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TribeCreations

Cut, Spiral Bit Warped In Kiln

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Hello everyone.

 

I'm new to this forum - read many interesting posts and gained a lot of tips from many of you. Really nice to know there's a place to reference things, and hopefully get some help now and again.

 

I'm also quite new to pottery. I have a wheel and kiln, and I make stuff purely for the therapeutic affect.

 

Anyway, back to the topic that I need help with.

 

I started making yarn/wool holders: basically a pot with a spiral cut out, with some holes (have attached pics). I used professional white stoneware clay, with a cone 6 glaze. When I bisque fired them, I noticed that some of the spiral bits warped. So when I bisque fired the second lot, I left clay support bits in-between the spiral area to stop it from warping. This method worked - I just needed to grind those bits away before applying glaze (as illustrated in pic). 

 

But sadly, after the glaze firing, all the spirals warped/collapsed. Does anybody know why? I thought that after the pots were bisque fired, they wouldn't warp again?

 

Am I using the wrong clay? Or should the pots be higher with a narrower cut spiral section?

 

Any input will be greatly appreciated.

 

Thank you. 

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Bunnybaer likes this

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Hi, and welcome to the forums.

 

That looks like a lot of weight being supported by not much clay at the top of the bowl. Yes, pots can look okay in the bisque and warp in the glaze firing for ^6. There is a fairly long thread on yarn bowls here http://community.ceramicartsdaily.org/topic/4837-knitting-bowls/?hl=%2Bknitting+%2Bbowl

TribeCreations likes this

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Thank you Min. I will have a look at the thread link you posted.

 

Marc - thanks for your input. Hmm, that does make sense. The yarn bowl I saw, that inspired me to have a go; was almost the same as mine - same amount of clay removed, with a big chunk out the top. But looking at the clay the person used, it looks like its got quite a bit of grain in it. Maybe I need to use a different clay?

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It is a beautiful design and if you want to stay with it I would suggest reversing your firing schedule.

 

Keep that big, main vertical vee shaped slice you cut out, wax any sides that touch the bowl and the sides of the bowl that touch it .. put it back in place during drying and firing for support. Since it is waxed it should not stick and since it is all dried together it should shrink at the right speed. Dry slowly.

Make your first firing the cone 5-6 without glaze... That way you can support it.

Then remove the support piece and glaze your bowl with a low firing glaze 05-06.

Not guaranteeing this will work, but worth a try before you give up on the design.

Rae Reich likes this

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Not quite sure what you mean Chris? What area/s should be waxed? 

 

I cut the spiral out when its leather hard, and I usually leave thin clay bits in-between as supports, which I grind away after its bisque fired. 

 

After doing some reading, it seems a lower firing temp will be better, and perhaps I should think about using Earthenware clay as apposed to stoneware as it doesn't need to be waterproof. 

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I'm not a cone 6 person but if you have seen this work before at cone 6 then I would suggest two changes that can work-thinker pot wall or a different clay body that has a higher slump temp. (as you said the body you saw was more toothy)-how about a cone 10 body since this will never hold water fired to cone 6.

Mark

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I'd been wondering about a variant of Chris's idea. Fire fully-cut and unglazed to cone 6

with the pot resting on its rim. Looking at the first picture most of the slumping seems to

be happening near the rim. (At the least you are reducing the spiral by its thickest 1800.)

 

Then low-fire glaze.  BTW glazing a vitrified pot can present problems, unless you are

using a "painting" glaze full of gums.

Chris Campbell likes this

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The only thing I could suggest is to throw the form with a thicker rim-- and maybe a change in direction at the rim, so it forms a kind of I-beam cross section.  That might make the rim stiff enough to hold its shape in the glaze firing, but as others have said, much depends on the clay body.

pattispots likes this

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Righty-ho! A bit of an update and still confused; seeking input.  :wacko: Not sure if this is still the right place for this topic anymore?

 

After chatting to the pottery supplier and showing them the yarn bowls that warped, and the clay that I used, they questioned whether my kiln was possibly over firing?

 

To check all options, I got samples of about ten different clay types to see which would be the best for making yarn bowls - withstand warping. Two came out OK, but the others warped ... and they shouldn't have because its stoneware clay and can handle high temperates over and above what I fired to. 

