Ron Roy on thu 15 oct 98
Hello the group,
There have been questions about my use of the word freeze - or frozen - to
describe what happens when a glaze becomes solid and can no longer adjust
to the further contraction of the clay on cooling. The literature on the
subject uses the work congeal to describe this but I found this confusing
so sought to make it more understandable. There are other words which might
be appropriate - harden, set, solidify.
I am most interested in hearing from those of you who are trying to
understand expansion of course because I want to talk in terms that lead to
understanding. I am certainly open to suggestions and welcome thoughts on
this no matter what your understanding of the subject is.
Please feel free to comment via the list or or off list. I will summerize
private messages for the list - RR
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 fri 16 oct 98
This is in expansion of Ron Roy's remarks.
A good way to look at glaze is the following:
Glaze is an amorphous material, it normally is not crystalline, so
when it cools it just slowly gets more and more viscous (thicker
and thicker, like molasses) until it just doesn't move any more.
In a way it is like butter in that it is liquid when hot and solid
when cool and changes very slowly from one form to the other.
Freezing can be confusing because that intimates a specific point
at which a liquid becomes a solid, a very specific point. But
glaze doesn't do that, just like butter, it just gets harder and
harder Because there is no set point at which it becomes to stiff
to move, Ron's choices of set, harden or solidify make sense.
Just for our purposes, I propose harden. That is an undefined
point which varies glaze to glaze, but gives everyone a clear idea
of what is happening.
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.
Cameron
--
**********************************************************
Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
get your free ezine: http://www.kilnman.com/ezine/ezine.html
THE place for total kiln and drier support
**********************************************************
Michael McDowell on fri 16 oct 98
Ron,
I have to say that I'm one who is uncomfortable with using the term "freeze"
as a substitute for "congeal" which you say is the standard in the
literature for describing the stage at which a cooling glaze is no longer
able to flow to adjust to contraction of the cooling clay beneath it. To me,
using the term "freeze" suggests a phase change (such as water changing to
ice) which might involve a major change in the dimensions of the glaze
itself. By my understanding, there is no such phase change at this stage,
it's just that the steadily increasing viscosity of the glaze as it cools
reaches a point where it can no longer adjust in "real time" to the further
contraction of the clay.
Still, I agree with you that "congeal" leaves something to be desired. How
about "stiffen"?
Michael McDowell
Whatcom County, WA USA
mmpots@memes.com
http://www2.memes.com/mmpots
David Hewitt on sun 18 oct 98
Ron,
For my two-penny worth, I suppose all can be considered as appropriately
meaningful, but personally I do not like 'congeal' and would prefer
'solidify'
David
In message , Ron Roy writes
>----------------------------Original message----------------------------
>Hello the group,
>
>There have been questions about my use of the word freeze - or frozen - to
>describe what happens when a glaze becomes solid and can no longer adjust
>to the further contraction of the clay on cooling. The literature on the
>subject uses the work congeal to describe this but I found this confusing
>so sought to make it more understandable. There are other words which might
>be appropriate - harden, set, solidify.
>
>I am most interested in hearing from those of you who are trying to
>understand expansion of course because I want to talk in terms that lead to
>understanding. I am certainly open to suggestions and welcome thoughts on
>this no matter what your understanding of the subject is.
>
>Please feel free to comment via the list or or off list. I will summerize
>private messages for the list - RR
>
>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
>
--
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
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