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cracking platters


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#1 Natania

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Posted 09 January 2013 - 10:51 AM

Hi,
I am trying to throw fairly large platters with a cone 5 B-Mix (which has no grog and is porcelain-like, although supposedly not as tempermental as is porcelain). They keep cracking in the middle as they dry. Not quite like S-cracks, at least I don't think so. I condense the bottoms a lot as I go. The few that haven't cracked are perhaps thinner on the bottom? Are the ones that have cracked perhaps a bit too thick and that is why they are cracking? Is it the absence of the grog in the clay? Should I let them dry on sand, and if so, how would I get them off the bat in time to do so? It seems that they are getting pretty dry by the time they release from the bat. Any advice is very welcome!
Thanks!

#2 neilestrick

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Posted 09 January 2013 - 11:11 AM

Good compression during throwing is key. You should also cut them loose from the bat as soon as you throw them, and again before they reach leather hard. If they are stuck to the bat they can't shrink, and will crack. Even thickness is also key. I cover the rims of my platters with plastic and let the middle dry uncovered. After 2-3 days they are ready to tim. I make 24" platters with porcelain following these procedures, and they never crack.
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#3 TJR

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Posted 09 January 2013 - 11:21 AM

Natania;
I had a problem like this a couple of years ago. It could be a wedging problem. After you wedge your clay into a cone, you have to rotate the bottom of the clay on the wheel head before you smack it down. For some reason, the bottom of the cone doesn't get compressed as well. Also, you have compress with a rib on the inside. I also wire cut them off the bat immediately after throwing. Next day, I flip them over on another bat. I also dry mine under plastic like Neil has explained.
TJR.

#4 Natania

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Posted 09 January 2013 - 12:54 PM

Natania;
I had a problem like this a couple of years ago. It could be a wedging problem. After you wedge your clay into a cone, you have to rotate the bottom of the clay on the wheel head before you smack it down. For some reason, the bottom of the cone doesn't get compressed as well. Also, you have compress with a rib on the inside. I also wire cut them off the bat immediately after throwing. Next day, I flip them over on another bat. I also dry mine under plastic like Neil has explained.
TJR.


Thanks. I'll try both those things. I do wire them off right after throwing, but not again a bit later. I'll cover the rims too. What do you mean, TJR, by rotating the cone of clay before throwing? Just compressing it on the wheel head manually before beginning to throw?
Thanks all!

#5 yedrow

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Posted 09 January 2013 - 10:45 PM

Ditto to what Neil and TJR said. Also, you might try coning the clay up pretty high and then down a few times, this helps organize the particles in the middle and against the bat. As a rule, cracks on greenware begin fairly early on in the process.

Joel.

#6 OffCenter

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Posted 10 January 2013 - 10:28 AM


Natania;
I had a problem like this a couple of years ago. It could be a wedging problem. After you wedge your clay into a cone, you have to rotate the bottom of the clay on the wheel head before you smack it down. For some reason, the bottom of the cone doesn't get compressed as well. Also, you have compress with a rib on the inside. I also wire cut them off the bat immediately after throwing. Next day, I flip them over on another bat. I also dry mine under plastic like Neil has explained.
TJR.


Thanks. I'll try both those things. I do wire them off right after throwing, but not again a bit later. I'll cover the rims too. What do you mean, TJR, by rotating the cone of clay before throwing? Just compressing it on the wheel head manually before beginning to throw?
Thanks all!


One reason the bottom of pots are more prone to cracking is that that clay is only worked from one side while throwing. That is why compression and working the bottom after the pot is off the wheel helps prevent cracking. Another thing that may help when all else fails is to center the clay and work it by pushing it up and down a few times (especially the top part of the cone), then cut if off and flip it over so that that well-worked top of the cone is now the bottom and will become bottom of the pot.

Jim
E pur si muove.

