Janet H Walker on thu 15 may 97
I have a 3-D reasoning puzzle that I just can't get my head around
so I am hoping some of the physicists or fluid flow folks can help.
In the books, like e.g. Fraser (Ceramic Faults & their Remedies)
they show diagrams of how particles get aligned on the surface for
various kinds of forming methods. You know, parallel to the surface
with slip casting and extrusion, with the middle part of those items
having particles that are all mixed up directionally w.r.t each other.
I use a slab roller and I'm trying to figure out how to think about
the particle alignment that is going on when one does multiple
passes through the rollers. In particular -- I read somewhere that
clay shrinks more in the dimension that it is stretched more during
forming. This suggests that one needs at least two passes -- one
forwards and one sideways -- to make a slab that will shrink evenly
sideways and lengthways. OK so far?
Now, what is going on inside and at the surface of the slab if you roll
it through the rollers, change the height adjustment, and roll it back?
Now, turn it 90 degrees and do the same thing. What is happening inside?
I try to imagine a clay particle being dragged hither and yon but just
am not getting a good intuitive feel about whether (a) multiple passes
are good, bad, or irrelevant and (b) whether alternating 90 degrees
between passes is good, bad, or irrelevant.
I should say, my goal is well behaved slabs that don't warp unless I
tell 'em to. Anyone who can enlighten me further about how these
pesky little particles slide around on each other has my lasting
admiration...
Jan Walker
Cambridge MA USA
Ron Roy on fri 16 may 97
Jan asked,
>Now, what is going on inside and at the surface of the slab if you roll
>it through the rollers, change the height adjustment, and roll it back?
>
>Now, turn it 90 degrees and do the same thing. What is happening inside?
>
>I try to imagine a clay particle being dragged hither and yon but just
>am not getting a good intuitive feel about whether (a) multiple passes
>are good, bad, or irrelevant and (b) whether alternating 90 degrees
>between passes is good, bad, or irrelevant.
>
>I should say, my goal is well behaved slabs that don't warp unless I
>tell 'em to. Anyone who can enlighten me further about how these
>pesky little particles slide around on each other has my lasting
>admiration...
Hi Jan,
What I imagine is going on: I don't think there is much particle alignment
going on in slab rolling. I think the problem is stretched and compressed
clay. Stretched clay has a memory and wants to "unstretch" during drying
and firing. Compressed clay - much less so. Clay which has been rolled is
compressed in the middle and stretched around the outside. As the clay
dries the outside (stretched clay) wants to shrink more than the inside
(compressed) clay and the slab has to warp or crack.
I can imagine what might happen if you took a number of rolled 15" circles
of slab and cut them in different ways. Would you see certain patterns of
warpage which might lead to some understanding. I can even imagine - if
different sized circles were cut out of some slabs that you might find
where the general border is between stretched and compressed clay.
What to do - well working with clays that have less plasticity will help.
Keeping at least two of the edges compressed (with pieces of wood?) as you
roll them will help - the ends could be discarded.
This all supposes the slabs are dried in a controlled way so that the
effect of uneven drying is not a contributing factor.
Wish I had time to do the experiments - the results would be interesting to
many who are trying to figurer this part out.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
ronroy@astral.magic.ca
Stern HQ on sat 17 may 97
I have just read an article on thixotropic cllay and I'm eage to to try
using it. Problem, what do yu add to very plastic clay to make it slump
and flow as it is handled? Can any one help and/or give me some
references to books that will help. TIA Jeni in Denver
On Fri, 16 May 1997, Ron Roy wrote:
> ----------------------------Original message----------------------------
> Jan asked,
> >Now, what is going on inside and at the surface of the slab if you roll
> >it through the rollers, change the height adjustment, and roll it back?
> >
> >Now, turn it 90 degrees and do the same thing. What is happening inside?
