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Why did my plates split ?

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I've been making mugs, bowls, & other similar shapes for several years.  Decided to try throwing some small plates - and my first three all split in half while drying.  I suspect it's because they dried too fast and/or unevenly.  (The rims were nearly bone-dry, while the rest was still a stiff leather-hard.)    Am I right - this is a drying issue - or is there something else that might cause this ?

It's Standard #112 stoneware.  Plates are approx. 5/16" thick, & the two smaller ones are btwn 5" and 6" across - the larger 7"-8".  The split on the large one followed the central stem of the leaf that was pressed into it - but the other two did not, so I don't think that was a primary factor.

 

20171202_210907a.jpg

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112 is a very forgiving clay, so they're most likely too thick. At least too thick for the speed at which you dried them. Plates should be under 1/4" thick, especially plates that small. Trim more next time. Also make sure you compress the bottom well during throwing. If you do a good job compressing and trimming, you shouldn't need to slow down the drying at all for something that size.

D.M.Ernst likes this

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As Neil says 112 is extremely forgiving. That said, I do know that when I first started doing patens(communion plates), I had trouble. In the beginning I used stiffer clay, and tried to throw thin, but it was difficult with the stiffness. So I tried using the clay from the outside portions of the bag, cut off from the center. As I let my clay sit all year round it freezes, and thaws causing the water to flow to the outside of the block. If I cut this off and wedge it well. . .very well, it is great for patens. I throw them with about 3/8  inch thickness on plywood bats, and let them set until the rims are leather hard, cut them from the bat then flip with a bat on top to remove the bottom. This allows me to let the bottoms stiffen up for trimming which I like to do just before complete leather hard. . . carefully.  The trimmed foot ring is about 1/4 inch deep, and final plate before firing 1/4, with center a tad thicker.

In the end, when using the 112 0% loss on patens out of probably 200. 

 

best,

Pres

D.M.Ernst likes this

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Yep- to fast drying and to thick of plates-All good ideas posted above that will fix this issue.I am unfamiliar with that clay as its an east coast/midwest body.

Any cracks in middle also can be compression cracks-like S  cracks

slow the drying down

 

one note others forgot to mention is if you really ,really, really  needed them -then the chances go way up for them cracking.

Edited by Mark C.

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Oh, one other thing that helps on plates is compression, compression, etc. This is important as the surface will have fewer weak spots in the span if you compress well. I have a very large slightly curved rib that I use to compress my plates.

 

best,

Pres

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While I agree the number one issue is likely too fast drying ... the rims fighting the rest of the plate ... I would also be looking at what method you are using to get the leaf impression and at what stage you are applying  that pressure to the plate. Looks like one crack is right up the central vein of the leaf.

Stephen likes this

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21 hours ago, Pres said:

...set until the rims are leather hard, cut them from the bat

How many of you cut at this stage and how many cut immediately after throwing?

Joe

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20 hours ago, Pres said:

Oh, one other thing that helps on plates is compression, compression, etc. This is important as the surface will have fewer weak spots in the span if you compress well. I have a very large slightly curved rib that I use to compress my plates.

best,

Pres

These are small plates (saucers), so used a small rib, but did compress a good bit in the process of getting them smooth - didn't want any spiral finger marks running through the leaves.

8 hours ago, Chris Campbell said:

While I agree the number one issue is likely too fast drying ... the rims fighting the rest of the plate ... I would also be looking at what method you are using to get the leaf impression and at what stage you are applying  that pressure to the plate. Looks like one crack is right up the central vein of the leaf.

Leaves are applied at soft-leather stage.  I mist the back of the leaf with water - it helps create a very thin layer of slip that brings out the texture better.  Then, work from center out,  I lightly rub the entire leaf with a modeling tool, so that it is uniformly pressed into the surface.   (The tool is essentially a very small spoon, similar to the picture below.  It's one of several leather-working tools that I've found uses for in my clay adventures.)  The process is very similar to what you would do with a piece of chalk or charcoal if you were using tissue-paper to make a rubbing of the leaf. 