 

Here's the firing schedule I used in my electric kiln which has a controller. 

 

Bisque Firing:  

1st ramp: 90 degrees celsius - 600 degrees celsius

2nd ramp: Full - 1000 degrees celsius

- soak 10 mins -

 

Glaze Firing:

1st ramp: 150 degrees celsius - 600 degrees

2nd ramp: Full - 1220 degrees celsius 

- soak 10 mins -

 

Cones used to measure the heat work:

Cone 6: 1222 degrees celsius (melted)

Cone 7: 1240 degrees celsius (half melted)

Cone 8: 1263 degrees celsius (quarter melted)

 

I put the cones on the middle shelf in the centre of the kiln. Attached is a pic of how the cones melted. This tells me the kiln is over firing, but to exactly what, I have no idea? And how to adjust the programme controller - I also have no idea what temperature to set it to in future?

 

The glaze firing result:

 

Most of the yarn bowls warped, glaze blistering/bubbles, holes, colours faded/darkened, uneven glaze distribution (even though it was glazed evenly). All the items are a waste  :(

 

Now, from the cone results, I presume the heat work in the kiln is higher than what the temperate is entered into the controller? Does this mean I need to lower the temperatures programmed in the controller? And if so, to what? I have no idea.

 

I'm new to pottery and have absolutely no clue when it comes to kiln firing ... but I'm keen and willing to learn. So any input will be greatly appreciated.

 

Sorry for all the pics, but I wanted to provide as much information as possible. 

 

 

 

 

 

 

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If that middle cone is 7, that's not half melted, it's down. It looks like a 7.5 to me. So yes, that's hot.

As to the warping, there is a lot of weight above a very empty space in that swirl. Can you move the swirls up the side of the bowl somewhat to lessen that effect? Many stoneware clays will indeed warp, if not quite so badly as porcelain.

Min likes this

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Your firing schedules look way to fast. Page 25 of this pdf http://www.psh.ca/pdf/700CompleteManual1.pdf show the firing profiles for slow and fast bisque and glaze used in the preset profiles for the Bartlett controller.

 

Also, the pots made with the groggy clay, did you burnish them with a rib after trimming? Pinholes can be a real pain to get rid of, your bisque schedule might have been part of the problem, groggy clay another part if used with a high viscosity glaze.

 

Your cones look like you went to around 7 and a bit, are you aiming for ^6? If so then use a 5, 6 and 7 in your cone pack.

Babs likes this

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Really no expert on this at all but here's my few pennyworth - my electric kiln seems to over fire too and I'll be trying to work this out next firing! Your 10 minute soak will add to the heatwork and I don't think it's necessary at all for bisque firing. Probably not necessary either for straightforward brush on glazes, but I may be wrong there. Your firing schedules look ok to me - Google ' Clayman Firing Schedules' and you'll see that Stafford Instruments suggest these. Not v different to what you're doing.

Bisque firing upside down is definitely worth a try. Is it totally dry before the bisque? Not sure if this could cause slumping, but it's a thought...

Why not try a white earthenware? I've never had slumping like this and I make pieces with lots cut out.

Pinholes - could your bisque have been a bit dusty? Give it a wipe over before glazing. Is the glaze a bit thick?

My inexpert thoughts! I really sympathise as I get some lovely bisqued pieces and glazing is where it tends to go wrong - so dispiriting, especially when you don't really know where to start changing things! I'm still in this position after 3 years - a lot still feels like trial & error!

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You definitely should consider slowing down the last 200 degrees of your glaze firing . . . drop to 80F per hour, not full ramp.  Slowing down will allow the glazes to melt better, perhaps curing some of your other surface issues.  If you do that, you could drop your 10 minute hold to 5 minutes -- time enough for the glazes to melt, but not so long as to increase the heatwork to the next cone. 

 

I hold my bisque for 10 minutes -- mostly to ensure the burnout of clay body materials that can affect glazing later on.

 

Agree that the swirl cut-outs on the bottom half make the bowls top heavy which is contributing to the warping.  Also, consider altering your design so the width of the cut-outs are thinner.  The slot only needs to be yarn-width. 