> >
> >I try to imagine a clay particle being dragged hither and yon but just
> >am not getting a good intuitive feel about whether (a) multiple passes
> >are good, bad, or irrelevant and (b) whether alternating 90 degrees
> >between passes is good, bad, or irrelevant.
> >
> >I should say, my goal is well behaved slabs that don't warp unless I
> >tell 'em to. Anyone who can enlighten me further about how these
> >pesky little particles slide around on each other has my lasting
> >admiration...
>
> Hi Jan,
>
> What I imagine is going on: I don't think there is much particle alignment
> going on in slab rolling. I think the problem is stretched and compressed
> clay. Stretched clay has a memory and wants to "unstretch" during drying
> and firing. Compressed clay - much less so. Clay which has been rolled is
> compressed in the middle and stretched around the outside. As the clay
> dries the outside (stretched clay) wants to shrink more than the inside
> (compressed) clay and the slab has to warp or crack.
>
> I can imagine what might happen if you took a number of rolled 15" circles
> of slab and cut them in different ways. Would you see certain patterns of
> warpage which might lead to some understanding. I can even imagine - if
> different sized circles were cut out of some slabs that you might find
> where the general border is between stretched and compressed clay.
>
> What to do - well working with clays that have less plasticity will help.
> Keeping at least two of the edges compressed (with pieces of wood?) as you
> roll them will help - the ends could be discarded.
>
> This all supposes the slabs are dried in a controlled way so that the
> effect of uneven drying is not a contributing factor.
>
> Wish I had time to do the experiments - the results would be interesting to
> many who are trying to figurer this part out.
>
> Ron Roy
> Toronto, Canada
> Evenings, call 416 439 2621
> Fax, 416 438 7849
> ronroy@astral.magic.ca
>
Gavin Stairs on fri 23 may 97
Hi Jan,
I just got back to the list from a long weekend +. Sorry if this is behind
the times.
When you roll a slab, the surface of the clay which is in contact with the
roller doesn't move at all. It gets stuck to the roller, or to the canvas
blanket. The clay in the middle gets squeezed, and buldges out in fron of
the roller. This buldge drags the surface clay which hasn't yet come into
contact with the roller along with it, and so the surface gets stretched
that way. Put it the opposite way, the clay in contact with the roller
gets dragged through the roller gap, and the stuff in the middle doesn't
have anywhere to go, so it gets stretched in the rolling direction. If you
were to imagine the clay all made up of little cubes of different colors,
you would see them get squeezed out into long, thin rectangles, except that
the rollers would drag the outside layer along more quickly, so that part
would drag the rectangles into diamonds at the outsides. If you want to
keep the rectangles square, you have to do many passes, lifting up the
blanket before each pass, so the clay can stretch out under it before going
through the rollers. If you really want rectangles, you have to reverse
the direction as well.
Now, all this stretches the clay in only one direction. If you want to
even the strain up, you have to roll sideways as well, just as you
suggested. If this is done evenly in both senses, then you will end up
with squares when looking down on the slab, and rectangles from the side.
What this may have to do with even maturing, I don't really know. It may
be that it will help to get square tiles and round bowls. I suspect that
even and thorough wedging or pugging will do more, and even drying will do
the most. However, it is certainly true that the greatest elastic recovery
will occur in the direction of the greatest strain. It will also help to
let the clay rest after rolling, if you can afford the time.
Gavin
=================================
Gavin Stairs
http://isis.physics.utoronto.ca/
Eleanora Eden on mon 26 may 97
Hi Ron and all,
>What to do - well working with clays that have less plasticity will help.
I am so glad you brought this point up. Somebody told me that the clay
that I can reclaim from my slip-casting body wouldn't be any good in the
slab roller. Now I've got lots of boxed slip-scrap and would like to use
it to make paperclay to make tiles. So are you saying here that that would
be better than using a throwing body?
TIA
Eleanora.....learning a new editing program is such a pain and I'm 2 weeks
behind..
