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docweathers

Cracking in 20 inch greenware platters

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You got it. Defloculating lets you add more and more clay without water. Flocculate only the stuff you want to use when ready to use it and it’s super sticky again. Turns into a creamy whipped texture. Very sticky, workable, creamy and has lots of density

Edited by Bill Kielb

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24 minutes ago, Bill Kielb said:

Super slip is just clay glue where it is  created with lots and lots of clay supersaturating it by using Darvan, clay, a blender and minimal water.  when you use it you add some fully saturated epsom salt to flocculate it. It’s super sticky, super dense and has less water so shrinkage is less. It is sort of pottery glue that allows folks  to stick their handles on quickly and move on to the next.

I refer to it as "Super Slip" when I tell students about it.  It just makes it sound more impressive, but I didn't realize that anyone else referred to it as such.  The recipe I went from, called it a joining slip.

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1 hour ago, Bill Kielb said:

Sorry, compression is my only thought at this point. I think we confirmed that for whatever reason your clay cannot take the forces of drying so I am at a loss really. You could not dry this any slower or more even it appears.

Compression and alignment go hand in hand.  Compressing the base aligns the particles through the base. Hamer and Hamer (The Potter's Dictionary of Materials and Technique) have some interesting points to make about the difference in drying shrinkage vertically through the thickness of the base as opposed to horizontal shrinkage across the base because the particles are aligned differently in walls vs. the base.  Somehow, there is uneven shrinkage from the rim to the center happening during drying in these plates.  As you wrote, it's always helpful to see how a piece is thrown when figuring out a solution.  I think it would be helpful to see a video of the throwing of these pieces. Is there a lip to these plates? How is it made? 

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2 hours ago, Bill Kielb said:

I am a compression person so my first inclination is to always make sure the clay is compressed to the same density especiallly in the center. I am not an alignment guy though. To me being as dense throughout means less space or holes in between the clay particles and strength in the areas that are compressed correctly will be greatest while those that are not will be least. Partical  size in claybodies varies considerably so less holes between particle means more strength and lower shrinkage. Alignment to me is a bit too unrealistic. To me, simply fill in all the open spaces with the variety of partical sizes available in the body. This will  actually displace the water occupying those holes.  

Anecdotally, I have seen spiral cracks in bodies not wedged and right out of the bag where after thrown they begin to delaminate in a spiral direction while drying. Polarizing or aligning things would appear  to have some real downside potential.

Simply pressing down on things is not necessarily compressing it as where unconstrained the clay will squirt out. Attachments similarly can benefit greatly with a joint that is outlined and compressed with a suitable smooth scribe. Super slip becomes better as a glueing agent as we seek to make it thicker in clay content and as dense as practical all for the same reason, more uniform shrinkage and greater strength. I always felt that scoring joints and then compressing the connection actually provided space for the joint to  fill with as much clay as possible, just like applying tile glue with a proper V notch spreader.

slow drying and evening out water content are two things to me so having something in a bag for a day or two to even things out makes sense. Three months, seems counter productive for something that is uniformly flat. For sculpture I understand the need for slow to maintain even drying  between differing thicknesses of clay.

I think your inadvertent keep them in a bag test proved there was a weak area in your finished clay (assuming reasonable storage and freedom to move during drying) so now the question would be what construction method was responsible for the loss of strength in those areas?

Your point is well taken, re squirting clay. To clarify,  when opening, I'm pressing from both the top and side of the opening, so the clay can't squirt anywhere. 

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2 hours ago, docweathers said:

Since I throw completely dry with some roller tools that I built, I can put a huge amount of pressure on the clay to compress it. If it's not compressed after what I do in it is not going to be compressed with anything.

 

What is this super slip you mentioned?

Have you made any videos of the roller tools in use?

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8 minutes ago, RuthB said:

Have you made any videos of the roller tools in use?

No I have not. They work just like your finger sliding over the clay only pressure being applied with a roller so you don't need water as a lubricant. All the clay moving strategies are exactly the same. The only disadvantage I can see is that you have to change tools more often since rollers are not as flexible as fingers. Attached are some pictures the roller tools and one of me throwing a jar with it. What you can't see in that picture is that there is another roller inside the jar. I keep adding new variants on the basic design. As you can see one of them is nothing more than a wallpaper seam roller. 

Demo of Roller tools (Small).JPG

P1120834.JPG

P1120835 (Small).JPG

RollerTools (Small).JPG

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doc, just FYI, if you throw on Duron,  commonly called  masonite, bats as you describe, there is no reason to wire them at all.  when dry, they will just separate.  usually within a day or two depending on the humidity level.   if you watch youtube videos you might have come across bill van gilder showing how to trim a plate on thick foam.   lots of good info in that one. 

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Grogged clay has been associated with cracking around each little piece of grog. Grog I think can weaken the clay structure..been discussed before bur dont know the relevance here.

Throw a platter with different clay go through same process and see!

Slow drying shouldnt worry uf drying evenly. "Found" a platter put high and safe....and forgotten.

