Gail Dapogny on thu 21 nov 02
Vince,
I would love to hear you talk a bit more on the subject of clay memory.
I of course have heard about it and tried to "respect" it in a somewhat
ignorant fashion, but always wonder in puzzlement (as I am working) whether
it is relevant to throwing, or tooling, or both, and how it works exactly,
and if it is correctable during, say, the tooling/finishing stage of work.
In other words, can those stress zones be undone or gotten rid of? I
suspect that If your clay goes out of round during throwing, you don't need
to just throw in the towel then and there, but at what point have you done
irreparable harm? Seems to be a fascinating subject.
Also...I was talking with Tom Turner last weekend, and he mentioned that he
feels that if flint (silica) is too fine (200 plus), porcelain will
readily warp. (At least, I think that's what he was saying; I may be
over-simplifying.) Be interesting to know what Ron thinks along these
lines.
Thanks--- Gail
>The most common causes of warping are 1) uneven support of wares in the
>firing,
>2) overfiring of a claybody, causing it to become excessively pyroplastic,
>and 3) clay memory from forming and drying, leaving stress zones in the
>wares which "relieve" themselves in the firing, causing serious warping.
>The importance of the latter cannot be overstated. There are so many things
>people do to leave undesirable clay memory in the wares - stresses which are
>stored in the rigid unfired clay, but which cause the clay to move around
>once it becomes pyroplastic in the firing.
>Best wishes -
>- Vince
Gail Dapogny
1154 Olden Road
Ann Arbor, MI 48103-3005
(734) 665-9816
gdapogny@umich.edu
http://www.silverhawk.com/ex99/dapogny (single historical photo - no longer
registered with Silverhawk)
Ron Roy on sun 24 nov 02
Hi Gail,
The "memory" effect can take place whenever clay is stretched - as in slab
making and throwing - even in coiling if you try hard enough.
Stretched clay alway wants to unstretch - during drying and firing - the
higher the clay is fired the more it wants to return to the unstretched
state. Rims on plates are a case in point - because the clay is stretched
during forming the rims will rise during drying because the clay there will
shrink more than the rest of the plate. Recompressing the rim helps to stop
it from happening. Slowing the drying of rims is necessary as well so the
plates driy more evenly.
When slabs are made the outside clay is stretched and the inside clay
compressed - if you try to make tiles with partly stretched clay and partly
compressed clay you are bound to get warping with a throwing type clay.
If the pot gets enough more heat on one side the clay will shrink more on
that side causing warping. Fast firing can cause the problem because the
load does not get a chance to even out properly. This Kind of warping is
not related to memory.
As for fine silica making porcelain warp more - finer particles tend to
aggravate the problem - and also will result in quicker melting - Tom
should certainly know. I would be interested to know where the silica was
from and what the actual mesh range was in each case.
RR
> I would love to hear you talk a bit more on the subject of clay memory.
>I of course have heard about it and tried to "respect" it in a somewhat
>ignorant fashion, but always wonder in puzzlement (as I am working) whether
>it is relevant to throwing, or tooling, or both, and how it works exactly,
>and if it is correctable during, say, the tooling/finishing stage of work.
>In other words, can those stress zones be undone or gotten rid of? I
>suspect that If your clay goes out of round during throwing, you don't need
>to just throw in the towel then and there, but at what point have you done
>irreparable harm? Seems to be a fascinating subject.
>
>Also...I was talking with Tom Turner last weekend, and he mentioned that he
>feels that if flint (silica) is too fine (200 plus), porcelain will
>readily warp. (At least, I think that's what he was saying; I may be
>over-simplifying.) Be interesting to know what Ron thinks along these
>lines.
>
>Thanks--- Gail
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Gail Dapogny on sun 24 nov 02
Ron,
What Tom said, as I recall, was that his past silica had been labled as 200
mesh but he happened to know that they sieve it routinely at about 325, and
he felt that it was primarily "dust". I think he said that he likes his
silica to be about 140 (that is, labled 140). He mentioned that several
clay companies have his formula (Laguna is what he is using), and that he
uses 6-tile kaolin and Kaopate 20. (I think it was 20; these numbers were
flying by pretty fast and I was brain-scrambling to take it all in!) He
mentioned that when you see formulas with the kaopate 20, it is almost
always his porcelain formula or very close.
He was specifically talking about using these kaolins (as well as the
silica that doesn't go to dust) because of potential warping problems. I
gathered that he was having very good success with resolving this problem.
Certainly were gorgeous pots.
I have his e-mail address -- tell me if you want it. He was very generous
with information. Oh, also, he had had his bodies tested by
dilatometer--passed with flying colors. I asked if it was your machine,
and he said that he had used the one at Ortons (close by). He gave a
little nod of admiration and respect to you when I mentioned being in touch
with you.
---Gail
>As for fine silica making porcelain warp more - finer particles tend to
>aggravate the problem - and also will result in quicker melting - Tom
>should certainly know. I would be interested to know where the silica was
>from and what the actual mesh range was in each case.
>
>RR
>
>
>> I would love to hear you talk a bit more on the subject of clay memory.
>>I of course have heard about it and tried to "respect" it in a somewhat
>>ignorant fashion, but always wonder in puzzlement (as I am working) whether
>>it is relevant to throwing, or tooling, or both, and how it works exactly,
>>and if it is correctable during, say, the tooling/finishing stage of work.
>>In other words, can those stress zones be undone or gotten rid of? I
>>suspect that If your clay goes out of round during throwing, you don't need
>>to just throw in the towel then and there, but at what point have you done
>>irreparable harm? Seems to be a fascinating subject.
>>
>>Also...I was talking with Tom Turner last weekend, and he mentioned that he
>>feels that if flint (silica) is too fine (200 plus), porcelain will
>>readily warp. (At least, I think that's what he was saying; I may be
>>over-simplifying.) Be interesting to know what Ron thinks along these
>>lines.
>>
>>Thanks--- Gail
>
>Ron Roy
>RR#4
>15084 Little Lake Road
>Brighton, Ontario
>Canada
>K0K 1H0
>Phone: 613-475-9544
>Fax: 613-475-3513
>
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Gail Dapogny
1154 Olden Road
Ann Arbor, MI 48103-3005
(734) 665-9816
gdapogny@umich.edu
http://www.silverhawk.com/ex99/dapogny (single historical photo - no longer
registered with Silverhawk)
Janet Price on sun 24 nov 02
>
>Stretched clay alway wants to unstretch - during drying and firing - the
>higher the clay is fired the more it wants to return to the unstretched
>state. Rims on plates are a case in point - because the clay is stretched
>during forming the rims will rise during drying because the clay there will
>shrink more than the rest of the plate. Recompressing the rim helps to stop
>it from happening. Slowing the drying of rims is necessary as well so the
>plates driy more evenly.
Plate rims may also rise when drying because the edges dry first and so
initially shrink proportionally more than the clay just inside the very
edge. This means that the circumference of the very edge of the plate needs
to be smaller than it can be if the rim's perfectly flat. The rim can
either crack or maintain its integrity by rising at the edge so that the
circumference of the very edge can be a bit smaller and handle this greater
initial shrinkage. From this standpoint slow drying of the rims of the
plates is also helpful.
Janet Price
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