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#7 GEP

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Posted 10 January 2013 - 11:13 AM

Natania,

In addition to all the good advice noted above, I wrote a blog post with a few more specific tips for keeping those platters from cracking in the middle:

http://www.goodeleph...out-cracks.html

Mea
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#8 Natania

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Posted 10 January 2013 - 11:35 AM

Natania,

In addition to all the good advice noted above, I wrote a blog post with a few more specific tips for keeping those platters from cracking in the middle:

http://www.goodeleph...out-cracks.html

Mea



Thanks everyone, I will try ALL of the above. I had to laugh reading the beginning of your post, Mea, since your experience mirrors my own journey toward realizing that I had a problem that needed addressing. I must admit also, that i am trying to skip the trimming process. I throw the floor fairly thinly, and use a profile tool to shape the edge of the bottom of the platter. I also bring the rim out quite wide, so although the platter isn't lifted up with a trimmed foot, I think the profile of the whole piece works well. Could this be contributing to my problem? Not sure why I would absolutely need to trim a foot? Also Mea, did you use melamine bats? If so, where did you get them?
Thanks!

#9 OffCenter

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Posted 10 January 2013 - 11:50 AM

Natania,

In addition to all the good advice noted above, I wrote a blog post with a few more specific tips for keeping those platters from cracking in the middle:

http://www.goodeleph...out-cracks.html

Mea


Nice platters!

Jim
E pur si muove.

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

#10 GEP

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Posted 10 January 2013 - 04:23 PM


Natania,

In addition to all the good advice noted above, I wrote a blog post with a few more specific tips for keeping those platters from cracking in the middle:

http://www.goodeleph...out-cracks.html

Mea



Thanks everyone, I will try ALL of the above. I had to laugh reading the beginning of your post, Mea, since your experience mirrors my own journey toward realizing that I had a problem that needed addressing. I must admit also, that i am trying to skip the trimming process. I throw the floor fairly thinly, and use a profile tool to shape the edge of the bottom of the platter. I also bring the rim out quite wide, so although the platter isn't lifted up with a trimmed foot, I think the profile of the whole piece works well. Could this be contributing to my problem? Not sure why I would absolutely need to trim a foot? Also Mea, did you use melamine bats? If so, where did you get them?
Thanks!


I don't use melamine bats, I throw platters on Hydrobats (made of hydrocal) then transfer them to melamine boards for drying. I bought some melamine boards from Home Depot, and got some others from taking apart some old furniture.

I don't think that the untrimmed bottoms are related to the cracking, I don't see any reason why you can't continue doing that.

Mea
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#11 neilestrick

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Posted 10 January 2013 - 07:59 PM

This just goes to show how many opinions there are in ceramics:
I have the opposite opinion about bats. Absorbent bats will dry the platter more evenly, because the bat will wick water from the bottom while the top air dries. As long as the platter is cut loose from the bat, sticking is not an issue. I use Medex bats from Northstar. Medex is an exterior grade MDF that lasts a good long time. I have bats that are 10+ years old and the holes still fit tight. They do hump slightly in the middle, but they do so evenly around, not like taco, so it doesn't warp the pot. My platters dry even enough that I never need to flip them over to get them to leather hard. I just wrap the lip with plastic 'caution' tape so it doesn't dry too quickly, and in 2-3 days they are ready to trim without ever flipping them or uncovering the lip. Plaster/hydrocal bats are ideal for this.

I think the untrimmed bottoms are related to the cracking. I think there is a good chance that the platters are slightly uneven, which is all it takes on a larger piece to cause cracking. If the flat area of the platter curves up even a little, and is a little thicker at the edges, it'll crack.
Neil Estrick
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#12 Mark C.

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Posted 10 January 2013 - 10:41 PM

I throw platters on Northstar blue plastic bats up to 15 inches then its medex as Nieil uses for bigger ones-The blue bats stored flat have never warped-the medex do a bit but I store them flat with the blues bats on top -that keeps them pretty flat. When I flip them it many times is on a huge 3/4 inch formica round which is real flat.I trend to cut with a thicker wire on larger forms-keeps them from sticking as much.
This all seems to be what works best for each really.

