owner of the studio says "Oh, that was in the kiln that didn't get up to temperature" or "That was in the kiln that underfired" or "Someone elses pot exploded".  Many variables, as you know. I guess the good part is that the entire kiln load wasn't mine, just one or a few pieces ;o)  So that is the upside.  

I am intensely glad you mentioned this.........and what about the "that was the careless or crazy fellow student who knocked someone else's piece "  (never spoken about) yes, there always is a risk in sharing a kiln. Just a question but have you noticed at your studio that anyone above mediocre tends to have more breakage? Underfired, not the right temp....somebody is watching the electric bills or not wanting to kiln sit. I just put a ad on craig's for a fellow potter with a gas kiln. I am in Denver.

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cgb    6

Just want to say that it's a beautiful bowl ;o) I'd still use it in my home, even if I couldn't sell it or give it away. I have a square plate that I made and the corner cracked. It's on display in my kitchen, one of my favorites!

Thanks..........I just took it out of the closet and will use it in my kitchen.

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Chris Campbell    1,086

That's what drives us to buy our own kilns ... Control freaks we are!!

I used to come into the public studio and find people holding my dried pieces then banging them back onto the shelf. Signs everywhere saying don't touch other work but some think it does not mean them. When you work thin there is just so much that can happen before firing that you see only after final firing.

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PotterGrl    3

 

If it was in a gas kiln, the uneven flame may be the cause on something that thin. If there is not much of a foot, the stress from slumping may has caused it. There are many variables.

 

Marcia

 

All of my stuff is fired at the studio where I take classes as well (stuff I make at home I have to take there).  I find it difficult to not be in control of the kilns. Sometimes a piece doesn't turn out and my teacher/owner of the studio says "Oh, that was in the kiln that didn't get up to temperature" or "That was in the kiln that underfired" or "Someone elses pot exploded".  Many variables, as you know. I guess the good part is that the entire kiln load wasn't mine, just one or a few pieces ;o)  So that is the upside.  

I am intensely glad you mentioned this.........and what about the "that was the careless or crazy fellow student who knocked someone else's piece "  (never spoken about) yes, there always is a risk in sharing a kiln. Just a question but have you noticed at your studio that anyone above mediocre tends to have more breakage? Underfired, not the right temp....somebody is watching the electric bills or not wanting to kiln sit. I just put a ad on craig's for a fellow potter with a gas kiln. I am in Denver.

 

I don't notice more damage among any certain people, but the turnover for the kilns is so great that I couldn't possibly know about all of the damage or kiln mishaps. We have 4 kilns and they're all fired 2-3 times a week each. I made a great piece once, and when I picked it up it had bits of bisque stuck all over the pot, embedded in the glaze. Apparently someone accidentally put greenware on the glazed shelf (maybe it had slip that made it look glazed?) and it was put in a glaze firing.  It exploded. My teacher has always taught us to never get attached to a pot until the final product is in hand ;o)  Anyways, I know they aren't conserving on electric bills. They have the kiln doctor come in whenever necessary. The owners are amazing. It's just like what others have said. Students still pick up other people's work even when they shouldn't.

 

Here's another complaint I have (if you don't mind ;o)  ).  The kiln shelves don't seem right to me.  They are so thickly coated in kiln wash, but it's all bumpy, holey, falling off in places and uneven. Plus the dots of dripped glaze everywhere. I nearly always need to take my dremel tool to the bottom of my pots to get off either dried on kiln wash or small glaze spots that my pot was set on. Is this normal? This is my only experience with kilns, so I don't know. Oh, and the kiln posts have so much stuff stuck to the ends that they don't stand straight. I do load and unload them somethings since I work there on Fridays, so I see this all the time. I never set a pot on a drip of glaze though ;o) I normally wont even use that shelf.

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PSC    54

If the kilns are fired often likely there isn't enough time for proper shelf maintenance. A bit if kiln wash on the bottom of a pot can happen...get a diamond file and it comes right off. The glaze bits on the bottom of a pot is caused cause someone isn't taking care of the shelves between firings. A chisel to chip the glaze off the kiln wash and a bit more kiln wash added to the shelves is all that is needed. The lumpy flaky kiln wash is normal for a busy kiln room.

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cgb    6

If the kilns are fired often likely there isn't enough time for proper shelf maintenance. A bit if kiln wash on the bottom of a pot can happen...get a diamond file and it comes right off. The glaze bits on the bottom of a pot is caused cause someone isn't taking care of the shelves between firings. A chisel to chip the glaze off the kiln wash and a bit more kiln wash added to the shelves is all that is needed. The lumpy flaky kiln wash is normal for a busy kiln room.

