Alan D. Scott on tue 5 feb 02
Having devoured Chapter 5 of Ron & John's new book, I've been thinking more
about thermal expansion of clay bodies. It seems to me that I should be
able to formulate a clay body with a calculated expansion using Insight to
match a given glaze. This may be approaching clay-glaze fit from the wrong
direction, but as I will be mixing my own clay bodies, why not make use of
Insight to fight the dragon from both ends? After all, the same materials
are used to formulate both clay body and glaze. Or, am I all wrong here???
Alan Scott
The Fourth Cup Clayworks
Parker, AZ
Jon Pacini on wed 6 feb 02
Greetings all ---As I understand it, in using a glaze calculation program, a
major factor that is assumed is that you end up with a 'homogenous vitrified
glass'. And that the COE is defined from that glass.
Clay bodies rarely fit that category. To what degree a body is vitrified
plays into the actual COE. Granted a body could be theoretically calculated
and you could see how adding or subtracting any given element altered the
calculated COE of that mix. But when fired at ^5 and /or at ^10, the clay
body will exhibit vastly different actual COE's and may not be related much
to the calculated one at either temperature.
If the glaze program takes into account the amount of vitrification that
occurs in any given clay recipe, at any given temperature, then you would
get a more accurate calculation. Does your program take that into account??
That would be the key question to ask of the Program Gurus.
Jon Pacini
Clay Manager
Laguna Clay Co
David Hewitt on thu 7 feb 02
Alan,
Yes, clay-glaze fit is dependant on both the body and the glaze and you
can change the fit by altering the clay body.
If you turn to my web site and look under Pottery Techniques /
Calculating Crazing you will see list of different clay bodies produced
by Potclays and their expansions. These I understand were obtained from
dilatometer tests rather than calculation, which is certainly the best
way to get such figures.
I would not give too much credence to the absolute values of calculated
clay body expansion figures, but they would be a guide as to the change,
and in which direction, a different formulation was taking you.
Out of interest I did take three of Potclays stated analyses of clay
bodies and calculate out their expansion.
1120 Buff body 4.92 x10-6/oC Linear
1104 Red St Thomas 4.94 x10-6/oC Linear
1146 Porcelain 5.33 x10-6/oC Linear
These would at least seem to be the right order.
Changes in a clay body, like changes in a glaze recipe, to alter glaze-
body fit, can of course alter the way the glaze appearance.
David
In message , Alan D. Scott writes
>Having devoured Chapter 5 of Ron & John's new book, I've been thinking mo=
>re
>about thermal expansion of clay bodies. It seems to me that I should be
>able to formulate a clay body with a calculated expansion using Insight t=
>o
>match a given glaze. This may be approaching clay-glaze fit from the wro=
>ng
>direction, but as I will be mixing my own clay bodies, why not make use o=
>f
>Insight to fight the dragon from both ends? After all, the same material=
>s
>are used to formulate both clay body and glaze. Or, am I all wrong here?=
>??
>
>Alan Scott
>The Fourth Cup Clayworks
>Parker, AZ
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP18 3DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Web site http://www.dhpot.demon.co.uk
Ron Roy on fri 8 feb 02
No way Alan,
It is well know that calculated expansion of recrystallized glazes is not
reliable - we can only count on usable numbers with super cooled liquids -
glossy glazes.
On top of that we have quartz and sometimes cristobalite in fired clay and
they have a very strong effect on expansion/contraction when they go
through their inversions. Look at the charts of the clays - the quartz
inversions happen around 573C. You very seldom see cristobalite in cone 6
pottery clays.
There are ways of altering bodies to help cure fit problems - like adding
silica to get more compression on the glaze from the quartz inversion - or
adding cristobalite to a body - for the same reason.
Trouble with changing a clay body is - if all your glazes have the same
fault - like crazing - then maybe - but it is rarely the case. You might be
setting up another fit fault in another glaze that was not crazing.
Clear as mud? thats usually the case with clay - best to keep words like
however and sometimes around.
Good to see you thinking though - RR
>Having devoured Chapter 5 of Ron & John's new book, I've been thinking more
>about thermal expansion of clay bodies. It seems to me that I should be
>able to formulate a clay body with a calculated expansion using Insight to
>match a given glaze. This may be approaching clay-glaze fit from the wrong
>direction, but as I will be mixing my own clay bodies, why not make use of
>Insight to fight the dragon from both ends? After all, the same materials
>are used to formulate both clay body and glaze. Or, am I all wrong here???
>
>Alan Scott
>The Fourth Cup Clayworks
>Parker, AZ
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
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