Mark
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#13 GEP

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Posted 11 January 2013 - 09:58 AM

This just goes to show how many opinions there are in ceramics:
I have the opposite opinion about bats. Absorbent bats will dry the platter more evenly, because the bat will wick water from the bottom while the top air dries. As long as the platter is cut loose from the bat, sticking is not an issue. I use Medex bats from Northstar. Medex is an exterior grade MDF that lasts a good long time. I have bats that are 10+ years old and the holes still fit tight. They do hump slightly in the middle, but they do so evenly around, not like taco, so it doesn't warp the pot. My platters dry even enough that I never need to flip them over to get them to leather hard. I just wrap the lip with plastic 'caution' tape so it doesn't dry too quickly, and in 2-3 days they are ready to trim without ever flipping them or uncovering the lip. Plaster/hydrocal bats are ideal for this.

I think the untrimmed bottoms are related to the cracking. I think there is a good chance that the platters are slightly uneven, which is all it takes on a larger piece to cause cracking. If the flat area of the platter curves up even a little, and is a little thicker at the edges, it'll crack.


I think we all agree that the platter needs to be evenly thick throughout to avoid cracking. But I think it's possible to throw a platter that way which then does not require trimming. I also think it's possible for someone to spend a lot of time trimming and still leave behind extra clay where the floor and wall meet. It depends on awareness and skill level. Anyhow, for Natania I would advise that Neil's point is valid, and it would be worthwhile to throw and platter the way you normally do, then slice it in half to see it's profile.

Mea
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#14 Natania

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Posted 11 January 2013 - 11:32 AM


This just goes to show how many opinions there are in ceramics:
I have the opposite opinion about bats. Absorbent bats will dry the platter more evenly, because the bat will wick water from the bottom while the top air dries. As long as the platter is cut loose from the bat, sticking is not an issue. I use Medex bats from Northstar. Medex is an exterior grade MDF that lasts a good long time. I have bats that are 10+ years old and the holes still fit tight. They do hump slightly in the middle, but they do so evenly around, not like taco, so it doesn't warp the pot. My platters dry even enough that I never need to flip them over to get them to leather hard. I just wrap the lip with plastic 'caution' tape so it doesn't dry too quickly, and in 2-3 days they are ready to trim without ever flipping them or uncovering the lip. Plaster/hydrocal bats are ideal for this.

I think the untrimmed bottoms are related to the cracking. I think there is a good chance that the platters are slightly uneven, which is all it takes on a larger piece to cause cracking. If the flat area of the platter curves up even a little, and is a little thicker at the edges, it'll crack.


I think we all agree that the platter needs to be evenly thick throughout to avoid cracking. But I think it's possible to throw a platter that way which then does not require trimming. I also think it's possible for someone to spend a lot of time trimming and still leave behind extra clay where the floor and wall meet. It depends on awareness and skill level. Anyhow, for Natania I would advise that Neil's point is valid, and it would be worthwhile to throw and platter the way you normally do, then slice it in half to see it's profile.

Mea


Trust me, people, the floor is thin. Perhaps too thin, but definitely not too thick. I've been throwing for many many years, although I've avoided platters up to now (not sure how). If there is a bit of extra clay where the floor meets the wall, it is perhaps 1/4th of an inch thicker than either the foor or the wall, but not more. I can feel how thin the floor is (and hear it when I touch the bone dry ones that haven't cracked). I am quite sure that this is not my problem. Actually, when I started the platter project, I had to wait so long for the platter to release from the plastic bats, they were quite hard and I think that this is probably where the difficulty lies. Wiring off twice may be the key. However, I will continue to skip trimming, get some hydro bats, wire off after throwing and again a bit later, dry under fabric and support the bottoms. Whew! Is there anything else I missed?

Thanks everyone for your help! I will post a picture if it works out and I become platter queen.