Oh no, more problems..........I know little about kilns.........but this information is very interesting if not another reason to buy a kiln!! 4 of my pieces were removed with the chisel.............I can see the tool marks on the pieces......I think clay is so enchanting, and most of the peeps we meet are nice.....that this tends to blur the reality of poorly maintained kilns and the disrespectful crazies who knock the bisque pieces either accidentally or deliberately.  I have noticed that some of the better pieces were broken. @ Pottergirl........... "Do not get attached", interesting........so the service provided is so poor or unreliable........that one should not get attached? Try talking to people who own a kiln and see if they have all these disgusting problems. . I spoke with a guy here who left a studio, he did not want to say why, and connected with a fellow potter in his area who had a kiln. So think outside the box, I sure am! But I thank everyone here, especially you PSC.........you must have your own kiln. Yes, we love our public studios......but there is definitely a down side and being worried or upset is not conducive to good art and or good pottery.

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PotterGrl    3

If the kilns are fired often likely there isn't enough time for proper shelf maintenance. A bit if kiln wash on the bottom of a pot can happen...get a diamond file and it comes right off. The glaze bits on the bottom of a pot is caused cause someone isn't taking care of the shelves between firings. A chisel to chip the glaze off the kiln wash and a bit more kiln wash added to the shelves is all that is needed. The lumpy flaky kiln wash is normal for a busy kiln room.

Thanks, PSC, very good to know how normal it is for busy kilns. I know that potters and even the studio owners don't make tons of money to just buy extra shelving. I can deal with it, I just wanted to see if it was normal. I have a good stone grinding dremel bit that takes care of the dried on white kiln wash chunks easily, and 3 diamond bits for the glaze bits.

 

Middlewest, a piece would have to be extremely adhered to a shelf to have to come off with a chisel. Even when I'm unloading a kiln, and there is a piece that dripped a ton, it wiggles off (leaving huge chunks on the shelf). Although maybe a chisel makes the pot foot not break in the process so you can save your pot. Often when you pull it off by hand, it takes off chunks of pot with it. Maybe chiseling is best. My teacher just told me to pull them off, which is why I do it that way.

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Chris Campbell    1,086

Very often the quality of a public studio is in the hands of an overworked person who is doing a lot of other jobs ... and probably teaching too. They don't have time to sit down and get shelves back to new condition so slap on a bit more kiln wash to protect future pieces. It also takes time to clean up kiln furniture.

In some wonderful storybook place, all the people who love to use the facilities would take an afternoon to pitch in and help do the nasty jobs ... cleaning shelves & kiln furniture, vacuuming out the kilns ... organizing the shelves, sorting out old work that will never be picked up, cleaning glaze buckets ... etc. :rolleyes:

 

Stop laughing ... it could happen .....

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Pugaboo    438

Believe it or not IT DOES HAPPEN. The studio I belong to has "workdays" when we are all asked to pitch in and clean, paint, organize, etc. not everybody shows up of course but enough do to help make the work go quickly. We all have lunch together and pretty much have a good time doing jobs nobody wants to do by ourselves.

 

It's actually kind of fun to find a piece all dusty and forgotten for whatever reason and we put it aside in an area for one of the older ladies to sit and pick glazes to dip them in and then we fire them. The older ladies have a harder time doing some of the chores but they really like to help so it works out for everyone. These unclaimed pieces, if they come out nice, get put in the studio gallery and sold with proceeds going towards the studio for supplies or such so we make out that way too.

 

When I joined the studio I actually volunteered to clean the kiln shelves. I wanted to learn this aspect of the craft as well thinking it would hold me in good stead if I ever got a kiln of my own and it has. Just having the director show me how to go about chiseling glaze chunks off a shelf gave me a healthy respect for proper glazing and dry footing of my work. The studio director IS super busy and he teaches classes as well so I think he really appreciates any help we can give him. So maybe something like this might be a good way to help improve your studios kiln shelves and furniture? Just a thought different studios have different ways of doing things.

 

Terry

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ayjay    119

We do it once a year, at our normal meeting time but in the week following our last official evening session, everything gets a good tidy up for about an hour and then we all go for a curry.

 

The effects don't last for long, there are two midweek daytime classes for "special needs" folk in the same space.

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Babs    386

How about even when you like the thin look, roll to the thickness you desire then, here's the skill part, compress the rim so that any invisible little cracks, lurking there to seduce you into firing them and giving them the chance to grow in the second firing, are cemented back into a solid state! then carefully shape outer edge of the thickened rim into an illusion of fragility!

