Snail Scott on fri 10 oct 03
Ron:
I am experimenting with some non-standard techniques,=20
and the temperature at which a glaze re-freezes (or=20
stays frozen when reheated) has become relevant. I=20
wonder if there are any useful rules of thumb for=20
guessing what that temperature might be, or will it=20
require separate random testing of each recipe? I'd=20
love to narrow down the pool of 'possibles' before=20
making tests, as they will be much more labor-
intensive than a simple glaze test and I'd like to=20
optimize the time spent.=20
Is it safe to say that glazes which mature at similar=20
temeratures will melt at similar temperatures? (I=20
have my doubts about that.) I know some fluxes have=20
a narrower melt range than others - will a larger=20
percentage of these fluxes be a useful indicator?=20
So far, I've only tested ash-deposit glaze (not a=20
recipe, just anagama pine fly-ash melt). It has=20
stayed frozen at the 1800=BAF limit I'm after, but=20
the clay is too brittle to withstand the rest of=20
the process. (Too much cristobalite, I'd guess.)
So althought the wood-fire look is perfect, I need=20
to find a finish that works with a more forgiving=20
fired clay body. ^6 would be great, but I suspect=20
that it creates a very narrow range, for the fired=20
glaze to stay frozen when reheated to 1800=BAF. Not=20
sure if any ^6-7 glazes are still frozen at that=20
temp. Semi-vitreous engobes and matte glazes of=20
the 'underfired gloss' type will suit my purposes,=20
so I'm hopeful, but can you recommend a starting=20
place - things to look for in a likely recipe?
Lacking glaze software, I'll be doing any needed=20
calculation by hand, so I'd welcome any hints
about where to begin.
-Snail
Ron Roy on sat 11 oct 03
Hi Snail,
There is no correlation between end temperature and set point (freezing) of
glazes at any temperature.
It has to do with the viscosity and the flux to refractory balance as far
as I can see.
Stiff glazes (high alumina) freeze sooner than those with high KnaO or B2O3
for instance.
I tried to use the expansion glazes in our book (chapter 5) to illustrate
but they are very close and it is not clear. Probably because they are
fairly well balanced glazes.
There is only one way I can see to get that information and that is to run
the dilatometer tests. Pricey at $75 US per run. Let me know if you want to
go that route.
On another note - you say cristobalite - what is your end temperature for
the clay? If it's cone 10 and a high iron body then I would guess yes - but
it is possible to formulate bodies at that temperature that will not have a
cristobalite problem.
The time firing between 1100C to the end and down again to 1100C is when
cristobalite forms - so crash cooling to 1100C will help somewhat.
If you have to contend with high cristobalite it will be a matter of
refiring slow enough so it is not a problem.
RR
>I am experimenting with some non-standard techniques,
>and the temperature at which a glaze re-freezes (or
>stays frozen when reheated) has become relevant. I
>wonder if there are any useful rules of thumb for
>guessing what that temperature might be, or will it
>require separate random testing of each recipe? I'd
>love to narrow down the pool of 'possibles' before
>making tests, as they will be much more labor-
>intensive than a simple glaze test and I'd like to
>optimize the time spent.
>
>Is it safe to say that glazes which mature at similar
>temeratures will melt at similar temperatures? (I
>have my doubts about that.) I know some fluxes have
>a narrower melt range than others - will a larger
>percentage of these fluxes be a useful indicator?
>
>So far, I've only tested ash-deposit glaze (not a
>recipe, just anagama pine fly-ash melt). It has
>stayed frozen at the 1800=BAF limit I'm after, but
>the clay is too brittle to withstand the rest of
>the process. (Too much cristobalite, I'd guess.)
>So althought the wood-fire look is perfect, I need
>to find a finish that works with a more forgiving
>fired clay body. ^6 would be great, but I suspect
>that it creates a very narrow range, for the fired
>glaze to stay frozen when reheated to 1800=BAF. Not
>sure if any ^6-7 glazes are still frozen at that
>temp. Semi-vitreous engobes and matte glazes of
>the 'underfired gloss' type will suit my purposes,
>so I'm hopeful, but can you recommend a starting
>place - things to look for in a likely recipe?
>
>Lacking glaze software, I'll be doing any needed
>calculation by hand, so I'd welcome any hints
>about where to begin.
>
> -Snail
>
>___________________________________________________________________________=
___
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.=
com.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
=46ax: 613-475-3513=20
Snail Scott on sun 12 oct 03
At 01:33 PM 10/11/03 -0500, Ron wrote:
>There is no correlation between end temperature and set point (freezing) of
>glazes at any temperature.
>
Interesting. No correlation at all? Wow!
>On another note - you say cristobalite - what is your end temperature for
>the clay?
>The time firing between 1100C to the end and down again to 1100C is when
>cristobalite forms - so crash cooling to 1100C will help somewhat.
It was fired to around ^13-14 in a three-day firing with a
long cool-down. Not my firing; my stuff was just along for
the ride, mooching on Paul's generosity (and his anagama).
Liked the look of the wood ash for my purposes, but I may
just have to find a different approach.
>If you have to contend with high cristobalite it will be a matter of
>refiring slow enough so it is not a problem.
Unfortunately, not an option. Other constraints dictate
a fast reheat and cool. I'm hoping that kyanite might
help, but haven't tried it yet. Kyanite seems unknown
to West Coast suppliers (they keep trying to give me
cryolite), so I may just mail-order some.
I suspect that I'm asking the impossible from this
particular combo of materials and processes. I'll just
have to look at other surfaces, I think.
-Snail
| |
|
