[Clayart] S crack remedy?

[Ken Chase]

Thank you
I’ve been watching you on YouTube and confused about one thing. When you cut and manipulate your pieces have they first had the bottom trimmed? And do you use wax resist to slow drying time on the rims?
Best,
Ken

Sent from my iPad

> On Nov 14, 2020, at 8:40 AM, Antoinette Badenhorst <porcelainbyantoinette at gmail.com> wrote:
> 
> I have to fully agree with Ron. Get to understand the behavior of clay particles. Then follow the logics. 
> 
> Best wishes,
> Antoinette Badenhorst
> www.porcelainbyAntoinette.com
> www.TeachinArt.com
> https://www.aic-iac.org/en/member/antoinette-badenhorst/
> 
> 
> 
>> On Nov 13, 2020, at 10:26 AM, ronroy at ca.inter.net wrote:
>> 
>> Yes all of that and - the clay particles in the wall of the pot get aligned differently that the particles across the bottom. This can result in different shrinkage rates. Paying more attention to bottom clay during throwing can make a difference. This is from The Dictionary of Materials and Techniques by Frank and Janet Hamer. 10 pages on cracks - pictures of different cracks and why they happen.
>> 
>> My theory about S cracks - the bottom clay gets twisted during centering and explains why the crack is twisted - happens more with hump thrown pots.
>> 
>> The overall problem can be traced to high shrinkage and/or not enough plasticity of the clay body and/or over aggressive centering techniques.
>> 
>> Many clay bodies can survive the problem but porcelains suffer the most.
>> 
>> RR
>> 
>> 
>> Quoting Snail Scott <claywork at flying-snail.com>:
>> 
>>> 
>>> 
>>>>>> On Nov 12, 2020, at 10:22 AM, Ken Chase <kchase235 at gmail.com> wrote:
>>>>> 
>>>>> Hello:
>>>>> Haven?t posted in a long time but read everything.
>>>>> Trying porcelain and made one I really like but has a
>>>>> Large s crack at the bottom. Is there a pre bisque repair? Thanks
>>> 
>>> 
>>> Hi, Ken!
>>> 
>>> There are a number of specific potential reasons why thrown work cracks across the bottom, but they all share the same underlying reason: tension.
>>> 
>>> Most commonly, the bottom has dried slower than the walls. This can be because 1. Water remained standing in the bottom while being thrown, not sponged up, possibly exacerbated by a prolonged period of work, allowing the bottom clay to become more saturated and delaying its drying. 2. The bottom was left sitting on an impervious or saturated bat surface while the walls had air contact, delaying the drying of the bottom. Removing the piece from the bat as soon as possible, then flipping it upside down, can usually cure this issue. If it cannot be flipped on its rim, flip it into a chuck instead, or transfer it onto a more absorbent surface like plaster. 3. The bottom was thrown quite thick and not trimmed thinner in a timely fashion, keeping it damp longer than the walls.
>>> 
>>> Work thrown 'off the hump' is often more vulnerable to cracks of this sort, as well. Throwing off the hump generally results in a lump of thicker clay at the bottom, calling reason 3. (above) to mind. However, hump-throwing can also leave the bottom 'uncompressed' and unworked, compared with work thrown directly on a bat or wheelhead, and the clay may retain greater spiral ?shear? tendency as a result. (I don?t believe that ?compression? in a literal sense is actually occurring, but the term is in common use, so I use it here. Regardless of the physics involved, working the bottom of a hump-thrown form by pressing on it does demonstrably seem to reduce bottom cracking.)
>>> 
>>> -Snail
>> 
>> 
>> 
>> Ron Roy
>> ronroy at ca.inter.net
>> Web page ronroy.net
>> 
>> 
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