#15 GEP

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Posted 11 January 2013 - 11:48 AM

Depending on the overall thickness of the platter, and extra 1/4 inch of clay where the floor meets the wall can be enough to cause the platter to crack. The extra thickness should be thrown out or trimmed off. One more thing ... if you throw the platter on a Hydrobat, you do not need to wire under them at all. When leather hard, they will pop right off.

Mea
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#16 Natania

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Posted 11 January 2013 - 12:00 PM

Depending on the overall thickness of the platter, and extra 1/4 inch of clay where the floor meets the wall can be enough to cause the platter to crack. The extra thickness should be thrown out or trimmed off. One more thing ... if you throw the platter on a Hydrobat, you do not need to wire under them at all. When leather hard, they will pop right off.

Mea



Oh good. The ad for the hydro bats said that, but I wasn't sure, given the issues involved. I ordered the bats today. I'll watch that corner when throwing to avoid extra clay.
Thanks again!

#17 Dinah

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Posted 13 January 2013 - 07:37 PM

You throw a very big and to your mind a thick piece. A bowl or a platter. You wire it off. Right after throwing.

Or, if it's wet and sloppy, no wiring off. Leave. Use a propane flame to tighten up if you're pressed for time or a heat gun. Come back after several hours, or overnight, and continue to throw and refine, then wire it off.

You...when it is drier, flip piece over onto another bat. Richard Phethean, a UK potter with a couple of books out there, has the best explanations of throwing, perhaps for more advanced, I've ever come across. In the meantime, put a piece of upholstery sponge you've trimmed to fit depth onto the receiving bat. Or a folded towel. Just check the depth. I have made bats from marine ply and used spar varnish to finish. I've also bought bats from Seattle Pottery Supply and Clay Arts in Tacoma. All work beautifully. Then, when piece is drier, you can turn foot ring. If it's that big a piece you'd be doing yourself a favor by turning a double foot ring.
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#18 Marcia Selsor

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Posted 13 January 2013 - 08:50 PM

Ditto to all of the above. My platter bats are home made plywood sealed with varithane or enamel paint 18-24". They are 30-40 + years old. I have a bat extender for my Bailey wheel ( a 3" thick bat ) that raises large bats above the splash pan. .. I flip the platters over and over as they dry, supporting the soft center with sponges to prevent sagging. The lips need the 24" bats and the bottoms need the 18" bats. As soon as the lips are hard enough to support the weight, I flip them over to dry the bottom. I rotate them daily. I sandwich the platter between two bats and flip them, sometimes using my head, literally.




Marcia

#19 Diane Puckett

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Posted 14 January 2013 - 01:44 PM


Depending on the overall thickness of the platter, and extra 1/4 inch of clay where the floor meets the wall can be enough to cause the platter to crack. The extra thickness should be thrown out or trimmed off. One more thing ... if you throw the platter on a Hydrobat, you do not need to wire under them at all. When leather hard, they will pop right off.

Mea



Oh good. The ad for the hydro bats said that, but I wasn't sure, given the issues involved. I ordered the bats today. I'll watch that corner when throwing to avoid extra clay.
Thanks again!


I am not crazy about my Hydrobat. Initially, I had to use the garden hose on it full pressure to remove lots of tiny pieces of the bat. The bottom of the first platter I made was covered with them. Once they were gone, the bat was covered with tiny holes, not a big issue as long as you plan to trim. I was told to moisten the bat before using it and to run a thin rib under the edge of the thrown platter so that it would later release. Those things do seem to help. Being super-absorbent, the bat can be difficult to clean, especially if I let clay dry on it. I generally take it outside, clean it with the garden hose, and let it dry in the sun.

It is fabulous for drying and wedging really wet clay.
Diane Puckett
Dry Ridge Pottery

#20 Natania

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Posted 14 January 2013 - 04:18 PM

Interesting. Thanks for the reply. I have ordered one but it hasn't arrived yet. I'll have to experiment when it gets here, and I will keep in mind that I may have ordered an expensive clay-drying surface...




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