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I also like a 10 minute hold on bisque fires. I pack enough work into the kiln and this short soak helps even things out.

 

Some other tips:

The curve of you bowl is could become stronger with just a slight outward flare.

The weak point is the area directly above the inner spiral. Any weight after that section is adding to the problem.

 

I love bad analogies. How heavy is a glass of water? Not too much. What if you had to hold it for an hour? And with an outstretched arm? The clay can hold some weight. The firing is like holding it an hour. This curve is like holding for an hour at arms length.

Chilly likes this

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Well, thank you all for the helpful replies. The supplier I bought the kiln from suggested those firing schedules, and they programmed it in for me. They asked me what clay and glaze I was generally using, and they based the schedules on that information. So I presumed all was OK!?

 

I dry the greenware out very well before bisque firing, so there's no way moisture can still be present. Before applying glaze to the bisque ware, I wipe each piece with a damp sponge (wearing surgical gloves) to remove any dust or debris. I then apply underglaze or glaze with a brush - ensuring even strokes, and not too thick either. I then leave the glazed items to rest for a day or two, and then I load the kiln and glaze fire.

 

Reply to Diesel Clay: Yes, it's a 7 cone, and I agree, it's way too hot. I did make other yarn bowls with a thin 'upside down question mark' cut-out, and those even slumped too. I've tried many designs, and the majority slump.

 

Reply to Min: Yes, I used a metal rib after trimming the pots to smooth out the bits of grogg. The glazes I'm using are all cone 6 (1222Ëšc/2232ËšF) - this is the top temperature I'm aiming for. My controller is programmed to reach that, but the cones are showing the headwork is going way over that. After chatting to many suppliers about which clay to use for the yarn bowls, it was suggested I use groggy clay, which I tried ... but it still slumped. I did try earthenware, but that too slumped. 

 

Reply to Celia: Now there's an idea - bisque firing upside down. Didn't think of that. I tried earthenware, but that slumped slightly too, and its more expensive than the stoneware clay I usually use. I'm very particular when preparing for glazing - I wear surgical gloves, wipe each item well, and ensure there's no dust or greasy hand marks on the items before applying glaze. I add water to the glaze to thin it out (single cream consistency) otherwise its too thick to apply. Maybe that's the problem - using water? If the glaze is too thick, it doesn't brush on evenly, so that's why I thin it out.  

 

Reply to bciskepottery: Good suggestion about slowing down the last 200Ëš. I will try and figure out how to do that and give it a go. I have done various cut-out experiments; from basic thin cuts to this extravagant one, and regardless of what clay I use, they all slump to a degree. So that leaves me to think it's the heatwork?

 

Reply to MatthewV: Oh right, yes I see the weak point area. A few people suggested making the walls thicker than normal to possibly add strength, which is what I did. I did think about maybe creating an outward flared bowl as apposed to curved to see if that helps.

 

I really don't understand why I'm having issues; when quite clearly, other potters are making yarn bowls with similar designs and cut-out's successfully.  :(

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Have you tried firing the ^6 clay with ^06 glazes and fire to ^06?

 

I use a speckled stoneware clay from Potclays in Stoke and fire it to both ^06 and ^6 and find it fits reasonably well for both temperatures.

TribeCreations likes this

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aaaaah, very nice pics,I love it... I made this mistake befor, too...cutting too much clay out of the wall of my bowl, I love spirals so much. Today I am waiting much longer, till the clay is leatherhard (we tell it in germany lederhart) the the cuttings are easier... I love the glaces... wonderful!

Here in germany we only use cones, if we are not sure, that the oven temparature is right...I don´t have cones here. I mostly try it, When I have much red glaces the I push the temparature to 1100°C. Most times I glaze by 1050°C.

pattispots likes this

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I have a manual controller (Me!) and I fire a slow ramp after a drying period of about 80c overnight.

I increase by only 150oC at a time. Taking it very slow so to fire to 1260oC it take about 8 hrs.  My kiln has an automatic switch which I put on when doing the last ramp.

I don't have much experience but I think that you should take it a bit slower.

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