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Cracked Bottom Help


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#1 Paasch E

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Posted 01 March 2013 - 05:30 PM

Hi guys, I was hoping you could help me out.

Just recently, I finished a bisque load that included 4 dip chillers. These are the types with the large basin bottom (for ice) and the bowl that sits on top (for dip). For some reason, every single dip chiller has cracked.

I don't understand it. Neither of my other wide-bottomed pieces cracked -- I had 2 wine chillers and 6 wide bowls. And I'm always careful about compressing bottoms. They were on different shelves in the kiln, and I brought everything in at the same time. For some reason dip chillers have been a large problem for me in the past, too. I'd say about a third of the ones I make crack. To try and combat this, I no longer store the small bowls on on them while drying or during firing (I fire them separately for both bisque and glaze firings).

I am just so frustrated. Does anyone have any ideas as to why these are so prone to cracking?

#2 Pres

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Posted 01 March 2013 - 06:00 PM

Hi guys, I was hoping you could help me out.

Just recently, I finished a bisque load that included 4 dip chillers. These are the types with the large basin bottom (for ice) and the bowl that sits on top (for dip). For some reason, every single dip chiller has cracked.

I don't understand it. Neither of my other wide-bottomed pieces cracked -- I had 2 wine chillers and 6 wide bowls. And I'm always careful about compressing bottoms. They were on different shelves in the kiln, and I brought everything in at the same time. For some reason dip chillers have been a large problem for me in the past, too. I'd say about a third of the ones I make crack. To try and combat this, I no longer store the small bowls on on them while drying or during firing (I fire them separately for both bisque and glaze firings).

I am just so frustrated. Does anyone have any ideas as to why these are so prone to cracking?


A crack is a crack is a crack is not so. It would be best if you could take a picture of your crack to show it we could tell what would have done it.

Simply retired teacher, not dead, living the dream. on and on and. . . . on. . . .                                                                                 http://picworkspottery.blogspot.com/


#3 neilestrick

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Posted 01 March 2013 - 06:03 PM

There are a lot of different reasons for cracks. Post a photo.

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#4 Paasch E

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Posted 01 March 2013 - 06:38 PM

Here you go. The 4th one broke down the middle when I threw them out the first time, but here are the other 3

one two three


#5 neilestrick

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Posted 01 March 2013 - 06:57 PM

Any chance we can see the rest of the pot?

Neil Estrick
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#6 mregecko

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Posted 01 March 2013 - 07:15 PM

So it looks like the crack is in the bottom of the basin then?

What is your drying process for these? It basically looks to me like an S crack, which can come with poor compression (which you said you're good about) or when your base and sides have different drying rates... Usually happens for me if the bottom is thinner and dries faster than the walls, causing the clay to pull apart outwardly from the thinnest spot (the center).

Whenever I have a crack I don't understand, I always end up breaking the pot in half to see a cross-section. That's the best way to learn what's going on.

My guess would be that your bottom is a little too thin and your sides are thicker.

BUT I could be entirely wrong ;-) Gotta love pottery.

#7 Mark C.

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Posted 01 March 2013 - 07:42 PM

This answer above
(My guess would be that your bottom is a little too thin and your sides are thicker.)
Is also my guess.
A topside photo would help. But from what you said and those photos I think the bottom may be a different thickness than say the side walls or outer bottoms.
Snap one of these in 1/2 to know what went wrong.
Mark

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#8 Paasch E

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Posted 01 March 2013 - 07:53 PM

So it looks like the crack is in the bottom of the basin then?

What is your drying process for these? It basically looks to me like an S crack, which can come with poor compression (which you said you're good about) or when your base and sides have different drying rates... Usually happens for me if the bottom is thinner and dries faster than the walls, causing the clay to pull apart outwardly from the thinnest spot (the center).

Whenever I have a crack I don't understand, I always end up breaking the pot in half to see a cross-section. That's the best way to learn what's going on.

My guess would be that your bottom is a little too thin and your sides are thicker.

BUT I could be entirely wrong ;-) Gotta love pottery.


Yes, the bottom of the basin. So, if I were to turn these upside-down as soon as they are stiff enough to manage it, would that also help? that would help them dry at a more similar rate, right?

#9 OffCenter

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Posted 01 March 2013 - 07:59 PM

The bottom doesn't seem "worked" at all. The other side of the bottom (inside the pot) was worked when you threw the pot, so afterwards you not only need to compress the bottom by going around the edge pushing in on the clay toward the center, but also you need to wet the bottom and use a rib to rub it smooth in one direction, then the other, etc. to work the bottom from the outside like it has been worked from the inside. Make it very slightly concave which helps keep it from cracking and keeps it from rocking when sitting on a flat surface. If even a very slight concaveness is not desired then make it concave but then push it back flat from the inside later.

Jim
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"But it does move," said Galileo under his breath.