Eleanora Eden 802 869-2003
Paradise Hill
Bellows Falls, VT 05101 eden@sover.net
Ron Roy on thu 29 may 97
> Somebody told me that the clay
>that I can reclaim from my slip-casting body wouldn't be any good in the
>slab roller. Now I've got lots of boxed slip-scrap and would like to use
>it to make paperclay to make tiles. So are you saying here that would
>be better than using a throwing body?
>Eleanora....
Hi Eleanora,
I have no experience with slip casting bodies so I don't know how it would
work - It is deflocculated so it needs less water to be plastic - the
question is - will it work for tiles? The best answer is - try it. I don't
know how "formable" casting scrap is but I am sure there are others on the
list who have tried this.
Typical throwing clays have an overall shrinkage of about 12% - I do know
this type of clay does warp easily and requires much care in drying to keep
it flat. One of the best solutions is to add enough grog to a throwing clay
- till the overall shrinkage is 9 or 10%. The grog serves two purposes. It
provides "channels" for the water to escape more evenly and it cuts down on
the amount of shrinkage - if there is enough of it.
I suggest you measure the actual shrinkage of that casting scrap (sure glad
I didn't leave out the "s" in scrap.)
The other factor of course is to be aware that adding material to a clay
body may affect the maturing temperature of the body - 30% grog certainly
would - so keep that in mind. This would not be a factor with paper however
but the body should be well matured if durability is desired.
I recommend you use some of your creativity to develop some testing
procedures that will give you the answers you need. Be glad to help if you
will share the results.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/ronroy.htm
Eleanora Eden on fri 6 jun 97
Hi Ron,
Thanks for your response. Yes I will try it. Called the Laguna folks (who
make the clay) and they said it should work. I know I can use this slip
fine for throwing...it is about as fun as porcelain in other words small
things are just the same but big is a drag...It will be fall before I have
more R&D time...more then.
Eleanora
At 11:22 AM 5/29/97 EDT, you wrote:
>----------------------------Original message----------------------------
>
>> Somebody told me that the clay
>>that I can reclaim from my slip-casting body wouldn't be any good in the
>>slab roller. Now I've got lots of boxed slip-scrap and would like to use
>>it to make paperclay to make tiles. So are you saying here that would
>>be better than using a throwing body?
>
>>Eleanora....
>
>Hi Eleanora,
>
>I have no experience with slip casting bodies so I don't know how it would
>work - It is deflocculated so it needs less water to be plastic - the
>question is - will it work for tiles? The best answer is - try it. I don't
>know how "formable" casting scrap is but I am sure there are others on the
>list who have tried this.
>
>Typical throwing clays have an overall shrinkage of about 12% - I do know
>this type of clay does warp easily and requires much care in drying to keep
>it flat. One of the best solutions is to add enough grog to a throwing clay
>- till the overall shrinkage is 9 or 10%. The grog serves two purposes. It
>provides "channels" for the water to escape more evenly and it cuts down on
>the amount of shrinkage - if there is enough of it.
>
>I suggest you measure the actual shrinkage of that casting scrap (sure glad
>I didn't leave out the "s" in scrap.)
>
>The other factor of course is to be aware that adding material to a clay
>body may affect the maturing temperature of the body - 30% grog certainly
>would - so keep that in mind. This would not be a factor with paper however
>but the body should be well matured if durability is desired.
>
>I recommend you use some of your creativity to develop some testing
>procedures that will give you the answers you need. Be glad to help if you
>will share the results.
>
>Ron Roy
>Toronto, Canada
>Evenings, call 416 439 2621
>Fax, 416 438 7849
>Studio: 416-752-7862.
>Email ronroy@astral.magic.ca
>Home page http://digitalfire.com/education/ronroy.htm
>
>
Eleanora Eden 802 869-2003
Paradise Hill
Bellows Falls, VT 05101 eden@sover.net
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