Ron Roy on mon 19 oct 98
Hi Cameron,
Thanks for your comments on my question about how to describe that state
when a cooling glaze can no longer adjust to the body. It just occurred to
me that "immobilized" could be another way of describing this state.
I have still not heard from anyone who is trying to understand about
expansion and contraction - the question is - what word would be best for
describing when a glaze hardens enough to become rigid?
The statement below seems to indicate that slow firing would be better than
fast - is that what you are saying Cameron?
>----------------------------Original message----------------------------
>There is another pint of equal interest. On heating the surface of
>a glaze will "sinter" which is a solid to solid joining. When the
>glaze surface has sintered it forms a barrier to prevent gasses
>from the glaze composition from leaving. This will cause bubbles
>and blisters later when the glaze surface has melted and the gas
>bubbles try to get out. It is always important to fire a glaze in
>such a way that the gas forming reactions are all complete before
>the surface 'sinters' or becomes 'crusted over' to the point that
>the gasses are trapped.
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
Cameron Harman on tue 20 oct 98
Roy,
Yes slow firing is better from the point at which the glaze begins
to out gas until it finishes. In some glazes this is a temperature
range from 1500 F to 1700 F ,say 800 C to 950 C. It must be
determined for each type of glaze, usually not too many types to
worry about. Obviously a low melting glaze is a different type
than a high melting glaze.
Cameron
--
**********************************************************
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Sue Ballou on wed 21 oct 98
I'm one of the people struggling to understand more about the chemistry of
glazes, expansion and contraction, etc., so I've been following these
explanations with interest. The term "freeze" works best for me. I
understand the concerns about change of state, but given other definitions of
'freeze' (as in body motion) I think it makes it very clear. Thanks to all of
you who understand this stuff for all your work explaining it to those of us
who don't.
Sue Ballou
Roger Keane on sat 24 oct 98
Ron Roy wrote:
> ----------------------------Original message----------------------------
>
>
> I have still not heard from anyone who is trying to understand about
> expansion and contraction - the question is - what word would be best for
> describing when a glaze hardens enough to become rigid?
>
> >----------------------------Original message----------------------------
> Hi Ron,
The point at which a cooling glaze becomes rigid is known as it's Transition
Point .
Regarding the glaze transitional point, you may be interested to know that it
is possible to have two glazes of exactly the same thermal expansion,one which
crazes and the other which fits. The former glaze crazes because of it low
transitional point and the other fits because of its higher transitional point.
Regards
Roger Keane
Keane Ceramics Pty Ltd
3971 Debenham Road
Somersby,NSW. 2250 Australia
David Hewitt on mon 26 oct 98
In message , Roger Keane writes
Roger,
Can you, please, tell us more about the Transition Point. Can this be
predicted or calculated from a glaze recipe?
David
>
>The point at which a cooling glaze becomes rigid is known as it's Transition
>Point .
>
>Regarding the glaze transitional point, you may be interested to know that it
>is possible to have two glazes of exactly the same thermal expansion,one which
>crazes and the other which fits. The former glaze crazes because of it low
>transitional point and the other fits because of its higher transitional point.
>
> Regards
>Roger Keane
>Keane Ceramics Pty Ltd
>3971 Debenham Road
>Somersby,NSW. 2250 Australia
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
Ron Roy on tue 27 oct 98
Hi Roger,
Yes I understand this - I have a dilatometer and regularly work on glazes -
but my problem is how to describe that transition point - when a glaze is
no longer a liquid in any sense - to potters who are struggeling to
undertand what happens as glaze and body cool and the resulting stresses
build up.
Do you have any information about the effect of faster or slower cooling on
the transtional point.
Would you care to expound on your statement about two glazes having the
same expansion - on the same body and having different fit results. I
happen to know this is true but there are different factors involved -
viscosity, cooling and body quartz for instance.
>The point at which a cooling glaze becomes rigid is known as it's Transition
>Point .
>
>Regarding the glaze transitional point, you may be interested to know that it
>is possible to have two glazes of exactly the same thermal expansion,one which
>crazes and the other which fits. The former glaze crazes because of it low
>transitional point and the other fits because of its higher transitional point.
>
> Regards
>Roger Keane
>Keane Ceramics Pty Ltd
>3971 Debenham Road
>Somersby,NSW. 2250 Australia
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
Roger Keane on thu 29 oct 98
Hi David,
A glaze Transition Point can not be predicted or calculated from a glaze recipe.
can only be observed on a glaze thermal expansion graph measured by a dilatomete
This is one reason why computer calculated glaze expansions are not accurate and
only be regarded as a usefull approximation.
Regards
Roger Keane
Keane Ceramics Pty Ltd
3971 Debenham Road
Somersby NSW 2250 Australia
David Hewitt wrote:
> ----------------------------Original message----------------------------
>
> Can you, please, tell us more about the Transition Point. Can this be
> predicted or calculated from a glaze recipe?
> David
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