Butch Welch on tue 16 dec 03
I am an infant in the process of mastering cone 6 glazes, but I have taken
my first few steps. The thing I feel most important to learn is what
parameters in the glazes best match to the clay I am using. Now I have the
ability to know what the calculated expansion coefficients of the various
glazes are. I have requested and received the calculated expansion
coefficients for three of Highwater Clay's cone six clay's. There is as wide
a variance in the clay expansion in the same manner as the wide variances in
the glazes.
My questions:
1. Has any one worked on developing the clay to match to the glaze.
2. Has anyone done any worked to develop a range of numbers to matching the
calculated expansion of the clay and the calculated expansion of the glaze.
3. I do not mix my own clay so I can' calculate the clay numbers on my own.
Has any one who manufactures their own clay used the calculated expansion
numbers to dictate the mix of the clay?
All questions, comments, and curses welcome.
If you think I am in left field speck up and say so.
Regards, Butch
Jon Pacini on thu 18 dec 03
Greetings all-------Hi Butch----
Butch Welch wrote----I am an infant in the process of mastering cone 6
glazes, but I have taken
my first few steps. The thing I feel most important to learn is what
parameters in the glazes best match to the clay I am using. Now I have the
ability to know what the calculated expansion coefficients of the various
glazes are. I have requested and received the calculated expansion
coefficients for three of Highwater Clay's cone six clay's. There is as wide
a variance in the clay expansion in the same manner as the wide variances in
the glazes.
My questions:
1. Has any one worked on developing the clay to match to the glaze.
2. Has anyone done any worked to develop a range of numbers to matching the
calculated expansion of the clay and the calculated expansion of the glaze.
3. I do not mix my own clay so I can' calculate the clay numbers on my own.
Has any one who manufactures their own clay used the calculated expansion
numbers to dictate the mix of the clay?
All questions, comments, and curses welcome.
If you think I am in left field speck up and say so. >clip
Here's my take on this, for what it's worth. There are two primary ways to
do COE's, Theoretical (Calculated) and Physical (Dilatometric) Is that a
word??
The big problem with using calculated COE's with clay is that the
calculation process makes an assumption that all the minerals involved
become a homogenous fused glass. This is something that glazes come very
close to attaining, but clays don't. At least you'd better hope they don't
or your pots will more closely resemble puddles. Though they do fuse to an
extent, and may even approach Zero absorption, clays more closely resemble
collections of crystal masses. Ball Milled Porcelains being an exception and
there are more than likely some others, but they are rare.
Something else that plays into this mix is how hot you're firing the clay
and what the atmosphere in the kiln was like during the firing. Reduced
bodies have a different COE from their oxidized counterpart, even when fired
to the same temperature. And the timing of the reduction can also play apart
in this dance. This is particularly true of Iron and Manganese bearing
bodies, as these materials can either be refractory in Oxidation or act as
fluxes in Reduction. Thus changing the way the body fuses and the
temperature at which the fusing takes place.
Then of course you've got variations in the firing temperature. This is not
only a problem from Kiln to Kiln, firing to firing, but many kilns show a
lot of variation from top to bottom--side to side ---ect. This may not so
much come into play with your own firings, but if your COE's are from a
Supplier, using a sample he or someone other than yourself fired under who
knows what conditions, then the COE you are presented with is of dubious
value. Is it better than nothing??--hard to say. If you're serious about
your numbers, it's of no value at all.
It has never been Laguna's policy to regularly test our stock bodies and
keep these on file. This is due to the magnitude of the variations involved
that I have outlined above. The COE's on the bodies I fire under my
conditions in the test kiln would have very little in common with the
results you would get.
The best thing to do is to test fire bodies under the conditions they are
going to be used, in the kiln they are going to be used in, test them on a
dilatometer and adjust them accordingly. This is done by industrial users, I
have the equipment here to do it, I have about one request a year by potters
to test one of their bodies. Laguna charges a fee of $50 to do this.
Best regards
Jon Pacini
Clay Manager
Laguna Clay Co
David Hewitt on fri 19 dec 03
Butch,
In message , Butch Welch writes
>I am an infant in the process of mastering cone 6 glazes, but I have taken
>my first few steps. The thing I feel most important to learn is what
>parameters in the glazes best match to the clay I am using. Now I have the
>ability to know what the calculated expansion coefficients of the various
>glazes are. I have requested and received the calculated expansion
>coefficients for three of Highwater Clay's cone six clay's. There is as wide
>a variance in the clay expansion in the same manner as the wide variances in
>the glazes.
You are definitely on the right track in getting the COE figures from
the manufacturers, but I would guess that the answers they have given
you are not calculated but from dilatometer tests.
>My questions:
>1. Has any one worked on developing the clay to match to the glaze.
In normal circumstances it is a case of selecting a suitable clay rather
than developing one specifically for a glaze, but perhaps someone has.
>2. Has anyone done any worked to develop a range of numbers to matching the
>calculated expansion of the clay and the calculated expansion of the glaze.
Yes, Mike Bailey and I did some work on this as few years back. It was
published it the time in Ceramic Review, but you can see the information
on my web site under 'Pottery Techniques/Calculating Crazing'.
Web:- http://www.dhpot.demon.co.uk
>3. I do not mix my own clay so I can' calculate the clay numbers on my own.
>Has any one who manufactures their own clay used the calculated expansion
>numbers to dictate the mix of the clay?
I am not a clay manufacturer, but I would expect manufacturers to carry
out dilatometer test to provide reliable figures. Calculated expansion
figures assume that all the oxides go into the melt, as in a shiny
glaze, which is not the case with a clay body.
>All questions, comments, and curses welcome.
>If you think I am in left field speck up and say so.
>Regards, Butch
--
David Hewitt
David Hewitt Pottery
South Wales UK
Web:- http://www.dhpot.demon.co.uk
John Hesselberth on fri 19 dec 03
Hi David,
Unfortunately, in this country very few clay body manufacturers have
ever run any of their clay bodies through a dilatometer. Tucker has
because you-know-who does their clay body formulation and he wouldn't
let a clay body out the door if he didn't know its properties as fired.
I couldn't tell from Jon's post whether Laguna does it regularly or
not--I think not. At least they have the equipment to do it. But many
of our clay body manufacturer's don't do it and some would even say
'huh, what's a dilatometer'? As always I would be delighted to have to
eat these words by having several clay body manufacturers challenge me,
but I don't think there is much danger of that.
Regards,
John
On Friday, December 19, 2003, at 06:13 AM, David Hewitt wrote:
> You are definitely on the right track in getting the COE figures from
> the manufacturers, but I would guess that the answers they have given
> you are not calculated but from dilatometer tests.
http://www.frogpondpottery.com
http://www.masteringglazes.com
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