Also, viewing your image,  if you are not going back in and compressing the rims of your pots, your claybody may be being put under stres in the cutting process.

Place pots on sticks/rods to enable even drying, perhaps plastic them for a couple of days to slow down the drying process.

Remember the crack lets the light in!

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We also have a cleaning day every quarter where members help. The kiln techs are very conscientious, volunteer members help with loading, and unloading, maintenance. It's the only way we could survive. We fire two gas kilns weekly and our bisque kilns are loaded and fired every couple of days, as soon as there's enough work.

Even so, pots get bumped, dropped, etc. we have classes so there's always another crop of beginners, so stuff sticks to the shelf. We've been lucky. We caught green ware before it could blow up in the glaze firing. If we suspect that a piece might run and stick to the shelf, we will put it on a piece of broken kiln shelf. It might become its own sculpture, but at least it won't damage the kiln shelves.

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Babs    386

I wonder, are you forming these bowls by coiling? One of the cracks running horizontally seems to suggest a weak construction.

How are you cutting the rim? SOme tools will tear the edge which results in cracks. Try firming up the rims after the trimming of them.

May get less cracks if you continually, well sometimes, compress the rims of the bowls as you are making them as this tends to strengthen the rims.

Are these bowls formed in a slump or hump mold? Could be cracking in the drying process, invisible little cracks not seen till the second firing.

Slow drying is good too. Turning the bowls upside down as soon as firm enough so that teh bowl dries more uniformly may help, or wrappping in plastic for a couple of days.

Good luck, nice pots

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cgb    6

whatttt???? here we go again, this was paper clay that sat covered for a long time, I was not surprised the bottom detached but if you look closely the upper rim looks like it was cracked and then pulled at an angle.......the rim glaze is actually brown not maroon or red........bottom is a experiment of mine with glazing.......and its a vivid brilliant blue.........

post-36904-0-93976300-1383350170_thumb.jpg

post-36904-0-93976300-1383350170_thumb.jpg

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bciskepottery    925

Looks like dunting during cool down of the bowl, possibly compounded by thicker glaze pooling in bottom of bowl vs. glaze thickness on sides and/or thicker bottom vs. thinner sides.  Could also be a difference in glaze thickness from glaze on the outside of the bowl vs. glaze on the inside. 

 

Are you constructing the ware so that there are stress points that will crack (seems to be part of the look you are trying to achieve)?  If so, you might be setting the stage for these types of cracks/dunts when fired. 

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Babs    386

Being a peasant potter myself, I wsn't commenting on getting a pristine, smooth pot. I am also self taught so experiment is my method... If the edges of  of the cracks are sharp ie glaze looks like it is sharp then I think your pots are subjected to a rapid cooling at crucial times. Some one technical can tell you which temps the ware is sensitive to a rapid cool.

Love your pots.

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cgb    6

Someone had asked me if the kiln was fully cooled down prior to the pieces being removed........I have no idea....but the pots are unloaded to an outside area immediately and that temp could be as low as 30 degrees, its outside.

I am rolling thinner than most ppls........very few pots that I construct.......have planned stress points. Brown pot photo above had many areas that could have cracked....but by some amazing miracle...........no cracks......maybe no mishandling in green ware, bisque or fired state...........I personally do not handle the piece through all the stages.......that is impossible.......my fees pay for that....

No, lol, I am not trying to create pots with serious cracks :) There is a sign in the studio (which is a gov taught facility ) "Careful with other's pots, the next pot cracked, may be your own"

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cgb    6

Oh, its fine Babs...........I am in love with a very primitive look.........am in a studio taught by a very talented skilled woman.....but I like to go my own route....I think that's the whole wonder and joy of clay. I could try rolling a little bit thicker and see if that changes anything.....like fewer serious cracks........the other odd thing is that the pots once home develop new cracks or problems......more temp changes?......the gas fired pieces are removed to the outside air.....then I bring them home into the heat.......so.....

at any rate do you have your works and pots in the subscribers gallery...? I would love to see them....:)

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Babs    386

Yeh, didn't mean to offend, just trying to work out the reason. If part of your pots are sticking to dirty shelves then when the rest of your pot moves during firing, the stuck section cannot and so another area of stress as it expands during heating and contracts during cooling. Opening the kiln before it cools to about 200degs celsius will also crack ware.

Will get around to posting pots but that part of life gets neglected for the doing stuff.

Meant cracks, check out wabi sabi pots now that is seriously seeking breakdown.. ;)

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