Took it out let it dry off and it was ok jyst saying!

But it was on its foot sitting on a number of slats of wood for all round circulation.

I'd fire just one of your mended platters  as this way you can put rest in bucket if your mending didn't do the job.

Spooze plus paper may be a good choice of mender.

Big post on it a while back re. Moistening pot etc.

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When throwing plates, platters, and other wide pieces, I recommend cutting the piece loose from the bat regardless of what the bat is made from, with the exception of plaster bats. Because wide pieces have further to move as they shrink, any sticking to the bat can be an issue. 4% shrinkage of a 3 inch wide cup is barely 1/8". 4% shrinkage of an 18" wide platter is almost 3/4". On plastic bats, they often require re-cutting after a day of drying because they tend to re-adhere.

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Hi, Doc! I'm thinking that just because these platters are intended for the wall, you are not giving enough love to their backsides. As Neil said, there's a lot of movement going on in such a wide form. Additional trimming on the back, even if you don't trim a footring, could help prevent the cracks you're getting by adding a little compression to the only (but large, area-wise) part of the piece that isn't compressed from both sides. 

I dry large flat-ish forms on slats for good air circulation, my slat shelf is a repurposed baby crib side (the part that lowers).

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21 minutes ago, Rae Reich said:

Hi, Doc! I'm thinking that just because these platters are intended for the wall, you are not giving enough love to their backsides. As Neil said, there's a lot of movement going on in such a wide form. Additional trimming on the back, even if you don't trim a footring, could help prevent the cracks you're getting by adding a little compression to the only (but large, area-wise) part of the piece that isn't compressed from both sides. 

I dry large flat-ish forms on slats for good air circulation, my slat shelf is a repurposed baby crib side (the part that lowers).

I'm thinking the same thing. The clay does stick to the bat and there may be some drag possibly increased by the wooden tools and the dry clay causing tension in the base. It sounds like it doesn't always happen, but often enough to be a drag ( no pun intended). As Rae suggests doing a little trimming on the back would remove this clay before it becomes a problem in drying. You may want to add 1/8" in thickness to your platters so that you have enough clay to trim off. Alternatively, don't throw them at all. Cut a piece the desired size from a slab and flip on to the bat. Attach to the wheel head and add a coil to the edge and throw just the rim.  Wire off and dry on slats. 

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Doc,

Fred Olsen and Vince Pitelka convinced me to use either canvas, tar paper, or a plastic tablecloth for bats when making large platters.  As soon as the platter is stiff enough to finish trimming the rim, a wire tool is run between the tar paper bat and the wheel bat.  Fred also only placed slip around the edges of the table cloth bat he used in making 1+ meter platters at a workshop in Beaumont, TX.  I use either canvas or tar paper bats for the my large platters (~ 12-15 inches at wet foot ring). 

see Vince's handouts here: http://www.vincepitelka.com/handoutsinformation/ 
Throwing on Canvas Bats as an Alternative to Rigid Bats: http://www.vincepitelka.com/wp-content/uploads/2019/02/Throwing-on-Canvas-Bats-as-an-Alternative-to-Rigid-Bats.pdf 

LT
 

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2 hours ago, Magnolia Mud Research said:

Doc,

Fred Olsen and Vince Pitelka convinced me to use either canvas, tar paper, or a plastic tablecloth for bats when making large platters.  As soon as the platter is stiff enough to finish trimming the rim, a wire tool is run between the tar paper bat and the wheel bat.  Fred also only placed slip around the edges of the table cloth bat he used in making 1+ meter platters at a workshop in Beaumont, TX.  I use either canvas or tar paper bats for the my large platters (~ 12-15 inches at wet foot ring). 

see Vince's handouts here: http://www.vincepitelka.com/handoutsinformation/ 
Throwing on Canvas Bats as an Alternative to Rigid Bats: http://www.vincepitelka.com/wp-content/uploads/2019/02/Throwing-on-Canvas-Bats-as-an-Alternative-to-Rigid-Bats.pdf 

LT
 

Great idea! I have made bats from the interfacing used  in sewing, available from a fabric store. It's available in various thickness and stiffnesses. I then brush porcelain slip on both sides. Great bats, easy to peel off, take no room to store. 

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ruth, that material is Pellon, a marvelous material for lots of uses.  i have a cut piece in the shape of a cat and when i press it into clay and remove it, the cat looks fuzzy and soft like a real cat.

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Doc, I’m new to this and want to ask you some questions about your wedging tool you made. I have rheumatoid arthritis and struggle working the clay on the wheel. A teacher suggested the strong arm tool but there is no way I can afford it.

i saw your invention and would love to know if I can get you to email me the plan or anything I need so I can see if my son can put one together for me.

i adore clay but as my hands are getting worse I’m limited to my throwing.

i know it’s a big ask but I pray you can help.

my email is vickycarter@msn.com

any centering, wedging etc tools I would be so very indebted to you. 

Thanks for letting me jump in here.

Victoria

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