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fw: tech data on slumping?

updated mon 9 aug 99

 

Deborah Bouchette on thu 5 aug 99

------------------
(reposting, since mailer daemon reported that the recipient mailbox was =
full)

-----Original Message-----
From: Deborah Bouchette =5BSMTP:principal=40aleatoric-art.com=5D
Sent: Sunday, August 01, 1999 11:26 PM
To: Clayart Listserv (E-mail)
Subject: Tech data on slumping?

How thin is too thin?

I've been through dozens of resources trying to find information on =
predicting
slumping in a clay body. Tichane's book is one of the few that even mention
slumping, but still not much of a help.

What I'm looking for is...if a piece slumps in a firing at its =22normal=22
maturation temperature, can you fire it lower and keep it from slumping =
(yes,
I'm trying this...)? If it's actually melting when it slumps, is this =
partial
or total vitrification, even though the clay is not getting to its =
maturation
temperature? Does having a glaze on (appropriate for the firing temp) =
increase
the risk of slumping (it seems to in my tests)? Does a mat or gloss glaze =
make
a difference (haven't tried this yet)?

Any gurus out there?

By the way, thanks Clayart for info on local reduction a while back...I now =
have
a cone 9-10 copper red out of my electric kiln by taking a Coleman recipe =
and
adding 1=25 silicon carbide fff to it. It's got some salt scumming that =
I'll try
to get rid of by putting a little vinegar in the batch, but my better half
actually =2Alikes=2A the scumming effect :-) I got a nice celadon, too--the=
color
was perfect but I didn't like the glaze texture. More tests, more tests. I
thought when I graduated that I was done with tests=21

---------------------
now 15 years in Oregon
Deborah Bouchette
principal=40aleatoric-art.com
http://www.aleatoric-art.com

Tom Wirt on sat 7 aug 99

>
> -----Original Message-----
> Subject: Tech data on slumping?
>
>
> I've been through dozens of resources trying to find information on
predicting
> slumping in a clay body. Tichane's book is one of the few that even
mention
> slumping, but still not much of a help.
>
> What I'm looking for is...if a piece slumps in a firing at its "normal"
> maturation temperature, can you fire it lower and keep it from slumping
(yes,
> I'm trying this...)? If it's actually melting when it slumps, is this
partial
> or total vitrification, even though the clay is not getting to its
maturation
> temperature? Does having a glaze on (appropriate for the firing temp)
increase
> the risk of slumping (it seems to in my tests)? Does a mat or gloss glaze
make
> a difference (haven't tried this yet)?
>
> Any gurus out there?
>


Deborah..
A couple of suggestions you might try. Find out the temp at which you're
getting slumping. A good way to do this with some repeatable consistency
(as opposed to throwing pots which may vary in thickness and construction),
is to use extruded (or rolled) rods of clay 1/4" - 3/8" in diameter It's
best if these are extruded for the sake of consistency. Suspend them on "V"
shaped wedges (point up) in the kiln at a fixed distance apart (probably
6"). Set a cone next to them for exact heat-work measurement.

Once you find the slump point, make up bars about 1" wide, 1/2" thick and
maybe 6-7" long. Fire normally (again with cone) and take from the kiln
while still hot (gloves or tongs), weigh them on a gram scale, place them
still hot in boiling water, boil for 10 minutes and then shut the heat off.
Cool to room temp in the water, remove, dry off, and re-weigh. Now you can
measure the absorbency by taking the final weight, subtracting the original
weight and dividing the remainder (if any) by the original weight. This
will give the % absorbency. A clay is fully mature when it reaches 0
absorbency. There will be some point in here where your slumping will
appear as a bent rod.
Now you will have some idea of exactly how matured (vitrified) you clay body
is at specific cone positions.

Don't assume that the maturation point suggested by the clay company is the
true maturation point of the clay, especially if it's one that's claimed to
have a wide firing temp. The wide firing range clays are typically overfired
at the top temp. And many of the clay companies have never run the slumping
tests suggested above.

I suspect you could add the glaze issue to the equation by glazing the
center of the rod and doing the slump test as above. I suppose its possible
the glaze could add to the slumping by changing the surface tension of the
clay and/or from extra weight added when you're right on the line.

Hope this gives some ideas.

Tom Wirt

Ron Roy on sun 8 aug 99

Hi Deborah,

Pots slump because they are not "engineered" properly or the clay is over
fired. Let me try and explain the first part. When forming - lets say a
bowl - and the form is such that the weight of the clay is not transferred
to the foot in a certain, even way - and the clay is vitrified enough to
exclude water - slumping will occur. The more the clay is vitrified the
more "engineered" the form has to be.

Pyro plastic clay is not very good at supporting any kind of weight so care
has to be taken so that the weight of the upper clay is transferred down to
the foot in a logical and even way - It is surprising what one can get away
with - like with vitrified porcelain - if you have engineered the weight
transfer properly.

One of the hardest forms to make and not have it slump would be a wide
shallow bowl with a small foot.

You might want to test the degree of absorbency of your clay to find out if
it is too tight.

We are assuming a certain degree of trimming skill here - both sides of the
ware should be similar in cross section.

Wish I could show you on a black board - it would be so simple that way.

Glaze on the pot will certainly increase the clay melt - the thinner the
clay the more effect. The higher the firing the more effect glazes have on
the clay. Some glazes may have some solubles that actually get well into
the body and will contribute more melting.

Anyway that's how I see it.

RR


>-----Original Message-----
>How thin is too thin?
>
>I've been through dozens of resources trying to find information on predicting
>slumping in a clay body. Tichane's book is one of the few that even mention
>slumping, but still not much of a help.
>
>What I'm looking for is...if a piece slumps in a firing at its "normal"
>maturation temperature, can you fire it lower and keep it from slumping (yes,
>I'm trying this...)? If it's actually melting when it slumps, is this partial
>or total vitrification, even though the clay is not getting to its maturation
>temperature? Does having a glaze on (appropriate for the firing temp) increase
>the risk of slumping (it seems to in my tests)? Does a mat or gloss glaze make
>a difference (haven't tried this yet)?

Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849

Web page: http://digitalfire.com/education/people/ronroy.htm