#10 mregecko

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Posted 01 March 2013 - 08:01 PM

Yes, the bottom of the basin. So, if I were to turn these upside-down as soon as they are stiff enough to manage it, would that also help? that would help them dry at a more similar rate, right?


Actually, if anything, I think that might be worse. My guess is the bottom is too thin and drying too fast. You need to make sure you have an even thickness and everything dries at the same rate. Break one or two in half, look at the difference between the wall thickness and the bottom thickness, and see what's going on.

If you think about it, clay shrinks when it dries. Thinner clay dries faster. If the base dries faster than the walls, that's going to create stresses in SOME direction. It's thicker near the walls (and thus stronger), so it will pull apart in the center.

#11 Paasch E

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Posted 01 March 2013 - 08:58 PM

I know all of this. Which is why I've always been good about compressing the bottoms (with a wooden rib). It still doesn't make sense that these are the only things that are cracking. Also I do "work" the bottoms. Usually I push in so it doesn't rock, then run a finger over the bottom to smooth out lines. After it comes out of the bisque kiln, I sand it to get anything that might have gotten on during the drying process off.

If this were simply an S-Crack situation, then why aren't my wine chillers - which are just as wide, or my brie bakers, which are wider, not cracking?

I throw on wooden bats, then when they're dry enough to come off the bat I transfer them to unfinished wooden shelves, still right-side up. Wouldn't having the bottom against the wood surface (thus keeping less air from hitting the bottom) make the bottoms dry slower than the sides?

#12 Diane Puckett

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Posted 01 March 2013 - 09:16 PM

I know you are compressing them, but it looks like throwing rings are visible in two of the photos. I would think that would not happen if the entire base was compressed. I also see straight lines. Is this from the wood grain? If so, the clay may be catching/sticking to the wood and then cracking as it dries. A sheet of newsprint under the pot can prevent that. Which, I know, does not explain why your wider pieces are not cracking.

Have you been neglecting your kiln goddess?
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#13 Paasch E

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Posted 01 March 2013 - 10:04 PM

I cracked them open. The bottoms are thinner, but not very much so. I'd say the difference is less than 2cm. Nevertheless I will endeavor to make them more uniform in the future. I will also try the newspaper trick as well.

Thank you for your ideas.

#14 Mark C.

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Posted 01 March 2013 - 10:13 PM

A few more ideas that work for me and may help you. You may being doing this already.
Cut them off right after throwing and for larger flat forms use a thicker wire so the pot does not reattach itself to the bat. This thicker wire does wonders sometimes..
I threw a run of larger chip and dips today with 13 inch wide bottoms and always cut them with a thick wire-
For small forms I never use thick wires.
Mark
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#15 Paasch E

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Posted 01 March 2013 - 10:52 PM

A few more ideas that work for me and may help you. You may being doing this already.
Cut them off right after throwing and for larger flat forms use a thicker wire so the pot does not reattach itself to the bat. This thicker wire does wonders sometimes..
I threw a run of larger chip and dips today with 13 inch wide bottoms and always cut them with a thick wire-
For small forms I never use thick wires.
Mark


Interesting. what wire do you use?

#16 Mark C.

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Posted 02 March 2013 - 02:51 AM

I have a collection of lengths and sizes. Many were made buy a tool maker that is no longer with us.
A good supplier will carry some selections in various diameters.
What clay body is that which you are having issues with. It looks course.
I use mostly porcelain.
Mark
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#17 Lindsay

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Posted 02 March 2013 - 05:11 AM

I may be stating the obvious, but use as little water as possible when throwing, S cracks are normally a result of unequal moisture in the thrown clay.

#18 weeble

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Posted 02 March 2013 - 06:33 AM

Wait, 2cm? If it really IS 2cm and not a typo that SHOULD be 2mm... 2cm is a huge difference in thickness....
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#19 Paasch E

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Posted 02 March 2013 - 06:50 PM

Wait, 2cm? If it really IS 2cm and not a typo that SHOULD be 2mm... 2cm is a huge difference in thickness....



SILLY ME. Yes, I meant mm, not cm. That's what I get for replying at 11pm.

Also, yes, I know not to leave water at the bottom while throwing.

Gee, could you imagine if my walls were 2 cm thicker than my bottoms? Either the bottoms would be paper thin or I would have some ridiculously heavy pottery...

#20 timbo_heff

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Posted 04 March 2013 - 10:55 AM

Jeff Zamek did a nice paper on s-cracks: his conclusion is that it is all about the power wedge aka: cone up and pushing down phase.
Perhaps with these pieces you are using more clay and therefore not engaging the clay enough at that stage.
Try really making sure you are involving all the clay down low in the cone. Really push it together at the bottom. If you have a wide cone then the bottom clay is not getting involved in the power wedge.
Cone up and push down a few more times than normal and see if that solves ( in combination with the normal compressing the bottom tricks of course).
I have not had a single s crack since paying heed to this advice (knocking vigorously on wood:)




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