 

image.png.0f98ed5b02f7ff7eb45100571fed2e21.png

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4 hours ago, Joe_L said:

How many of you cut at this stage and how many cut immediately after throwing?

Joe

Neither, plaster batts for the win. No cutting and the base dries out from the plaster absorbing moisture, plates will pop off when ready to trim the bottoms. When I did have to use wood batts I would wire them off straight after throwing, think leaving them on impedes the pot shrinking and contracting during the first phase of drying. #1 for uneven drying being the most likely culprit.

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What concerns me is the width of these cracks, as well as the uniformity on all three pieces. Not hairline, nor S cracks, but rather large (as clay goes). Measure the width next time after you throw: and measure at bone dry: compare. I would expect to see shrinkage in the 10% range. The crack pattern indicates drag to me: which is why I think Min is correct.

Given the shape; you essentially have the same surface contact as tile. So here is a tile trick that will solve the drag issue. Go to the Dollar Store and get the cheapest and thinnest roll of wax paper you can find. Cut into pieces slightly larger than your plates, and lay the thrown pieces on them after you cut them from the wheel. (when they can be handled safely) Then as they shrink, the wax paper will move with them.  Old tile trick.... which should work for you. Might need very minor touch up on the bottom when bone dry.

Nerd

Side note: suspect Nep SY was used as the body flux: which rapidly accelerates drying of stoneware. How long after they were thrown, did the rims dry? (guessing fast)

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7 hours ago, Joe_L said:

How many of you cut at this stage and how many cut immediately after throwing?

Joe

I cut with a thicker wire for thicker forms thiner wire for smaller forms right after throwing them-all my plates-dinner and salad -pie plates and chip and dishes as well as platters-all off which have been made is last few weeks.With porcelain clay it drys fast and I let the flat stuff dry a bit slow before flipping it to dry backsides then trim as soon as the form can support itself upside down well .

D.M.Ernst and glazenerd like this

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Cut them right away after throwing, then again as they set up a bit if you're using plastic bats. Wood/pressboard bats should only need one cut.

With plates that small, with that clay body, you should be able to dry them just about any awful way you can think of as long as they're not stuck to a bat or too thick.

GEP likes this

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5 hours ago, Marcia Selsor said:

I cut right away. I dry slow sandwich flipping regularly as they dry.

Question: at what stage did you make the impression? Was the plate trimmed yet?

Marcia

 

Leaves were applied before trimming - probably "cheddar cheese hard".  When I'm putting them on mugs, I trim first, and apply the leaves after I attach the handle - then wait 'til it's completely dry before I pull them off.  Since they covered a significant portion of the plate, and the rim was already drying much faster than the rest, I peeled them off after trimming.

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For those the wire cut the plates off the bats and move them right away, How do you keep the plates from distorting when lifting them to place them on a ware board?

I wire mine after turning too but I leave them on the bat until the next day

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1 hour ago, RonSa said:

For those the wire cut the plates off the bats and move them right away, How do you keep the plates from distorting when lifting them to place them on a ware board?

I wire mine after turning too but I leave them on the bat until the next day

Cut them loose right away, but leave them on the bat until they stiffen up enough to flip. On wood bats they'll pop off once they stiffen up. On plastic bats you'll have to cut them again before leather hard.

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Interestingly there's less variation in method than I thought there might be, with most people cutting straight away and leaving on the bat .  I do this too, usually no problems

I've been having some trouble recently with bowls not popping off even after quite a few days, but I think this is caused by very low temperatures in the studio shed at the moment slowing evaporation (sometimes 2-3C) so I moved the bats to the house and propped the bats on yoghurt pots so the bat could dry a little from underneath (keeping the bowls somewhat covered to even the drying). Seemed to help.

Joe

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