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grayfree

The dreaded S crack

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Pressing too hard's not good - there's a risk you might remove clay instead of truly compressing it. Do you turn your thrown ware? (Or trim, as I believe people the other side of the Atlantic call it!) Stresses can be set up in the clay if the pot's too dry, and/or the tool's not sharp enough, and/or you put too much pressure on the unsupported base. The turnings should have the texture of grated Cheddar - any crumblier and you're creating a drag which will exacerbate any underlying compression problem. Good luck!

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Oh, bad luck, Grayfree! We had -17 (Celsius) here in the middle of the UK last winter (a record!), and even though I keep a heater on frost-setting overnight in the studio, next morning I found a whole day's mugs standing with frost crystals on the surface. I feared the worst so scrapped them, saving just one for the sake of experiment, and guess what? It was fine. (Grr!) The surface was very finely dimpled but I processed it as usual (turned a foot-ring, pulled a handle, fired and glazed it) and it was indistinguishable from the normal run. I must just have been lucky that the surface frost hadn't got into the body properly - or unlucky, as I consigned the rest of the day's work to the recycling bin!!

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I am no expert, but heres my 1/2 penny's worth..... I used to get lots of S cracks. I read in a PMI, I think, of a potter who threw mainly tall, thin necked bottles, that she didn't add a foot to, just cleaned up the edges. She said that to prevent an S Crack in a tall, bottle shape, that "coning up was vital". My tallest bottle shape at the time was probably all of five inches, but she got me into the habit of coning up, on each piece, as part of my centering process. I will still get an S crack on occasion, but most of the time, I can trace that back to either a 'fought' over piece - one that didn't 'take' right away and I had to fight - and therefore used too much water; or a too thin bottom. (I'm trying to be consistent - not too thin, not too thick.)

 

Another thing that helped me tremendously -- I throw on Hydro Bats, almost exclusively. You don't have to cut the piece off... it will 'pop' off after a day or two. Especially helpful on wide pieces... plates, etc. I love them.

 

Darla

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Thank you Darla I cone everything and wedge everything and I do use hydrobats. And Thanks again Lucille I tried your technique for making platters and love it. Now They are drying in my newest idea pizza boxes. I am out of town so they will be left alone no fussing for 5 days.

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As most have said compression does help a lot. Also keep any water out of the bottom. Starting out I got a few S cracks but following those rules and gradually drying out pieces I haven't had any. I dry pieces either on a bat loosely covered in a plastic bag or on a wire rack with a plastic bag over the top, leaving the bottom exposed through the rack.

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Outside of issues with the clay itself, you can get rid of most of your 's' cracks with good compression, as everyone is saying. Another trick is to snip the piece from the bat as soon as you can. This is especially true with wider pieces like pie plates and platters. I find that it helps to allow the wider pieces to dry a little and then cut them. I also try to not let them get very far into leather hard before I remove them from a plastic bat. The outer part of a wide form has more clay in contact with the bat (better stick) than the inner part (the old 2Ï€r thing), edit [this makes the inner part of your pie plate susceptible to the drying forces on the exposed surface, while the outer 'ring' holds with a stronger bond. . Also, if you are getting small cracks, in the range of an inch or so, it could be a pugging or wedging issue. You should always slap your clay down so that the grain of the plug of clay is perpendicular to the bat. For instance, if you pug out a tube of clay it should attach to the bat by the rounded side, not the flat side. I've known a few long term potters who've never figured that one out :) good luck!

 

Oh, also, you can solve many of your drying issues by thinking of the bottom of a pot as being two surfaces of different lengths and speeds of motion, a plate like || and a bowl like )). If the bottoms are both the same length in the beginning the exposed side will soon enough be shorter since it will loose material (H20) the quickest and thus there will be less of it. The softer part will spread/give up cohesion to maintain equal lengths.

 

Yikes, one more edit: Don't let wide flat forms sit in a draft. Cover them with a towel or otherwise keep their drying slow and even. A pie plate should be on the bisque rack waiting to be fired in 24 to 48 hours, or so. Keep the drying slow and get them off the bat as soon as possible.

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