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While looking into shrinkage rates and cracks and plasticity I found a picture I immediately recognised.

This picture exactly matches something I've encountered. I thought maybe it was the result of some 60 mesh silica sand I was trying but now I suspect not.

As someone looking for any clue along the way this small tidbit helps a bit.

Thanks Tony and anyone else at Digitalfire.

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40 minutes ago, neilestrick said:

I would say cracks like that are caused by thickness. Plates that are too thick love to crack like that.

Please excuse the lack of information on my part.

The detail I failed to explain is towards the end of the caption - "Another issue is the jagged edges of the disk, it is more difficult to cut a clean line in the plastic clay."

I hope to try this DFAC test someday soon. I've come across some minor cracking that I'd rather not encourage and ultimately eliminate if possible.

*For all those well thought folks out there who are somehow able to post with respect and consideration while inculding language that is clear and concise - my hats off to you. I consistently forget or otherwise neglect to include information to help represent my intentions. This sometimes prevents me from posting at all.

So again :) thanks for your communication skills! The rest of us are trying.

Edited by C.Banks
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Informational post:

large particle grog was first introduced into brick recipes to help keep the interior body oxidized: which in turn help stop the problem of carbon coring. Stoneware clay is actually a modified brick recipe, that has advanced in formulation the last. 50 years. There are posts from time to time of folks trying to identify cups/ plates, etc made in the 40's and 50's. Carbon spots are present, along with visible vermiculite; a commonly used grog in brick.

high plasticity clay means higher shrinkage rates. The larger the surface contact when drying: the more prone to cracking they become. As Neil pointed: that problem is compounded by weight, then add large grog to the equation: then those chunks can hang up on the surface and stick: causing a tear. Too much grog can compromise the mechanical properties as well.

10 years ago I could not tell you anything about clay except porcelain or stoneware. The last six years I have spent hundreds and hundreds of hours studying it. Read enough boring material that would put Mrs. Folgers asleep. One thing about this forum: there are many well informed and experienced people who are willing to share and teach. Almost like college without the expense. 

For the record: I am mildly dyslexic after an accident 18 years ago- head trauma. Sometimes I have to type and retype to make a sentence . My brain has it perfectly, but by the time it reaches my fingers: not so much. Tis what it is: I just keep moving forward.

Tom  and on some days mot. :)  

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The only reason I sound so good (sic) is because I edit things. Often after the fact. Don't worry, after a while you get a sense of what sort of information people might need to answer your particular question. And those of us that have been around a while will ask questions to clarify things. Its okay to practice trying to answer questions, as well as your throwing. No one starts out really good at this. Don't let it keep you from posting.  Errors and clarifications lead to good discussions that increase everyone's knowledge base.

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10 hours ago, C.Banks said:

This sometimes prevents me from posting at all.

No, no, no, don't hold back! Guaranteed there's someone else wondering the same thing and just as worried about how to express it. Near as I can tell, we're all in the same boat. Some are way more advanced than others, or better with the written word, but there is never a whiff of superiority or disdain at any level from anyone. The occasional person who gets offended for some reason, or is offensive, tends to go away right quick--probably because it just isn't part of the forum's culture. This is the best info exchange and mutual support group on the 'net, as far as I am concerned--no drama, no trash, no opinions on "outside issues"----just people sharing questions, answers, interests, and experiences,  in a true spirit of give & take. Not short on humor, either.

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On 1/25/2019 at 8:05 PM, Callie Beller Diesel said:

The only reason I sound so good (sic) is because I edit things


Hindsight is one of my greatest assests too!

On 1/25/2019 at 8:39 PM, LeeU said:

there is never a whiff of superiority or disdain at any level from anyone

Social media would be a far better place if this quality was shared more widely.

Edited by C.Banks
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  • 2 weeks later...

A couple of thoughts on cracking - I do lots of dinnerware and large platters.  Years ago I would cut the clay off from the bag, smack it around a little and put it on the wheel (this is circa the 70's and 80's).  I never had a problem with cracking.  Fast forward to today and if I don't wedge the clay really well before throwing a dinner plate or platter, I will most definitely get some that crack.  I resisted this for a long time, as I hate wedging.  It is worse with some clays but I find this to be true with all three of the clays I use.  So if you are making flat bottomed pieces, it is worth the effort of wedging!

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